muscles

67
CHAPTER 4
Chapter Outline
Epithelial Tissue
Simple Squamous Epithelium
Stratified Squamous Epithelium
Transitional Epithelium
Simple Cuboidal Epithelium
Simple Columnar Epithelium
Ciliated Epithelium
Glands
Unicellular glands
Multicellular glands
Connective Tissue
Blood
Areolar Connective Tissue
Adipose Tissue
Fibrous Connective Tissue
Elastic Connective Tissue
Bone
Cartilage
Muscle Tissue
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
Nerve Tissue
Membranes
Epithelial Membranes
Serous membranes
Mucous membranes
Connective Tissue Membranes
Aging and Tissues
68
BOX 4–1 CYSTIC FIBROSIS
BOX 4–2 VITAMIN C AND COLLAGEN
BOX 4–3 COSMETIC COLLAGEN
Student Objectives
• Describe the general characteristics of each of the
four major categories of tissues.
• Describe the functions of the types of epithelial
tissues with respect to the organs in which they
are found.
• Describe the functions of the connective tissues,
and relate them to the functioning of the body or
a specific organ system.
• Explain the differences, in terms of location and
function, among skeletal muscle, smooth muscle,
and cardiac muscle.
• Name the three parts of a neuron and state the
function of each. Name the organs made of nerve
tissue.
• Describe the locations of the pleural membranes,
the pericardial membranes, and the peritoneummesentery.
State the function of serous fluid in
each of these locations.
• State the locations of mucous membranes and the
functions of mucus.
• Name some membranes made of connective
tissue.
• Explain the difference between exocrine and
endocrine glands, and give an example of each.
Tissues and Membranes
69
New Terminology
Bone (BOWNE)
Cartilage (KAR-ti-lidj)
Chondrocyte (KON-droh-sight)
Collagen (KAH-lah-jen)
Connective tissue (kah-NEK-tiv TISH-yoo)
Elastin (eh-LAS-tin)
Endocrine gland (EN-doh-krin GLAND)
Epithelial tissue (EP-i-THEE-lee-uhl TISH-yoo)
Exocrine gland (EK-so-krin GLAND)
Hemopoietic (HEE-moh-poy-ET-ik)
Matrix (MAY-triks)
Mucous membrane (MEW-kuss MEM-brayn)
Muscle tissue (MUSS-uhl TISH-yoo)
Myocardium (MY-oh-KAR-dee-um)
Nerve tissue (NERV TISH-yoo)
Neuron (NYOOR-on)
Neurotransmitter (NYOOR-oh-TRANS-mih-ter)
Osteocyte (AHS-tee-oh-sight)
Plasma (PLAZ-mah)
Secretion (see-KREE-shun)
Serous membrane (SEER-us MEM-brayn)
Synapse (SIN-aps)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Atissue is a group of cells with similar structure
and function. The tissue contributes to the functioning
of the organs in which it is found. You may recall
that in Chapter 1 the four major groups of tissues were
named and very briefly described. These four groups
are epithelial, connective, muscle, and nerve tissue.
This chapter presents more detailed descriptions of
the tissues in these four categories. For each tissue, its
functions are related to the organs of which it is a part.
Also in this chapter is a discussion of membranes,
which are sheets of tissues. As you might expect, each
type of membrane has its specific locations and functions.
EPITHELIAL TISSUE
Epithelial tissues are found on surfaces as either coverings
(outer surfaces) or linings (inner surfaces).
Because they have no capillaries of their own, epithelial
tissues receive oxygen and nutrients from the
blood supply of the connective tissue beneath them.
Many epithelial tissues are capable of secretion and
may be called glandular epithelium, or more simply,
glands.
Classification of the epithelial tissues is based on
the type of cell of which the tissue is made, its characteristic
shape, and the number of layers of cells. There
are three distinctive shapes: squamous cells are flat,
cuboidal cells are cube shaped, and columnar cells
are tall and narrow. “Simple” is the term for a single
layer of cells, and “stratified” means that many layers
of cells are present (Fig. 4–1).
SIMPLE SQUAMOUS EPITHELIUM
Simple squamous epithelium is a single layer of flat
cells (Fig. 4–2). These cells are very thin and very
smooth—these are important physical characteristics.
The alveoli (air sacs) of the lungs are simple squamous
epithelium. The thinness of the cells permits the diffusion
of gases between the air and blood.
Another location of this tissue is capillaries, the
smallest blood vessels. Capillary walls are only one cell
thick, which permits the exchange of gases, nutrients,
and waste products between the blood and tissue fluid.
The interior surface of capillaries is also very smooth
(and these cells continue as the lining of the arteries,
veins, and heart); this is important because it prevents
abnormal blood clotting within blood vessels.
70 Tissues and Membranes
Columnar
Cuboidal
Squamous
Simple columnar
Simple cuboidal
Simple squamous
Stratified squamous
Shapes Simple Stratified
Figure 4–1. Classification of epithelial tissues
based on the shape of the cells and the number of
layers of cells.
QUESTION: Which of these might be best for efficient
diffusion, and why?
STRATIFIED SQUAMOUS EPITHELIUM
Stratified squamous epithelium consists of many
layers of mostly flat cells, although lower cells are
rounded. Mitosis takes place in the lowest layer to
continually produce new cells to replace those worn
off the surface (see Fig. 4–2). This type of epithelium
makes up the epidermis of the skin, where it is called
“keratinizing” because the protein keratin is produced,
and the surface cells are dead. Stratified squamous
epithelium of the non-keratinizing type lines the oral
cavity, the esophagus, and, in women, the vagina. In
these locations the surface cells are living and make up
the mucous membranes of these organs. In all of its
body locations, this tissue is a barrier to microorganisms
because the cells of which it is made are very
close together. The more specialized functions of the
epidermis will be covered in the next chapter.
TRANSITIONAL EPITHELIUM
Transitional epithelium is a type of stratified epithelium
in which the surface cells change shape from
round to squamous. The urinary bladder is lined with
transitional epithelium. When the bladder is empty,
the surface cells are rounded (see Fig. 4–2). As the
Tissues and Membranes 71
Free surface
Stratified squamous
Connective tissues
Transitional
Simple squamous
Example: Lung (approximately 430X)
Alveolar sacs
Example: Esophagus
(approximately 430X)
B
Free surface
Example: Urinary bladder (approximately 430X)
A C
Figure 4–2. Epithelial tissues. (A) Simple squamous. (B) Stratified squamous.
(C) Transitional.
QUESTION: Which two of these tissues seem to be most related in structure?
bladder fills, these cells become flattened. Transitional
epithelium enables the bladder to fill and stretch without
tearing the lining.
SIMPLE CUBOIDAL EPITHELIUM
Simple cuboidal epithelium is a single layer of cubeshaped
cells (Fig. 4–3). This type of tissue makes up
the functional units of the thyroid gland and salivary
glands. These are examples of glandular epithelium;
their function is secretion. In these glands the
cuboidal cells are arranged in small spheres and secrete
into the cavity formed by the sphere. In the thyroid
gland, the cuboidal epithelium secretes the thyroid
hormones; thyroxine is an example. In the salivary
glands the cuboidal cells secrete saliva. Cuboidal
epithelium also makes up portions of the kidney
tubules. Here the cells have microvilli (see Fig. 1–1),
and their function is the reabsorption of useful materials
back to the blood.
SIMPLE COLUMNAR EPITHELIUM
Columnar cells are taller than they are wide and are
specialized for secretion and absorption. The stomach
lining is made of columnar epithelium that
secretes gastric juice for digestion. The lining of the
small intestine (see Fig. 4–3) secretes digestive
enzymes, but these cells also absorb the end products
of digestion from the cavity of the intestine into the
blood and lymph. To absorb efficiently, the columnar
cells of the small intestine have microvilli, which you
72 Tissues and Membranes
Thyroid secretions (hormones)
Simple cuboidal
Simple columnar
Goblet cells
Cilia
Connective tissue
Ciliated
Example: Small intestine
(approximately 430X)
B
Example: Trachea (approximately 430X)
A C
Example: Thyroid gland (approximately 430X)
Figure 4–3. Epithelial tissues. (A) Simple cuboidal. (B) Simple columnar. (C) Ciliated.
QUESTION: What is the function of the cilia that line the trachea?
may recall are folds of the cell membrane on their free
surfaces (see Fig. 3–2). These microscopic folds
greatly increase the surface area for absorption.
Yet another type of columnar cell is the goblet cell,
which is a unicellular gland. Goblet cells secrete
mucus and are found in the lining of the intestines
and the lining of parts of the respiratory tract such as
the trachea. Mucous membranes will be described in a
later section.
CILIATED EPITHELIUM
Ciliated epithelium consists of columnar cells that
have cilia on their free surfaces (see Fig. 4–3). Recall
from Chapter 3 that the function of cilia is to sweep
materials across the cell surface. Ciliated epithelium
lines the nasal cavities, larynx, trachea, and large
bronchial tubes. The cilia sweep mucus, with trapped
dust and bacteria from the inhaled air, toward the
pharynx to be swallowed. Bacteria are then destroyed
by the hydrochloric acid in the stomach. The air that
reaches the lungs is almost entirely free of pathogens
and particulate pollution.
Another location of ciliated epithelium in women is
the lining of the fallopian tubes. The cilia here sweep
the ovum, which has no means of self-locomotion,
toward the uterus.
The epithelial tissues are summarized in Table 4–1.
GLANDS
Glands are cells or organs that secrete something;
that is, they produce a substance that has a function
either at that site or at a more distant site.
Unicellular Glands
Unicellular means “one cell.” Goblet cells are an
example of unicellular glands. As mentioned earlier,
goblet cells are found in the lining of the respiratory
and digestive tracts. Their secretion is mucus (see also
Box 4–1: Cystic Fibrosis).
Multicellular Glands
Most glands are made of many similar cells, or of a
variety of cells with their secretions mingled into a
collective secretion. Multicellular glands may be
divided into two major groups: exocrine glands and
endocrine glands.
Exocrine glands have ducts (tubes) to take the
secretion away from the gland to the site of its function.
Salivary glands, for example, secrete saliva that is
carried by ducts to the oral cavity. Sweat glands secrete
sweat that is transported by ducts to the skin surface,
where it can be evaporated by excess body heat. The
gastric glands of the stomach lining contain different
kinds of cells (see Fig. 16–5), which produce
Tissues and Membranes 73
Table 4–1 TYPES OF EPITHELIAL TISSUE
Type Structure Location and Function
Simple squamous
Stratified squamous
Transitional
Cuboidal
Columnar
Ciliated
One layer of flat cells
Many layers of cells; surface cells
flat; lower cells rounded;
lower layer undergoes mitosis
Many layers of cells; surface cells
change from rounded to flat
One layer of cube-shaped cells
One layer of column-shaped cells
One layer of columnar cells with
cilia on their free surfaces
• Alveoli of the lungs—thin to permit diffusion of gases
• Capillaries—thin to permit exchanges of materials;
smooth to prevent abnormal blood clotting
• Epidermis—surface cells are dead; a barrier to pathogens
• Lining of esophagus, vagina—surface cells are living; a
barrier to pathogens
• Lining of urinary bladder—permits expansion without
tearing the lining
• Thyroid gland—secretes thyroxine
• Salivary glands—secrete saliva
• Kidney tubules—permit reabsorption of useful materials
back to the blood
• Lining of stomach—secretes gastric juice
• Lining of small intestine—secretes enzymes and absorbs
end products of digestion (microvilli present)
• Lining of trachea—sweeps mucus and dust to the pharynx
• Lining of fallopian tube—sweeps ovum toward uterus
hydrochloric acid and the enzyme pepsin. Both of
these secretions are part of gastric juice.
Endocrine glands are ductless glands. The secretions
of endocrine glands are a group of chemicals
called hormones, which enter capillaries and are circulated
throughout the body. Hormones then bring
about specific effects in their target organs. These
effects include aspects of growth, use of minerals and
other nutrients, and regulation of blood pressure, and
will be covered in more detail in Chapter 10.
Examples of endocrine glands are the thyroid gland,
adrenal glands, and pituitary gland.
The pancreas is an organ that is both an exocrine
and an endocrine gland. The exocrine portions secrete
digestive enzymes that are carried by ducts to the
duodenum of the small intestine, their site of action.
The endocrine portions of the pancreas, called pancreatic
islets or islets of Langerhans, secrete the hormones
insulin and glucagon directly into the blood.
CONNECTIVE TISSUE
There are several kinds of connective tissue, some of
which may at first seem more different than alike. The
types of connective tissue include areolar, adipose,
fibrous, and elastic tissue as well as blood, bone, and
cartilage; these are summarized in Table 4–2. A characteristic
that all connective tissues have in common is
the presence of a matrix in addition to cells. The
matrix is a structural network or solution of nonliving
intercellular material. Each connective tissue
has its own specific kind of matrix. The matrix of
blood, for example, is blood plasma, which is mostly
water. The matrix of bone is made primarily of calcium
salts, which are hard and strong. As each type of
connective tissue is described in the following sections,
mention will be made of the types of cells present
as well as the kind of matrix.
BLOOD
Although blood is the subject of Chapter 11, a brief
description will be given here. Blood consists of cells
and plasma; cells are the living portion. The matrix of
blood is plasma, which is about 52% to 62% of the
total blood volume in the body. The water of plasma
contains dissolved salts, nutrients, gases, and waste
products. As you might expect, one of the primary
functions of plasma is transport of these materials
within the body.
Blood cells are produced from stem cells in the red
bone marrow, the body’s primary hemopoietic tissue
(blood-forming tissue), which is found in flat and
irregular bones such as the hip bone and vertebrae.
The blood cells are red blood cells, platelets, and the
five kinds of white blood cells: neutrophils,
eosinophils, basophils, monocytes, and lymphocytes
(see Figs. 4–4 and 11–2). Lymphocytes mature and
divide in lymphatic tissue, which makes up the spleen,
74 Tissues and Membranes
BOX 4–1 CYSTIC FIBROSIS
defensin, a bacterium called Pseudomonas aeruginosa
stimulates the lung cells to produce copious
thick mucus, an ideal growth environment for bacteria.
Defensive white blood cells cannot get
through the thick mucus, and their activity mistakenly
destroys lung tissue. A person with CF has
clogged bronchial tubes, frequent episodes of
pneumonia, and, ultimately, lungs that cannot carry
out gas exchange. CF is a chronic, progressive disease
that is eventually fatal unless a lung transplant
is performed.
CF is one of several disorders believed to be correctable
by gene therapy, but because it involves
human subjects, this kind of work proceeds very
slowly.
Cystic fibrosis (CF) is a genetic disorder (there are
many forms) of certain exocrine glands including
the salivary glands, the sweat glands, the pancreas,
and the mucous glands of the respiratory tract.
In the pancreas, thick mucus clogs the ducts and
prevents pancreatic enzymes from reaching the
small intestine, thus impairing digestion, especially
of fats. But the most serious effects of CF are in the
lungs. The genetic mistake in CF often involves a
gene called CFTR, which codes for chloride ion
channels (proteins) in the membranes of epithelial
cells. In the lungs, the defective channels are
destroyed (by proteasomes), which causes a change
in the composition of the tissue fluid around the
cells. This change inactivates defensin, a natural
antibiotic produced by lung tissue. In the absence of
the lymph nodes, and the thymus gland. The thymus
also contains stem cells, but they produce only a
subset of lymphocytes. Stem cells are present in the
spleen and lymph nodes as well, though the number of
lymphocytes they produce is a small fraction of the
total.
The blood cells make up 38% to 48% of the total
blood, and each type of cell has its specific function.
Red blood cells (RBCs) carry oxygen bonded to the
iron in their hemoglobin. White blood cells (WBCs)
destroy pathogens by phagocytosis, the production of
antibodies, or other chemical methods, and provide us
with immunity to some diseases. Platelets prevent
blood loss; the process of blood clotting involves
platelets.
AREOLAR CONNECTIVE TISSUE
The cells of areolar (or loose) connective tissue are
called fibroblasts. A blast cell is a “producing” cell,
and fibroblasts produce protein fibers. Collagen fibers
are very strong; elastin fibers are elastic, that is, able
to return to their original length, or recoil, after being
stretched. These protein fibers and tissue fluid make
up the matrix, or non-living portion, of areolar connective
tissue (see Fig. 4–4). Also within the matrix are
mast cells that release inflammatory chemicals when
tissue is damaged, and many white blood cells, which
are capable of self-locomotion. Their importance here
is related to the locations of areolar connective tissue.
Areolar tissue is found beneath the dermis of the
Tissues and Membranes 75
Blood
Areolar
Adipose
White blood cell
Fibroblast
Collagen fibers
Elastin fiber
Adipocytes
(Approximately 430X)
Red blood cells
White blood cell
(Approximately 300X)
(Approximately 150X)
A
B
C
Platelets
Figure 4–4. Connective tissues. (A) Blood. (B) Areolar. (C) Adipose.
QUESTION: What is the matrix of blood, and what is found in adipocytes?
skin and beneath the epithelial tissue of all the body
systems that have openings to the environment. Recall
that one function of white blood cells is to destroy
pathogens. How do pathogens enter the body? Many
do so through breaks in the skin. Bacteria and viruses
also enter with the air we breathe and the food we eat,
and some may get through the epithelial linings of the
respiratory and digestive tracts and cause tissue damage.
Areolar connective tissue with its mast cells and
many white blood cells is strategically placed to intercept
pathogens before they get to the blood and circulate
throughout the body.
ADIPOSE TISSUE
The cells of adipose tissue are called adipocytes and
are specialized to store fat in microscopic droplets.
True fats are the chemical form of long-term energy
storage. Excess nutrients have calories that are not
wasted but are converted to fat to be stored for use
when food intake decreases. Any form of excess calories,
whether in the form of fats, carbohydrates, or
amino acids from protein, may be changed to triglycerides
and stored. The amount of matrix in adipose
tissue is small and consists of tissue fluid and a few collagen
fibers (see Fig. 4–4).
Most fat is stored subcutaneously in the areolar
connective tissue between the dermis and the muscles.
This layer varies in thickness among individuals; the
more excess calories consumed, the thicker the layer.
As mentioned in Chapter 2, adipose tissue also cushions
organs such as the eyes and kidneys.
Recent research has discovered that adipose tissue
does much more than provide a cushion or store
energy. Adipose tissue is now considered an endocrine
tissue, because it produces at least one hormone.
Leptin is an appetite-suppressing hormone secreted
by adipocytes to signal the hypothalamus in the brain
that fat storage is sufficient (see also Chapter 17).
When leptin secretion diminishes, appetite increases.
Adipocytes secrete at least two chemicals that help
regulate the use of insulin in glucose and fat metabolism.
Adipose tissue is also involved in inflammation,
the body’s first response to injury, in that it produces
cytokines, chemicals that activate white blood cells.
Our adipose tissue is not simply an inert depository of
76 Tissues and Membranes
Fibrous tissue
Collagen
fibers
Fibroblasts
Example: Trachea
(approximately 430X)
B
A
C
(Approximately 430X)
Haversian canal
Canaliculi
Matrix
Osteocytes
Bone
Chondrocytes
Matrix
Cartilage
Example: Tendons
(approximately 430X)
Figure 4–5. Connective tissues. (A) Fibrous. (B) Cartilage. (C) Bone.
QUESTION: What is the matrix of fibrous tissue, and of bone?
fat, rather it is part of the complex systems that ensure
we are nourished properly or that protect us from
pathogens that get through the skin.
FIBROUS CONNECTIVE TISSUE
Fibrous connective tissue consists mainly of parallel
(regular) collagen fibers with a few fibroblasts scattered
among them (Fig. 4–5). This parallel arrangement
of collagen provides great strength, yet is
flexible. The locations of this tissue are related to
the need for flexible strength. The outer walls of arteries
are reinforced with fibrous connective tissue,
because the blood in these vessels is under high pressure.
The strong outer wall prevents rupture of the
artery (see also Box 4–2: Vitamin C and Collagen).
Tendons and ligaments are made of fibrous connective
tissue. Tendons connect muscle to bone; ligaments
connect bone to bone. When the skeleton is moved,
these structures must be able to withstand the great
mechanical forces exerted upon them.
Fibrous connective tissue has a relatively poor
blood supply, which makes repair a slow process. If
you have ever had a severely sprained ankle (which
Tissues and Membranes 77
Table 4–2 TYPES OF CONNECTIVE TISSUE
Type Structure Location and Function
Blood
Areolar (loose)
Adipose
Fibrous
Elastic
Bone
Cartilage
Within blood vessels
• Plasma—transports materials
• RBCs—carry oxygen
• WBCs—destroy pathogens
• Platelets—prevent blood loss
Subcutaneous
• Connects skin to muscles; WBCs destroy pathogens
Mucous membranes (digestive, respiratory, urinary,
reproductive tracts)
• WBCs destroy pathogens
Subcutaneous
• Stores excess energy
• Produces chemicals that influence appetite, use
of nutrients, and inflammation
Around eyes and Kidneys
• Cushions
Tendons and ligaments (regular)
• Strong to withstand forces of movement of joints
Dermis (irregular)
• The strong inner layer of the skin
Walls of large arteries
• Helps maintain blood pressure Around alveoli in lungs
• Promotes normal exhalation
Bones
• Support the body
• Protect internal organs from mechanical injury
• Store excess calcium
• Contain and protect red bone marrow
Wall of trachea
• Keeps airway open
On joint surfaces of bones
• Smooth to prevent friction
Tip of nose and outer ear
• Support
Between vertebrae
• Absorb shock
Plasma (matrix) and red blood cells,
white blood cells, and platelets
Fibroblasts and a matrix of tissue
fluid, collagen, and elastin fibers
Adipocytes that store fat (little matrix)
Mostly collagen fibers (matrix) with
few fibroblasts
Mostly elastin fibers (matrix) with few
fibroblasts
Osteocytes in a matrix of calcium
salts and collagen
Chondrocytes in a flexible protein
matrix
means the ligaments have been overly stretched), you
know that complete healing may take several months.
An irregular type of fibrous connective tissue forms
the dermis of the skin and the fasciae (membranes)
around muscles. Although the collagen fibers here are
not parallel to one another, the tissue is still strong.
The dermis is different from other fibrous connective
tissue in that it has a good blood supply (see also Box
4–3: Cosmetic Collagen).
ELASTIC CONNECTIVE TISSUE
As its name tells us, elastic connective tissue is primarily
elastin fibers. One of its locations is in the walls
of large arteries. These vessels are stretched when the
heart contracts and pumps blood, then they recoil, or
snap back, when the heart relaxes. This recoil helps
keep the blood moving away from the heart, and is
important to maintain normal blood pressure.
Elastic connective tissue is also found surrounding
the alveoli of the lungs. The elastic fibers are stretched
during inhalation, then recoil during exhalation to
squeeze air out of the lungs. If you pay attention to
your breathing for a few moments, you will notice that
normal exhalation does not require “work” or energy.
This is because of the normal elasticity of the lungs.
BONE
The prefix that designates bone is “osteo,” so bone
cells are called osteocytes. The matrix of bone is
made of calcium salts and collagen and is strong, hard,
78 Tissues and Membranes
BOX 4–2 VITAMIN C AND COLLAGEN
Many people take extra vitamin C, for various reasons.
Vitamin C has several functions, and an important
one is the synthesis of collagen.
Imagine the protein collagen as a ladder with
three uprights and rungs that connect adjacent
uprights. Vitamin C is essential for forming the
“rungs,” without which the uprights will not stay
together as a strong unit. Collagen formed in the
absence of vitamin C is weak, and the effects of
weak collagen are dramatically seen in the disease
called scurvy.
In 1753 James Lind, a Scottish surgeon, recommended
to the British Navy that lime juice be taken
on long voyages to prevent scurvy among the
sailors. Scurvy is characterized by bleeding gums
and loss of teeth, poor healing of wounds, fractures,
and bleeding in the skin, joints, and elsewhere in
the body. The lime juice did prevent this potentially
fatal disease, as did consumption of fresh fruits and
vegetables, although at the time no one knew why.
Vitamin C was finally isolated in the laboratory in
1928.
BOX 4–3 COSMETIC COLLAGEN
system as foreign tissue. More seriously, an autoimmune
response may be triggered in some individuals,
and the immune system may begin to destroy
the person’s own connective tissue.
In an effort to avoid these problems, some cosmetic
surgeons now use the person’s own collagen
and fat, which may be extracted from the thigh,
hip, or abdomen. The long-term consequences and
outcomes of such procedures have yet to be evaluated.
We might remember that for many years the
use of silicone injections had been considered safe.
Silicone injections are now banned by the FDA,
since we now know that they carry significant risk
of serious tissue damage.
Collagen is the protein that makes tendons, ligaments,
and other connective tissues strong. In 1981,
the Food and Drug Administration (FDA) approved
the use of cattle collagen by injection for cosmetic
purposes, to minimize wrinkles and scars. Indeed,
collagen injected below the skin will flatten out
deep facial wrinkles and make them less prominent,
and many people have had this seemingly simple
cosmetic surgery.
There are, however, drawbacks. Injected collagen
lasts only a few months; the injections must be
repeated several times a year, and they are expensive.
Some people have allergic reactions to the cattle
collagen, which is perceived by the immune
and not flexible. In the shafts of long bones such as the
femur, the osteocytes, matrix, and blood vessels are
in very precise arrangements called haversian systems
or osteons (see Fig. 4–5). Bone has a good blood
supply, which enables it to serve as a storage site for
calcium and to repair itself relatively rapidly after a
simple fracture. Some bones, such as the sternum
(breastbone) and pelvic bone, contain red bone marrow,
the primary hemopoietic tissue that produces
blood cells.
Other functions of bone tissue are related to the
strength of bone matrix. The skeleton supports the
body, and some bones protect internal organs from
mechanical injury. A more complete discussion of
bone is found in Chapter 6.
CARTILAGE
The protein–carbohydrate matrix of cartilage does
not contain calcium salts, and also differs from that of
bone in that it contains more water, which makes it
resilient. It is firm, yet smooth and flexible. Cartilage
is found on the joint surfaces of bones, where its
smooth surface helps prevent friction. The tip of the
nose and external ear are supported by flexible cartilage.
The wall of the trachea, the airway to the lungs,
contains firm rings of cartilage to maintain an open air
passageway. Discs of cartilage are found between the
vertebrae of the spine. Here the cartilage is a firm
cushion; it absorbs shock and permits movement.
Within the cartilage matrix are the chondrocytes,
or cartilage cells (see Fig. 4–5). There are no capillaries
within the cartilage matrix, so these cells are nourished
by diffusion through the matrix, a slow process.
This becomes clinically important when cartilage is
damaged, for repair will take place very slowly or not
at all. Athletes sometimes damage cartilage within the
knee joint. Such damaged cartilage is usually surgically
removed in order to preserve as much joint mobility as
possible.
MUSCLE TISSUE
Muscle tissue is specialized for contraction. When
muscle cells contract, they shorten and bring about
some type of movement. There are three types of
muscle tissue: skeletal, smooth, and cardiac (Table
4–3). The movements each can produce have very different
purposes.
SKELETAL MUSCLE
Skeletal muscle may also be called striated muscle or
voluntary muscle. Each name describes a particular
aspect of this tissue, as you will see. The skeletal muscle
cells are cylindrical, have several nuclei each, and
appear striated, or striped (Fig. 4–6). The striations
are the result of the precise arrangement of the contracting
proteins within the cells.
Skeletal muscle tissue makes up the muscles that
are attached to bones. These muscles are supplied
with motor nerves, and thus move the skeleton. They
also produce a significant amount of heat, which is
important to help maintain the body’s constant temperature.
Each muscle cell has its own motor nerve
ending. The nerve impulses that can then travel to the
muscles are essential to cause contraction. Although
we do not have to consciously plan all our movements,
the nerve impulses for them originate in the cerebrum,
the “thinking” part of the brain.
Let us return to the three names for this tissue:
“skeletal” describes its location, “striated” describes its
appearance, and “voluntary” describes how it functions.
The skeletal muscles and their functioning are
the subject of Chapter 7.
SMOOTH MUSCLE
Smooth muscle may also be called involuntary muscle
or visceral muscle. The cells of smooth muscle
have tapered ends, a single nucleus, and no striations
(see Fig. 4–6). Although nerve impulses do bring
about contractions, this is not something most of us
can control, hence the name involuntary. The term visceral
refers to internal organs, many of which contain
smooth muscle. The functions of smooth muscle are
actually functions of the organs in which the muscle is
found.
In the stomach and intestines, smooth muscle contracts
in waves called peristalsis to propel food
through the digestive tract. In the walls of arteries and
veins, smooth muscle constricts or dilates the vessels
to maintain normal blood pressure. The iris of the eye
has two sets of smooth muscle fibers to constrict or
dilate the pupil, which regulates the amount of light
that strikes the retina.
Other functions of smooth muscle are mentioned
in later chapters. This is an important tissue that you
will come across again and again in our study of the
human body.
Tissues and Membranes 79
80
Skeletal muscle
(Approximately 430X)
A
Smooth muscle
(Approximately 430X)
B
Cardiac muscle
(Approximately 430X)
C
Intercalated
discs
Figure 4–6. Muscle tissues. (A) Skeletal. (B) Smooth. (C) Cardiac.
QUESTION: Which kinds of muscle cells have striations? What forms these striations?
Table 4–3 TYPES OF MUSCLE TISSUE
Type Structure Location and Function Effect of Nerve Impulses
Skeletal
Smooth
Cardiac
Large cylindrical cells with
striations and several
nuclei each
Small tapered cells with no
striations and one nucleus
each
Branched cells with faint striations
and one nucleus
each
Attached to bones
• Moves the skeleton and
produces heat
Walls of arteries
• Maintains blood pressure
Walls of stomach and intestines
• Peristalsis
Iris of eye
• Regulates size of pupil
Walls of the chambers of the heart
• Pumps blood
Essential to cause contraction
(voluntary)
Bring about contraction or
regulate the rate of contraction
(involuntary)
Regulate only the rate of contraction
CARDIAC MUSCLE
The cells of the heart, cardiac muscle, are shown in
Fig. 4–6. They are branched, have one nucleus each,
and have faint striations. The cell membranes at the
ends of these cells are somewhat folded and fit into
matching folds of the membranes of the next cells.
(Interlock the fingers of both hands to get an idea of
what these adjacent membranes look like.) These
interlocking folds are called intercalated discs, and
permit the electrical impulses of muscle contraction to
pass swiftly from cell to cell. This enables the heart to
beat in a very precise wave of contraction from the
upper chambers to the lower chambers. Cardiac muscle
as a whole is called the myocardium, and forms
the walls of the four chambers of the heart. Its function,
therefore, is the function of the heart, to pump
blood. The contractions of the myocardium create
blood pressure and keep blood circulating throughout
the body, so that the blood can carry out its many
functions.
Cardiac muscle cells have the ability to contract by
themselves. Thus the heart maintains its own beat.
The role of nerve impulses is to increase or decrease
the heart rate, depending upon whatever is needed
by the body in a particular situation. We will return to
the heart in Chapter 12.
NERVE TISSUE
Nerve tissue consists of nerve cells called neurons
and some specialized cells found only in the nervous
system. The nervous system has two divisions: the
central nervous system (CNS) and the peripheral
nervous system (PNS). The brain and spinal cord are
the organs of the CNS. They are made of neurons and
specialized cells called neuroglia. The CNS and the
neuroglia are discussed in detail in Chapter 8. The
PNS consists of all of the nerves that emerge from
the CNS and supply the rest of the body. These nerves
are made of neurons and specialized cells called
Schwann cells. The Schwann cells form the myelin
sheath to electrically insulate neurons.
Neurons are capable of generating and transmitting
electrochemical impulses. There are many different
kinds of neurons, but they all have the same basic
structure (Fig. 4–7). The cell body contains the
nucleus and is essential for the continuing life of the
neuron. An axon is a process (the term “process” here
means “something that sticks out,” a cellular extension)
that carries impulses away from the cell body; a
neuron has only one axon. Dendrites are processes
that carry impulses toward the cell body; a neuron
may have several dendrites. A nerve impulse travels
along the cell membrane of a neuron, and is electrical,
but where neurons meet there is a small space called a
synapse, which an electrical impulse cannot cross. At
a synapse, between the axon of one neuron and the
dendrite or cell body of the next neuron, impulse
transmission depends upon chemicals called neurotransmitters.
A summary of nerve tissue is found in
Table 4–4, and each of these aspects of nerve tissue is
covered in more detail in Chapter 8.
Nerve tissue makes up the brain, spinal cord, and
Tissues and Membranes 81
Table 4–4 NERVE TISSUE
Part Structure Function
Neuron (nerve cell)
Cell body
Axon
Dendrites
Synapse
Neurotransmitters
Neuroglia
Schwann cells
Contains the nucleus
Cellular process (extension)
Cellular process (extension)
Space between axon of one neuron and the
dendrite or cell body of the next neuron
Chemicals released by axons
Specialized cells in the central nervous
system
Specialized cells in the peripheral nervous
system
• Regulates the functioning of the neuron
• Carries impulses away from the cell body
• Carry impulses toward the cell body
• Transmits impulses from one neuron
to others
• Transmit impulses across synapses
• Form myelin sheaths and other functions
• Form the myelin sheaths around neurons
peripheral nerves. As you can imagine, each of these
organs has very specific functions. For now, we will
just mention the categories of the functions of nerve
tissue. These include feeling and interpreting sensation,
initiation of movement, the rapid regulation of
body functions such as heart rate and breathing, and
the organization of information for learning and
memory.
MEMBRANES
Membranes are sheets of tissue that cover or line surfaces
or that separate organs or parts (lobes) of organs
from one another. Many membranes produce secretions
that have specific functions. The two major categories
of membranes are epithelial membranes and
connective tissue membranes.
EPITHELIAL MEMBRANES
There are two types of epithelial membranes, serous
and mucous. Each type is found in specific locations
within the body and secretes a fluid. These fluids are
called serous fluid and mucus.
Serous Membranes
Serous membranes are sheets of simple squamous
epithelium that line some closed body cavities and
cover the organs in these cavities (Fig. 4–8). The
pleural membranes are the serous membranes of the
thoracic cavity. The parietal pleura lines the chest wall
and the visceral pleura covers the lungs. (Notice that
line means “on the inside” and cover means “on the
outside.” These terms cannot be used interchangeably,
because each indicates a different location.) The pleural
membranes secrete serous fluid, which prevents
friction between them as the lungs expand and recoil
during breathing.
The heart, in the thoracic cavity between the lungs,
has its own set of serous membranes. The parietal
pericardium lines the fibrous pericardium (a connective
tissue membrane), and the visceral pericardium,
or epicardium, is on the surface of the heart muscle
(see also Fig. 12–1). Serous fluid is produced to prevent
friction as the heart beats.
In the abdominal cavity, the peritoneum is the
serous membrane that lines the cavity. The mesentery,
or visceral peritoneum, is folded over and covers
the abdominal organs. Here, the serous fluid prevents
friction as the stomach and intestines contract and
slide against other organs (see also Fig. 16–4).
Mucous Membranes
Mucous membranes line the body tracts (systems)
that have openings to the environment. These are the
respiratory, digestive, urinary, and reproductive tracts.
The epithelium of a mucous membrane (mucosa)
varies with the different organs involved. The mucosa
of the esophagus and of the vagina is stratified squamous
epithelium; the mucosa of the trachea is ciliated
epithelium; the mucosa of the stomach is columnar
epithelium.
82 Tissues and Membranes
Figure 4–7. Nerve tissue of the central nervous system
(CNS).
QUESTION: How many processes does the central neuron
have, and what are they called?
Cell body
Neuron
Neurons Nucleus
(Approximately 250X)
Neuroglia
83
Mucous Membranes
Parietal Pleura
Visceral Pleura
Mesentery
Peritoneum
Figure 4–8. Epithelial membranes. Mucous membranes line body tracts that open to the
environment. Serous membranes are found within closed body cavities such as the thoracic
and abdominal cavities. See text for further description.
QUESTION: Name another organ covered by mesentery.
The mucus secreted by these membranes keeps the
lining epithelial cells wet. Remember that these are
living cells, and if they dry out, they will die. In the
digestive tract, mucus also lubricates the surface to
permit the smooth passage of food. In the respiratory
tract the mucus traps dust and bacteria, which are then
swept to the pharynx by ciliated epithelium.
CONNECTIVE TISSUE MEMBRANES
Many membranes are made of connective tissue.
Because these will be covered with the organ systems
of which they are a part, their locations and functions
are summarized in Table 4–5.
AGING AND TISSUES
As mentioned in the previous chapter, aging takes
place at the cellular level, but of course is apparent in
the groups of cells we call tissues. In muscle tissue, for
example, the proteins that bring about contraction
deteriorate and are not repaired or replaced. The same
is true of collagen and elastin, the proteins of connective
tissue such as the dermis of the skin. Other aspects
of the aging of tissues will be more meaningful to you
in the context of the functions of organs and systems,
so we will save those for the following chapters.
SUMMARY
The tissues and membranes described in this chapter
are more complex than the individual cells of which
they are made. However, we have only reached an
intermediate level with respect to the structural and
functional complexity of the body as a whole. The following
chapters are concerned with the organ systems,
the most complex level. In the descriptions of the
organs of these systems, you will find mention of the
tissues and their contributions to each organ and
organ system.
84 Tissues and Membranes
Table 4–5 CONNECTIVE TISSUE
MEMBRANES
Membrane Location and Function
Superficial
fascia
Periosteum
Perichondrium
Synovial
Deep fascia
Meninges
Fibrous
pericardium
• Between the skin and muscles;
adipose tissue stores fat
• Covers each bone; contains
blood vessels that enter the
bone
• Anchors tendons and ligaments
• Covers cartilage; contains capillaries,
the only blood supply for
cartilage
• Lines joint cavities; secretes synovial
fluid to prevent friction
when joints move
• Covers each skeletal muscle;
anchors tendons
• Cover the brain and spinal cord;
contain cerebrospinal fluid
• Forms a sac around the heart;
lined by the serous parietal pericardium
STUDY OUTLINE
A tissue is a group of cells with similar structure
and function. The four main groups of
tissues are epithelial, connective, muscle, and
nerve.
Epithelial Tissue—found on surfaces; have no
capillaries; some are capable of secretion;
classified as to shape of cells and number of
layers of cells (see Table 4–1 and Figs. 4–1,
4–2, and 4–3)
1. Simple squamous—one layer of flat cells; thin and
smooth. Sites: alveoli (to permit diffusion of gases);
capillaries (to permit exchanges between blood and
tissues).
2. Stratified squamous—many layers of mostly flat
cells; mitosis takes place in lowest layer. Sites: epidermis,
where surface cells are dead (a barrier to
pathogens); lining of mouth; esophagus; and vagina
(a barrier to pathogens).
3. Transitional—stratified, yet surface cells are
rounded and flatten when stretched. Site: urinary
bladder (to permit expansion without tearing the
lining).
4. Simple cuboidal—one layer of cube-shaped cells.
Sites: thyroid gland (to secrete thyroid hormones);
salivary glands (to secrete saliva); kidney tubules (to
reabsorb useful materials back to the blood).
5. Simple columnar—one layer of column-shaped
cells. Sites: stomach lining (to secrete gastric juice);
small intestinal lining (to secrete digestive enzymes
and absorb nutrients—microvilli increase surface
area for absorption).
6. Ciliated—columnar cells with cilia on free surfaces.
Sites: trachea (to sweep mucus and bacteria to the
pharynx); fallopian tubes (to sweep ovum to
uterus).
7. Glands—epithelial tissues that produce secretions.
• Unicellular—one-celled glands. Goblet cells
secrete mucus in the respiratory and digestive
tracts.
• Multicellular—many-celled glands.
Exocrine glands have ducts; salivary glands
secrete saliva into ducts that carry it to the oral
cavity.
Endocrine glands secrete hormones directly
into capillaries (no ducts); thyroid gland secretes
thyroxine.
Connective Tissue—all have a non-living
intercellular matrix and specialized cells (see
Table 4–2 and Figs. 4–4 and 4–5)
1. Blood—the matrix is plasma, mostly water; transports
materials in the blood. Red blood cells carry
oxygen; white blood cells destroy pathogens and
provide immunity; platelets prevent blood loss, as
in clotting. Blood cells are made in red bone marrow.
2. Areolar (loose)—cells are fibroblasts, which produce
protein fibers: collagen is strong, elastin is
elastic; the matrix is collagen, elastin, and tissue
fluid. White blood cells and mast cells are also
present. Sites: below the dermis and below the
epithelium of tracts that open to the environment
(to destroy pathogens that enter the body).
3. Adipose—cells are adipocytes that store fat; little
matrix. Sites: between the skin and muscles (to
store energy); around the eyes and kidneys (to
cushion). Also involved in appetite, use of insulin,
and inflammation.
4. Fibrous—mostly matrix, strong collagen fibers;
cells are fibroblasts. Regular fibrous sites: tendons
(to connect muscle to bone); ligaments (to connect
bone to bone); poor blood supply, slow healing.
Irregular fibrous sites: dermis of the skin and the
fascia around muscles.
5. Elastic—mostly matrix, elastin fibers. Sites: walls of
large arteries (to maintain blood pressure); around
alveoli (to promote normal exhalation).
6. Bone—cells are osteocytes; matrix is calcium salts
and collagen, strong and not flexible; good blood
supply, rapid healing. Sites: bones of the skeleton
(to support the body and protect internal organs
from mechanical injury).
7. Cartilage—cells are chondrocytes; protein matrix is
firm yet flexible; no capillaries in matrix, very slow
healing. Sites: joint surfaces of bones (to prevent
friction); tip of nose and external ear (to support);
wall of trachea (to keep air passage open); discs
between vertebrae (to absorb shock).
Muscle Tissue—specialized to contract and
bring about movement (see Table 4–3 and
Fig. 4–6)
1. Skeletal—also called striated or voluntary muscle.
Cells are cylindrical, have several nuclei, and have
striations. Each cell has a motor nerve ending;
nerve impulses are essential to cause contraction.
Site: skeletal muscles attached to bones (to move
the skeleton and produce heat).
2. Smooth—also called visceral or involuntary muscle.
Cells have tapered ends, one nucleus each, and
no striations. Contraction is not under voluntary
control. Sites: stomach and intestines (peristalsis);
walls of arteries and veins (to maintain blood pressure);
iris (to constrict or dilate pupil).
3. Cardiac—cells are branched, have one nucleus
each, and faint striations. Site: walls of the four
chambers of the heart (to pump blood; nerve
impulses regulate the rate of contraction).
Nerve Tissue—neurons are specialized to
generate and transmit impulses (see Table
4–4 and Fig. 4–7)
1. Cell body contains the nucleus; axon carries
impulses away from the cell body; dendrites carry
impulses toward the cell body.
2. A synapse is the space between two neurons; a neurotransmitter
carries the impulse across a synapse.
Tissues and Membranes 85
3. Specialized cells in nerve tissue are neuroglia in the
CNS and Schwann cells in the PNS.
4. Sites: brain; spinal cord; and peripheral nerves (to
provide sensation, movement, regulation of body
functions, learning, and memory).
Membranes—sheets of tissue on surfaces, or
separating organs or lobes
1. Epithelial membranes (see Fig. 4–8)
• Serous membranes—in closed body cavities; the
serous fluid prevents friction between the two
layers of the serous membrane.
Thoracic cavity—partial pleura lines chest
wall; visceral pleura covers the lungs.
Pericardial sac—parietal pericardium lines
fibrous pericardium; visceral pericardium (epicardium)
covers the heart muscle.
Abdominal cavity—peritoneum lines the
abdominal cavity; mesentery covers the
abdominal organs.
• Mucous membranes—line body tracts that open
to the environment: respiratory, digestive, urinary,
and reproductive. Mucus keeps the living
epithelium wet; provides lubrication in the digestive
tract; traps dust and bacteria in the respiratory
tract.
2. Connective tissue membranes—see Table 4–5.
86 Tissues and Membranes
REVIEW QUESTIONS
1. Explain the importance of each tissue in its location:
(pp. 70, 73, 79)
a. Simple squamous epithelium in the alveoli of
the lungs
b. Ciliated epithelium in the trachea
c. Cartilage in the trachea
2. Explain the importance of each tissue in its location:
(pp. 77, 79)
a. Bone tissue in bones
b. Cartilage on the joint surfaces of bones
c. Fibrous connective tissue in ligaments
3. State the functions of red blood cells, white blood
cells, and platelets. (p. 75)
4. Name two organs made primarily of nerve tissue,
and state the general functions of nerve tissue.
(p. 81)
5. State the location and function of cardiac muscle.
(p. 81)
6. Explain the importance of each of these tissues in
the small intestine: smooth muscle and columnar
epithelium. (pp. 72, 79)
7. State the precise location of each of the following
membranes: (p. 82)
a. Peritoneum
b. Visceral pericardium
c. Parietal pleura
8. State the function of: (pp. 74, 82, 84)
a. Serous fluid
b. Mucus
c. Blood plasma
9. State two functions of skeletal muscles. (p. 79)
10. Name three body tracts lined with mucous membranes.
(p. 82)
11. Explain how endocrine glands differ from exocrine
glands. (pp. 73–74)
12. State the function of adipose tissue: (p. 76)
a. Around the eyes
b. Between the skin and muscles
13. State the location of: (p. 84)
a. Meninges
b. Synovial membranes
14. State the important physical characteristics of collagen
and elastin, and name the cells that produce
these protein fibers (p. 75)
1. A friend suffers a knee injury involving damage to
bone, cartilage, and ligaments. What can you tell
your friend about the healing of these tissues?
2. Stratified squamous keratinizing epithelium is an
excellent barrier to pathogens in the epidermis of
the skin. Despite the fact that it is such a good barrier,
this tissue would not be suitable for the lining
of the trachea or small intestine. Explain why.
3. Many tissues have protective functions, but it is
important to be specific about the kind of protection
provided. Name at least three tissues with a
protective function, and state what each protects
the body (or parts of the body) against.
4. Why is blood classified as a connective tissue?
What does it connect? What kinds of connections
does it make?
Tissues and Membranes 87
FOR FURTHER THOUGHT
88
CHAPTER 5
Chapter Outline
The Skin
Epidermis
Stratum germinativum
Stratum corneum
Langerhans cells
Melanocytes
Dermis
Hair follicles
Nail follicles
Receptors
Glands
Blood vessels
Subcutaneous Tissue
Aging and the Integumentary System
BOX 5–1 BURNS
BOX 5–2 PREVENTING SKIN CANCER: COMMON
SENSE AND SUNSCREENS
BOX 5–3 COMMON SKIN DISORDERS
BOX 5–4 ADMINISTERING MEDICATIONS
Student Objectives
• Name the two major layers of the skin and the
tissue of which each is made.
• State the locations and describe the functions of
the stratum germinativum and stratum corneum.
• Describe the function of Langerhans cells.
• Describe the function of melanocytes and melanin.
• Describe the functions of hair and nails.
• Name the cutaneous senses and explain their
importance.
• Describe the functions of the secretions of sebaceous
glands, ceruminous glands, and eccrine
sweat glands.
• Describe how the arterioles in the dermis respond
to heat, cold, and stress.
• Name the tissues that make up the subcutaneous
tissue, and describe their functions.
The Integumentary System
89
New Terminology
Arterioles (ar-TEER-ee-ohls)
Ceruminous gland/Cerumen (suh-ROO-mi-nus
GLAND/suh-ROO-men)
Dermis (DER-miss)
Eccrine sweat gland (EK-rin SWET GLAND)
Epidermis (EP-i-DER-miss)
Hair follicle (HAIR FAH-li-kull)
Keratin (KER-uh-tin)
Melanin (MEL-uh-nin)
Melanocyte (muh-LAN-oh-sight)
Nail follicle (NAIL FAH-li-kull)
Papillary layer (PAP-i-LAR-ee LAY-er)
Receptors (ree-SEP-turs)
Sebaceous gland/Sebum (suh-BAY-shus
GLAND/SEE-bum)
Stratum corneum (STRA-tum KOR-nee-um)
Stratum germinativum (STRA-tum JER-min-ah-
TEE-vum)
Subcutaneous tissue (SUB-kew-TAY-nee-us TISHyoo)
Vasoconstriction (VAY-zoh-kon-STRIK-shun)
Vasodilation (VAY-zoh-dye-LAY-shun)
Related Clinical Terminology
Acne (AK-nee)
Alopecia (AL-oh-PEE-she-ah)
Biopsy (BYE-op-see)
Carcinoma (KAR-sin-OH-mah)
Circulatory shock (SIR-kew-lah-TOR-ee SHAHK)
Contusion (kon-TOO-zhun)
Decubitus ulcer (dee-KEW-bi-tuss UL-ser)
Dehydration (DEE-high-DRAY-shun)
Dermatology (DER-muh-TAH-luh-gee)
Eczema (EK-zuh-mah)
Erythema (ER-i-THEE-mah)
Histamine (HISS-tah-meen)
Hives (HIGH-VZ)
Inflammation (IN-fluh-MAY-shun)
Melanoma (MEL-ah-NOH-mah)
Nevus (NEE-vus)
Pruritus (proo-RYE-tus)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
The integumentary system consists of the skin, its
accessory structures such as hair and sweat glands, and
the subcutaneous tissue below the skin. The skin is
made of several different tissue types and is considered
an organ. Because the skin covers the surface of the
body, one of its functions is readily apparent: It separates
the internal environment of the body from the
external environment and prevents the entry of many
harmful substances. The subcutaneous tissue directly
underneath the skin connects it to the muscles and has
other functions as well.
THE SKIN
The two major layers of the skin are the outer
epidermis and the inner dermis. Each of these layers
is made of different tissues and has very different functions.
EPIDERMIS
The epidermis is made of stratified squamous keratinizing
epithelial tissue and is thickest on the palms
and soles. The cells that are most abundant are called
keratinocytes, and there are no capillaries present
between them. Although the epidermis may be further
subdivided into four or five sublayers, two of these are
of greatest importance: the innermost layer, the stratum
germinativum, and the outermost layer, the stratum
corneum (Fig. 5–1).
Stratum Germinativum
The stratum germinativum may also be called the
stratum basale. Each name tells us something about
this layer. To germinate means “to sprout” or “to
grow.” Basal means the “base” or “lowest part.” The
stratum germinativum is the base of the epidermis, the
innermost layer in which mitosis takes place. New
cells are continually being produced, pushing the
older cells toward the skin surface. These cells produce
the protein keratin, and as they get farther away
from the capillaries in the dermis, they die. As dead
cells are worn off the skin’s surface, they are replaced
by cells from the lower layers. Scattered among the
keratinocytes of the stratum germinativum are very
different cells called Merkel cells (or Merkel discs);
these are receptors for the sense of touch (Fig. 5–2).
The living keratinocytes are able to synthesize
antimicrobial peptides called defensins; these and
other chemicals are produced following any injury to
the skin, as part of the process of inflammation.
Defensins rupture the membranes of pathogens such
as bacteria that may enter by way of breaks in the skin.
The living portion of the epidermis also produces a
vitamin; the cells have a form of cholesterol that, on
exposure to ultraviolet light, is changed to vitamin D
(then modified by the liver and kidneys to the most
active form, called 1,25-D, or calcitriol, which is considered
a hormone). This is why vitamin D is sometimes
referred to as the “sunshine vitamin.” People
who do not get much sunlight depend more on nutritional
sources of vitamin D, such as fortified milk. But
sunlight is probably the best way to get vitamin D, and
15 minutes a day a few times a week is often enough.
Vitamin D is important for the absorption of calcium
and phosphorus from food in the small intestine; this
function has been known for years. Recent research,
however, suggests that vitamin D is also involved in
maintaining muscle strength, especially in elderly people,
in the functioning of insulin, and in some immune
responses, where it may be protective for some types
of cancer.
Stratum Corneum
The stratum corneum, the outermost epidermal
layer, consists of many layers of dead cells; all that is
left is their keratin. The protein keratin is relatively
waterproof, and though the stratum corneum should
not be thought of as a plastic bag encasing the body, it
does prevent most evaporation of body water. Also of
importance, keratin prevents the entry of water. Without
a waterproof stratum corneum, it would be impossible
to swim in a pool or even take a shower without
damaging our cells.
The stratum corneum is also a barrier to pathogens
and chemicals. Most bacteria and other microorganisms
cannot penetrate unbroken skin. The flaking of
dead cells from the skin surface helps remove microorganisms,
and the fatty acids in sebum help inhibit
their growth. Most chemicals, unless they are corrosive,
will not get through unbroken skin to the living
tissue within. One painful exception is the sap of poison
ivy. This resin does penetrate the skin and initiates
an allergic reaction in susceptible people. The inflammatory
response that characterizes allergies causes
90 The Integumentary System
blisters and severe itching. The importance of the
stratum corneum becomes especially apparent when it
is lost (see Box 5–1: Burns).
Certain minor changes in the epidermis are
undoubtedly familiar to you. When first wearing new
shoes, for example, the skin of the foot may be subjected
to friction. This will separate layers of the epidermis,
or separate the epidermis from the dermis,
and tissue fluid may collect, causing a blister. If the
skin is subjected to pressure, the rate of mitosis in the
stratum germinativum will increase and create a
thicker epidermis; we call this a callus. Although calluses
are more common on the palms and soles, they
may occur on any part of the skin.
The Integumentary System 91
Sebaceous gland
Receptor for touch
(encapsulated)
Hair follicle
Receptor for pressure(encapsulated)
Pore
Stratum germinativum
Stratum corneum
Epidermis
Dermis
Papillary layer
with capillaries
Pilomotor
muscle
Subcutaneous
tissue
Fascia of
muscle
Adipose tissue
Eccrine sweat gland
Free nerve ending
Nerve
Arteriole
Venule
Figure 5–1. Skin. Structure of the skin and subcutaneous tissue.
QUESTION: Which layers of the integumentary system have blood vessels?
92
Stratum corneum
Langerhans cell
Mitosis
Capillary
Stratum germinativum
Sensory neuron
Dermis
Melanocyte
Merkel cell
Figure 5–2. The epidermis, showing
the different kinds of cells present
and the blood supply in the
upper dermis.
QUESTION: Which type of cell
shown is capable of self-locomotion,
and what does it carry?
BOX 5–1 BURNS
Burns of the skin may be caused by flames, hot
water or steam, sunlight, electricity, or corrosive
chemicals. The severity of burns ranges from minor
to fatal, and the classification of burns is based on
the extent of damage.
First-Degree Burn—only the superficial epidermis
is burned, and is painful but not blistered. Lightcolored
skin will appear red due to localized
vasodilation in the damaged area. Vasodilation is
part of the inflammatory response that brings more
blood to the injured site.
Second-Degree Burn—deeper layers of the epidermis
are affected. Another aspect of inflammation
is that damaged cells release histamine,
which makes capillaries more permeable. More
plasma leaves these capillaries and becomes tissue
fluid, which collects at the burn site, creating blisters.
The burned skin is often very painful.
Third-Degree Burn—the entire epidermis is
charred or burned away, and the burn may extend
into the dermis or subcutaneous tissue. Often such
a burn is not painful at first, if the receptors in the
dermis have been destroyed.
Extensive third-degree burns are potentially lifethreatening
because of the loss of the stratum
corneum. Without this natural barrier, living tissue is
exposed to the environment and is susceptible to
infection and dehydration.
Bacterial infection is a serious problem for burn
patients; the pathogens may get into the blood
(septicemia) and quickly spread throughout
the body. Dehydration may also be fatal if medical
intervention does not interrupt and correct the
following sequence: Tissue fluid evaporates from
the burned surface, and more plasma is pulled
out of capillaries into the tissue spaces. As more
plasma is lost, blood volume and blood pressure
decrease. This is called circulatory shock; eventually
the heart simply does not have enough blood
to pump, and heart failure is the cause of death. To
prevent these serious consequences, third-degree
burns are covered with donor skin or artificial skin
until skin grafts of the patient’s own skin can be put
in place.
(Continued on following page)
The Integumentary System 93
BOX 5–1 BURNS (Continued)
Normal skin
First-degree burn
Second-degree burn
Third-degree burn
Box Figure 5–A Normal skin section and representative sections showing first-degree, seconddegree,
and third-degree burns.
Langerhans Cells
Within the epidermis are Langerhans cells, which
are also called dendritic cells because of their
branched appearance when they move (see Fig. 5–2).
These cells originate in the red bone marrow, and are
quite mobile. They are able to phagocytize foreign
material, such as bacteria that enter the body through
breaks in the skin. With such ingested pathogens, the
Langerhans cells migrate to lymph nodes and present
the pathogen to lymphocytes, a type of white blood
cell. This triggers an immune response such as the
production of antibodies (antibodies are proteins that
label foreign material for destruction). Because of its
position adjacent to the external environment, the skin
is an important component of the body’s protective
responses, though many of the exact aspects of this
have yet to be determined. Immunity is covered in
Chapter 14.
Melanocytes
Another type of cell found in the lower epidermis is
the melanocyte, which is also shown in Fig. 5–2.
Melanocytes produce another protein, a pigment
called melanin. People of the same size have approximately
the same number of melanocytes, though these
cells may differ in their level of activity. In people with
dark skin, the melanocytes continuously produce large
amounts of melanin. The melanocytes of lightskinned
people produce less melanin. The activity of
melanocytes is genetically regulated; skin color is one
of our hereditary characteristics.
In all people, melanin production is increased by
exposure of the skin to ultraviolet rays, which are part
of sunlight and are damaging to living cells. As more
melanin is produced, it is taken in by the epidermal
cells as they are pushed toward the surface. This gives
the skin a darker color, which prevents further exposure
of the living stratum germinativum to ultraviolet
rays. People with dark skin already have good protection
against the damaging effects of ultraviolet rays;
people with light skin do not (see Box 5–2: Preventing
Skin Cancer: Common Sense and Sunscreens).
Melanin also gives color to hair, though its protective
function is confined to the hair of the head. Two parts
of the eye obtain their color from melanin: the iris and
the interior choroid layer of the eyeball (the eye is discussed
in Chapter 9).The functions of the epidermis
and its cells are summarized in Table 5–1.
94 The Integumentary System
BOX 5–2 PREVENTING SKIN CANCER: COMMON SENSE AND SUNSCREENS
A B
Anyone can get skin cancer, and the most important
factor is exposure to sunlight. Light-skinned
people are, of course, more susceptible to the
effects of ultraviolet (UV) rays, which may trigger
mutations in living epidermal cells.
Squamous cell carcinoma and basal cell carcinoma
(see A in Box Fig. 5–B) are the most common
forms of skin cancer. The lesions are visible as
changes in the normal appearance of the skin, and
a biopsy (microscopic examination of a tissue specimen)
is used to confirm the diagnosis. These lesions
usually do not metastasize rapidly, and can be
completely removed using simple procedures
Malignant melanoma (see B in Box Fig. 5–B) is
a more serious form of skin cancer, which begins in
melanocytes. Any change in a pigmented spot or
mole (nevus) should prompt a person to see a doctor.
Melanoma is serious not because of its growth
in the skin, but because it may metastasize very
rapidly to the lungs, liver, or other vital organ.
Researchers are testing individualized vaccines for
people who have had melanoma. The purpose of
the vaccine is to stimulate the immune system
strongly enough to prevent a second case, for such
recurrences are often fatal.
Although the most common forms of skin cancer
are readily curable, prevention is a better strategy.
We cannot, and we would not want to, stay out of
the sun altogether (because sunlight may be the
best way to get sufficient vitamin D), but we may
be able to do so when sunlight is most damaging.
During the summer months, UV rays are especially
intense between 10 A.M. and 2 P.M. If we are or
must be outdoors during this time, dermatologists
recommend use of a sunscreen.
Sunscreens contain chemicals such as PABA
(para-amino benzoic acid) that block UV rays and
prevent them from damaging the epidermis. An
SPF (sun protection factor) of 15 or higher is considered
good protection. Use of a sunscreen on
exposed skin not only helps prevent skin cancer but
also prevents sunburn and its painful effects. It is
especially important to prevent children from getting
severely sunburned, because such burns have
been linked to the development of skin cancer years
later.
Box Figure 5–B (A) Classic basal cell carcinoma on face. (B) Melanoma in finger web. (From
Goldsmith, LA, Lazarus, GS, and Tharp, MD: Adult and Pediatric Dermatology: A Color Guide to
Diagnosis and Treatment. FA Davis, 1997, pp 137 and 144, with permission.)
DERMIS
The dermis is made of an irregular type of fibrous
connective tissue, irregular meaning that the fibers are
not parallel, but run in all directions. Fibroblasts produce
both collagen and elastin fibers. Recall that collagen
fibers are strong, and elastin fibers are able to
recoil after being stretched. Strength and elasticity are
two characteristics of the dermis. With increasing age,
however, the deterioration of the elastin fibers causes
the skin to lose its elasticity. We can all look forward
to at least a few wrinkles as we get older.
The uneven junction of the dermis with the epidermis
is called the papillary layer (see Fig. 5–1). Capillaries
are abundant here to nourish not only the
dermis but also the stratum germinativum. The epidermis
has no capillaries of its own, and the lower, living
cells depend on the blood supply in the dermis for
oxygen and nutrients.
Within the dermis are the accessory skin structures:
hair and nail follicles, sensory receptors, and several
types of glands. Some of these project through the
epidermis to the skin surface, but their active portions
are in the dermis.
Hair Follicles
Hair follicles are made of epidermal tissue, and the
growth process of hair is very similar to growth of the
epidermis. At the base of a follicle is the hair root,
which contains cells called the matrix, where mitosis
takes place (Fig. 5–3). The new cells produce keratin,
get their color from melanin, then die and become
incorporated into the hair shaft, which is pushed
toward the surface of the skin. The hair that we comb
and brush every day consists of dead, keratinized cells.
The rate of hair growth averages 0.3 to 0.4 in./month
(8 to 10 mm).
Compared to some other mammals, humans do not
have very much hair. The actual functions of human
hair are quite few. Eyelashes and eyebrows help to
keep dust and perspiration out of the eyes, and the
hairs just inside the nostrils help to keep dust out of
the nasal cavities. Hair of the scalp does provide insulation
from cold for the head. The hair on our bodies,
however, no longer serves this function, but we have
the evolutionary remnants of it. Attached to each hair
follicle is a small, smooth muscle called the pilomotor
The Integumentary System 95
Hair shaft
Follicle
Hair root
B
Capillary
Venule
Fat cells
A
Figure 5–3. Structure of a hair follicle. (A) Longitudinal
section. (B) Cross-section.
QUESTION: What is the hair shaft made of?
Table 5–1 EPIDERMIS
Part Function
Stratum corneum
(keratin)
Stratum germinativum
(stratum
basale)
Langerhans cells
Merkel cells
Melanocytes
Melanin
• Prevents loss or entry of water
• If unbroken, prevents entry of
pathogens and most chemicals
• Continuous mitosis produces
new cells to replace worn-off
surface cells
• Produces antimicrobial
defensins
• Cholesterol is changed to vitamin
D on exposure to UV rays
• Phagocytize foreign material
and stimulate an immune
response by lymphocytes
• Receptors for sense of touch
• Produce melanin on exposure
to UV rays
• Protects living skin layers from
further exposure to UV rays
or arrector pili muscle. When stimulated by cold or
emotions such as fear, these muscles pull the hair follicles
upright. For an animal with fur, this would trap
air and provide greater insulation. Because people do
not have thick fur, all this does for us is give us “goose
bumps.”
Nail Follicles
Found on the ends of fingers and toes, nail follicles
produce nails just as hair follicles produce hair. Mitosis
takes place in the nail root at the base of the nail (Fig.
5–4), and the new cells produce keratin (a stronger
form of this protein than is found in hair) and then die.
Although the nail itself consists of keratinized dead
cells, the flat nail bed is living epidermis and dermis.
This is why cutting a nail too short can be quite
painful. Nails protect the ends of the fingers and toes
from mechanical injury and give the fingers greater
ability to pick up small objects. Fingernails are also
good for scratching. This is more important than it
may seem at first. An itch might mean the presence of
an arthropod parasite, mosquito, tick, flea, or louse.
These parasites (all but the tick are insects) feed on
blood, and all are potential vectors of diseases caused
by bacteria, viruses, or protozoa. A quick and vigorous
scratch may kill or at least dislodge the arthropod and
prevent the transmission of the disease. Fingernails
grow at the rate of about 0.12 in./month (3 mm), and
growth is a little faster during the summer months.
Receptors
Most sensory receptors for the cutaneous senses
are found in the dermis (Merkel cells are in the stratum
germinativum, as are some nerve endings). The
cutaneous senses are touch, pressure, heat, cold, and
pain. For each sensation there is a specific type of
receptor, which is a structure that will detect a particular
change. For pain, heat, and cold, the receptors are
free nerve endings. For touch and pressure, the
receptors are called encapsulated nerve endings,
which means there is a cellular structure around the
sensory nerve ending (see Fig. 5–1). The purpose of
these receptors and sensations is to provide the central
nervous system with information about the external
environment and its effect on the skin. This information
may stimulate responses, such as washing a
painful cut finger, scratching an insect bite, or
responding to a feeling of cold by putting on a sweater.
The sensitivity of an area of skin is determined by
how many receptors are present. The skin of the fingertips,
for example, is very sensitive to touch because
there are many receptors per square inch. The skin of
the upper arm, with few touch receptors per square
inch, is less sensitive.
When receptors detect changes, they generate
nerve impulses that are carried to the brain, which
interprets the impulses as a particular sensation.
Sensation, therefore, is actually a function of the brain
(we will return to this in Chapters 8 and 9).
Glands
Glands are made of epithelial tissue. The exocrine
glands of the skin have their secretory portions in the
dermis. Some of these are shown in Fig. 5–1.
Sebaceous Glands. The ducts of sebaceous glands
open into hair follicles or directly to the skin surface.
Their secretion is sebum, a lipid substance that we
commonly refer to as oil. As mentioned previously,
sebum inhibits the growth of bacteria on the skin surface.
Another function of sebum is to prevent drying of
skin and hair. The importance of this may not be readily
apparent, but skin that is dry tends to crack more
easily. Even very small breaks in the skin are potential
entryways for bacteria. Decreased sebum production is
another consequence of getting older, and elderly people
often have dry and more fragile skin.
Adolescents may have the problem of overactive
sebaceous glands. Too much sebum may trap bacteria
96 The Integumentary System
Nail root
Nail bed Cuticle
Nail body
Free edge of nail
Lunula
Figure 5–4. Structure of a fingernail shown in longitudinal
section.
QUESTION: The nail bed is which part of the skin?
within hair follicles and create small infections.
Because sebaceous glands are more numerous around
the nose and mouth, these are common sites of pimples
in young people (see also Box 5–3: Common Skin
Disorders).
Ceruminous Glands. These glands are located in the
dermis of the ear canals. Their secretion is called
cerumen or ear wax (which includes the sebum
secreted in the ear canals). Cerumen keeps the outer
surface of the eardrum pliable and prevents drying.
The Integumentary System 97
BOX 5–3 COMMON SKIN DISORDERS
Impetigo—a bacterial infection often caused by
streptococci or staphylococci. The characteristic
pustules (pus-containing lesions) crust as they heal;
the infection is contagious to others.
Eczema—a symptom of what is more properly
called atopic dermatitis. This may be an allergic
reaction, and is more common in children than
adults; the rash is itchy (pruritus) and may blister
or ooze. Eczema may be related to foods such as
fish, eggs, or milk products, or to inhaled allergens
such as dust, pollens, or animal dander. Prevention
depends upon determining what the child is allergic
to and eliminating or at least limiting exposure.
Warts—caused by a virus that makes epidermal
cells divide abnormally, producing a growth on the
skin that is often raised and has a rough or pitted
surface. Warts are probably most common on the
hands, but they may be anywhere on the skin.
Plantar warts on the sole of the foot may become
quite painful because of the constant pressure of
standing and walking.
Fever blisters (cold sores)—caused by the herpes
simplex virus, to which most people are exposed
as children. An active lesion, usually at the edge
of the lip (but may be anywhere on the skin), is
painful and oozes. If not destroyed by the immune
system, the virus “hides out” and becomes dormant
in nerves of the face. Another lesion, weeks
or months later, may be triggered by stress or
another illness.
A B
C D
Box Figure 5–C (A) Impetigo. (B) Eczema of atopic dermatitis. (C) Warts on back
of hands. (D) Fever blister on finger, localized but severe. (From Goldsmith, LA,
Lazarus, GS, and Tharp, MD: Adult and Pediatric Dermatology: A Color Guide
to Diagnosis and Treatment. FA Davis, 1997, pp 80, 241, 306, and 317, with permission.)
However, if excess cerumen accumulates in the ear
canal, it may become impacted against the eardrum.
This might diminish the acuity of hearing by preventing
the eardrum from vibrating properly.
Sweat Glands. There are two types of sweat glands,
apocrine and eccrine. Apocrine glands are most
numerous in the axillae (underarm) and genital areas
and are most active in stressful and emotional situations.
Although their secretion does have an odor, it is
barely perceptible to other people. Animals such as
dogs, however, can easily distinguish among people
because of their individual scents. If the apocrine
secretions are allowed to accumulate on the skin, bacteria
metabolize the chemicals in the sweat and produce
waste products that have distinct odors that
many people find unpleasant.
Eccrine glands are found all over the body but are
especially numerous on the forehead, upper lip, palms,
and soles. The secretory portion of these glands is
simply a coiled tube in the dermis. The duct of this
tube extends to the skin’s surface, where it opens into
a pore.
The sweat produced by eccrine glands is important
in the maintenance of normal body temperature. In a
warm environment, or during exercise, more sweat is
secreted onto the skin surface, where it is then evaporated
by excess body heat. Recall that water has a high
heat of vaporization, which means that a great deal of
heat can be lost in the evaporation of a relatively small
amount of water. Although this is a very effective
mechanism of heat loss, it has a potentially serious disadvantage.
Loss of too much body water in sweat may
lead to dehydration, as in heat exhaustion or even
after exercise on a hot and humid day. Increased
sweating during exercise or on warm days should
always be accompanied by increased fluid intake.
Those who exercise regularly know that they must
replace salt as well as water. Sodium chloride is also
lost in sweat, as are small amounts of urea (a nitrogenous
waste product of amino acid metabolism). This
excretory function of the skin is very minor, however;
the kidneys are primarily responsible for removing
waste products from the blood and for maintaining
the body’s proper salt-to-water proportion.
Blood Vessels
Besides the capillaries in the dermis, the other blood
vessels of great importance are the arterioles.
Arterioles are small arteries, and the smooth muscle
in their walls permits them to constrict (close) or
dilate (open). This is important in the maintenance of
body temperature, because blood carries heat, which is
a form of energy.
In a warm environment the arterioles dilate
(vasodilation), which increases blood flow through the
dermis and brings excess heat close to the body surface
to be radiated to the environment. In a cold environment,
however, body heat must be conserved if possible,
so the arterioles constrict. The vasoconstriction
decreases the flow of blood through the dermis and
keeps heat within the core of the body. This adjusting
mechanism is essential for maintaining homeostasis.
Regulation of the diameter of the arterioles in
response to external temperature changes is controlled
by the nervous system. These changes can often be
seen in light-skinned people. Flushing, especially in
the face, may be observed in hot weather. In cold, the
skin of the extremities may become even paler as blood
flow through the dermis decreases. In people with dark
skin, such changes are not as readily apparent because
they are masked by melanin in the epidermis.
Vasoconstriction in the dermis may also occur during
stressful situations. For our ancestors, stress usually
demanded a physical response: Either stand and
fight or run away to safety. This is called the “fight or
flight response.” Our nervous systems are still programmed
to respond as if physical activity were necessary
to cope with the stress situation. Vasoconstriction
in the dermis will shunt, or redirect, blood to more
vital organs such as the muscles, heart, and brain. In
times of stress, the skin is a relatively unimportant
organ and can function temporarily with a minimal
blood flow. You have probably heard the expression
“broke out in a cold sweat,” and may even have felt it
in a stressful situation. Such sweating feels cold
because vasoconstriction in the dermis makes the skin
relatively cool.
Blood flow in the dermis may be interrupted by
prolonged pressure on the skin. For example, a hospital
patient who cannot turn over by herself may
develop a decubitus ulcer, also called a pressure ulcer
or pressure sore. The skin is compressed between the
object outside, such as a bed, and a bony prominence
within, such as the heel bone or the sacrum in the
lower back. Without its blood supply the skin dies,
and the dead tissue is a potential site for bacterial
infection.
The functions of dermal structures are summarized
in Table 5–2.
98 The Integumentary System
SUBCUTANEOUS TISSUE
The subcutaneous tissue may also be called the
superficial fascia, one of the connective tissue membranes.
Made of areolar connective tissue and adipose
tissue, the superficial fascia connects the dermis to the
underlying muscles. Its other functions are those of its
tissues, as you may recall from Chapter 4.
Areolar connective tissue, or loose connective tissue,
contains collagen and elastin fibers and many
white blood cells that have left capillaries to wander
around in the tissue fluid between skin and muscles.
These migrating white blood cells destroy pathogens
that enter the body through breaks in the skin. Mast
cells are specialized connective tissue cells found in
areolar tissue; they produce histamine, leukotrienes,
and other chemicals that help bring about inflammation,
the body’s response to damage (inflammation is
described in Chapters 10 and 14).
The cells (adipocytes) of adipose tissue are specialized
to store fat, and our subcutaneous layer of fat
stores excess nutrients as a potential energy source.
This layer also cushions bony prominences, such as
when sitting, and provides some insulation from cold.
For people, this last function is relatively minor,
because we do not have a thick layer of fat, as do animals
such as whales and seals. As mentioned in
Chapter 4, adipose tissue is involved in the onset or
cessation of eating and in the use of insulin by body
cells, and it contributes to inflammation by producing
cytokines, chemicals that activate white blood
cells.
Just as the epidermis forms a continuous sheet that
covers the body, the subcutaneous tissue is a continuous
layer, though it is internal. If we consider the epidermis
as the first line of defense against pathogens,
we can consider the subcutaneous tissue a secondary
line of defense. There is, however, a significant
anatomic difference. The cells of the epidermis are
very closely and tightly packed, but the cells and
protein fibers of subcutaneous tissue are farther apart,
and there is much more tissue fluid. If we imagine the
epidermis as a four-lane highway during rush hour
with bumper-to-bumper traffic, the superficial fascia
would be that same highway at three o’clock in the
morning, when it is not crowded and cars can move
much faster. This is an obvious benefit for the migrating
white blood cells, but may become a disadvantage
because some bacterial pathogens, once established,
can spread even more rapidly throughout subcutaneous
tissue.
Group A streptococcus, for example, is a cause of
necrotizing fasciitis. Necrotizing means “to cause
death,” and fasciitis is the inflammation of a fascia, in
this case the superficial fascia and the deep fasciae
around muscles. Necrotizing fasciitis is an extremely
serious infection and requires surgical removal of the
infected tissue, or even amputation of an infected
limb, to try to stop the spread of the bacteria. The
body’s defenses have been overwhelmed because what
is usually an anatomic benefit, the “looseness” of areolar
connective tissue, has been turned against us and
become a detriment.
The functions of subcutaneous tissue are summarized
in Table 5–3. Box 5–4: Administering Medications,
describes ways in which we give medications
through the skin.
The Integumentary System 99
Table 5–2 DERMIS
Part Function
Papillary layer
Hair (follicles)
Nails (follicles)
Receptors
Sebaceous glands
Ceruminous
glands
Eccrine sweat
glands
Arterioles
• Contains capillaries that nourish
the stratum germinativum
• Eyelashes and nasal hair keep
dust out of eyes and nasal
cavities
• Scalp hair provides insulation
from cold for the head
• Protect ends of fingers and
toes from mechanical injury
• Detect changes that are felt as
the cutaneous senses: touch,
pressure, heat, cold, and pain
• Produce sebum, which prevents
drying of skin and hair and
inhibits growth of bacteria
• Produce cerumen, which prevents
drying of the eardrum
• Produce watery sweat that is
evaporated by excess body heat
to cool the body
• Dilate in response to warmth
to increase heat loss
• Constrict in response to cold
to conserve body heat
• Constrict in stressful situations
to shunt blood to more vital
organs
100 The Integumentary System
Table 5–3 SUBCUTANEOUS TISSUE
Part Function
Areolar connective tissue
Adipose tissue
• Connects skin to muscles
• Contains many WBCs to destroy pathogens that enter breaks in the skin
• Contains mast cells that release histamine, leukotrienes, and other
chemicals involved in inflammation
• Contains stored energy in the form of true fats
• Cushions bony prominences
• Provides some insulation from cold
• Contributes to appetite
• Contributes to use of insulin
• Produces cytokines that activate WBCs
BOX 5–4 ADMINISTERING MEDICATIONS
You have probably seen television or print advertisements
for skin patches that supply nicotine, and
you know that their purpose is to help smokers give
up cigarettes. This method of supplying a medication
is called transdermal administration. The name
is a little misleading because the most difficult part
of cutaneous absorption of a drug is absorption
through the stratum corneum of the epidermis.
Because such absorption is slow, skin patches are
useful for medications needed in small but continuous
amounts, and over a prolonged period of time.
You would expect that such patches should be
worn where the epidermis is thin. These sites
include the upper arm and the chest. The recommended
site for a patch to prevent motion sickness
is the skin behind the ear. Also available in patch
form are medications for birth control, overactive
bladder, high blood pressure, and both systemic
and localized pain relief.
Medications may also be injected through the
skin. Some injections are given subcutaneously, that
is, into subcutaneous tissue (see Box Fig. 5–D).
Subcutaneous adipose tissue has a moderate blood
supply, so the rate of absorption of the drug will be
moderate, but predictable. Insulin is given subcutaneously.
Other injections are intramuscular, and
absorption into the blood is rapid because muscle
tissue has a very good blood supply. Most injectable
vaccines are given intramuscularly, to promote
rapid absorption to stimulate antibody production.
Stratum
corneum
Dermis
Adipose
tissue
Skeletal
muscle
Patch
Subcutaneous
Intramuscular
Box Figure 5–D The skin, subcutaneous tissue, and muscle sites for the delivery of
medications.
AGING AND THE
INTEGUMENTARY SYSTEM
The effects of age on the integumentary system are
often quite visible. Both layers of skin become thinner
and more fragile as mitosis in the epidermis slows and
fibroblasts in the dermis die and are not replaced;
repair of even small breaks or cuts is slower. The skin
becomes wrinkled as collagen and elastin fibers in the
dermis deteriorate. Sebaceous glands and sweat glands
become less active; the skin becomes dry, and temperature
regulation in hot weather becomes more difficult.
Hair follicles become inactive and hair on the
scalp and body thins. Melanocytes die and are not
replaced; the hair that remains becomes white. There
is often less fat in the subcutaneous tissue, which may
make an elderly person more sensitive to cold. It is
important for elderly people (and those who care for
them) to realize that extremes of temperature may be
harmful and to take special precautions in very hot or
very cold weather.
SUMMARY
The integumentary system is the outermost organ
system of the body. You have probably noticed that
many of its functions are related to this location. The
skin protects the body against pathogens and chemicals,
minimizes loss or entry of water, blocks the
harmful effects of sunlight, and produces vitamin D.
Sensory receptors in the skin provide information
about the external environment, and the skin helps
regulate body temperature in response to environmental
changes. The subcutaneous tissue is a secondary
line of defense against pathogens, a site of fat
storage and of the other metabolic functions of adipose
tissue.
The Integumentary System 101
STUDY OUTLINE
The integumentary system consists of the
skin and its accessory structures and the
subcutaneous tissue. The two major layers of
the skin are the outer epidermis and the
inner dermis.
Epidermis—made of stratified squamous
epithelium; no capillaries; cells called keratinocytes
(see Figs. 5–1 and 5–2 and Table
5–1)
1. Stratum germinativum—the innermost layer
where mitosis takes place; new cells produce keratin
and die as they are pushed toward the surface.
Defensins are antimicrobial peptides produced
when the skin is injured. Vitamin D is formed from
cholesterol on exposure to the UV rays of sunlight.
2. Stratum corneum—the outermost layers of dead
cells; keratin prevents loss and entry of water and
resists entry of pathogens and chemicals.
3. Langerhans cells—phagocytize foreign material,
take it to lymph nodes, and stimulate an immune
response by lymphocytes.
4. Melanocytes—in the lower epidermis, produce
melanin. UV rays stimulate melanin production;
melanin prevents further exposure of the stratum
germinativum to UV rays by darkening the
skin.
Dermis—made of irregular fibrous connective
tissue; collagen provides strength, and
elastin provides elasticity; capillaries in the
papillary layer nourish the stratum germinativum
(see Fig. 5–1 and Table 5–2)
1. Hair follicles—mitosis takes place in the hair root;
new cells produce keratin, die, and become the hair
shaft. Hair of the scalp provides insulation from
cold for the head; eyelashes keep dust out of eyes;
nostril hairs keep dust out of nasal cavities (see
Figs. 5–1 and 5–3).
2. Nail follicles—at the ends of fingers and toes; mitosis
takes place in the nail root; the nail itself is dead,
keratinized cells. Nails protect the ends of the fingers
and toes, enable the fingers to pick up small
objects, and provide for efficient scratching (see
Fig. 5–4).
3. Receptors—detect changes in the skin: touch, pressure,
heat, cold, and pain; provide information
about the external environment that initiates appropriate
responses; sensitivity of the skin depends on
the number of receptors present.
4. Sebaceous glands—secrete sebum into hair follicles
or to the skin surface; sebum inhibits the growth of
bacteria and prevents drying of skin and hair.
5. Ceruminous glands—secrete cerumen in the ear
canals; cerumen prevents drying of the eardrum.
6. Apocrine sweat glands—modified scent glands in
axillae and genital area; activated by stress and
emotions.
7. Eccrine sweat glands—most numerous on face,
palms, soles. Activated by high external temperature
or exercise; sweat on skin surface is evaporated
by excess body heat; potential disadvantage is dehydration.
Excretion of small amounts of NaCl and
urea is a very minor function.
8. Arterioles—smooth muscle permits constriction
or dilation. Vasoconstriction in cold temperatures
decreases dermal blood flow to conserve heat in
the body core. Vasodilation in warm temperatures
increases dermal blood flow to bring heat to the
surface to be lost. Vasoconstriction during stress
shunts blood away from the skin to more vital
organs, such as muscles, to permit a physical
response, if necessary.
Subcutaneous Tissue—also called the superficial
fascia; connects skin to muscles (see Fig.
5–1 and Table 5–3)
1. Areolar tissue—also called loose connective tissue;
the matrix contains tissue fluid and WBCs that
destroy pathogens that get through breaks in the
skin; mast cells produce chemicals that bring about
inflammation.
2. Adipose tissue—stores fat as potential energy;
cushions bony prominences; provides some insulation
from cold. Other functions: contributes to
appetite, the use of insulin, and the activation of
WBCs.
102 The Integumentary System
REVIEW QUESTIONS
1. Name the parts of the integumentary system.
(p. 90)
2. Name the two major layers of skin, the location
of each, and the tissue of which each is made.
(pp. 90, 95)
3. In the epidermis: (pp. 90, 93)
a. Where does mitosis take place?
b. What protein do the new cells produce?
c. What happens to these cells?
d. What is the function of Langerhans cells?
4. Describe the functions of the stratum corneum.
(p. 90)
5. Name the cells that produce melanin. What is
the stimulus? Describe the function of melanin
(p. 94)
6. Where, on the body, does human hair have important
functions? Describe these functions. (p. 95)
7. Describe the functions of nails. (p. 96)
8. Name the cutaneous senses. Describe the importance
of these senses. (p. 96)
9. Explain the functions of sebum and cerumen.
(pp. 96–97)
10. Explain how sweating helps maintain normal
body temperature. (p. 98)
11. Explain how the arterioles in the dermis respond
to cold or warm external temperatures and to
stress situations. (p. 98)
12. What vitamin is produced in the skin? What is the
stimulus for the production of this vitamin?
(p. 90)
13. Name the tissues of which the superficial fascia is
made. Describe the functions of these tissues.
(p. 99)
The Integumentary System 103
FOR FURTHER THOUGHT
1. The epidermis has no capillaries of its own; the
stratum corneum is made of dead cells and doesn’t
need a blood supply at all. Explain why the epidermis
is affected by a decubitus ulcer. Name another
group of people, besides hospital or nursing home
patients, that is especially susceptible to developing
pressure ulcers.
2. Going out in the sun stimulates quite a bit of activity
in the skin, especially on a hot summer day.
Describe what is happening in the skin in response
to sunlight.
3. Ringworm is a skin condition characterized by
scaly red patches, often circular or oval in shape. It
is not caused by a worm, but rather by certain
fungi. What is the food of these fungi; that is, what
are they digesting?
4. There are several kinds of cells in the epidermis.
Which cells exert their functions when they are
dead? Which cells must be living in order to function?
5. Wearing a hat in winter is a good idea. What happens
to the arterioles in the dermis in a cold environment?
How does this affect heat loss? Is the
head an exception? Explain.
104
CHAPTER 6
Chapter Outline
Functions of the Skeleton
Types of Bone Tissue
Classification of Bones
Embryonic Growth of Bone
Factors That Affect Bone Growth and
Maintenance
The Skeleton
Skull
Vertebral Column
Rib Cage
The Shoulder and Arm
The Hip and Leg
Joints—Articulations
The Classification of Joints
Synovial Joints
Aging and the Skeletal System
BOX 6–1 FRACTURES AND THEIR REPAIR
BOX 6–2 OSTEOPOROSIS
BOX 6–3 HERNIATED DISC
BOX 6–4 ABNORMALITIES OF THE CURVES
OF THE SPINE
BOX 6–5 ARTHRITIS
Student Objectives
• Describe the functions of the skeleton.
• Explain how bones are classified, and give an
example of each type.
• Describe how the embryonic skeleton model is
replaced by bone.
• Name the nutrients necessary for bone growth,
and explain their functions.
• Name the hormones involved in bone growth and
maintenance, and explain their functions.
• Explain what is meant by “exercise” for bones, and
explain its importance.
• Name all the bones of the human skeleton (be able
to point to each on diagrams, skeleton models, or
yourself).
• Describe the functions of the skull, vertebral column,
rib cage, scapula, and pelvic bone.
• Explain how joints are classified. For each type,
give an example, and describe the movement possible.
• Describe the parts of a synovial joint, and explain
their functions.
The Skeletal System
105
New Terminology
Appendicular (AP-en-DIK-yoo-lar)
Articulation (ar-TIK-yoo-LAY-shun)
Axial (AK-see-uhl)
Bursa (BURR-sah)
Diaphysis (dye-AFF-i-sis)
Epiphyseal disc (e-PIFF-i-SEE-al DISK)
Epiphysis (e-PIFF-i-sis)
Fontanel (FON-tah-NELL)
Haversian system (ha-VER-zhun SIS-tem)
Ligament (LIG-uh-ment)
Ossification (AHS-i-fi-KAY-shun)
Osteoblast (AHS-tee-oh-BLAST)
Osteoclast (AHS-tee-oh-KLAST)
Paranasal sinus (PAR-uh-NAY-zuhl SIGH-nus)
Periosteum (PER-ee-AHS-tee-um)
Suture (SOO-cher)
Symphysis (SIM-fi-sis)
Synovial fluid (sin-OH-vee-al FLOO-id)
Related Clinical Terminology
Autoimmune disease
(AW-toh-im-YOON di-ZEEZ)
Bursitis (burr-SIGH-tiss)
Cleft palate (KLEFT PAL-uht)
Fracture (FRAK-chur)
Herniated disc (HER-nee-ay-ted DISK)
Kyphosis (kye-FOH-sis)
Lordosis (lor-DOH-sis)
Osteoarthritis (AHS-tee-oh-ar-THRY-tiss)
Osteomyelitis (AHS-tee-oh-my-uh-LYE-tiss)
Osteoporosis (AHS-tee-oh-por-OH-sis)
Rheumatoid arthritis
(ROO-muh-toyd ar-THRY-tiss)
Rickets (RIK-ets)
Scoliosis (SKOH-lee-OH-sis)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Imagine for a moment that people did not have
skeletons. What comes to mind? Probably that each of
us would be a little heap on the floor, much like a jellyfish
out of water. Such an image is accurate and
reflects the most obvious function of the skeleton: to
support the body. Although it is a framework for the
body, the skeleton is not at all like the wooden beams
that support a house. Bones are living organs that
actively contribute to the maintenance of the internal
environment of the body.
The skeletal system consists of bones and other
structures that make up the joints of the skeleton. The
types of tissue present are bone tissue, cartilage, and
fibrous connective tissue, which forms the ligaments
that connect bone to bone.
FUNCTIONS OF THE SKELETON
1. Provides a framework that supports the bo


dy; the
muscles that are attached to bones move the skeleton.
2. Protects some internal organs from mechanical
injury; the rib cage protects the heart and lungs, for
example.
3. Contains and protects the red bone marrow, the
primary hemopoietic (blood-forming) tissue.
4. Provides a storage site for excess calcium. Calcium
may be removed from bone to maintain a normal
blood calcium level, which is essential for blood
clotting and proper functioning of muscles and
nerves.
TYPES OF BONE TISSUE
Bone was described as a tissue in Chapter 4. Recall
that bone cells are called osteocytes, and the matrix
of bone is made of calcium salts and collagen. The
calcium salts are calcium carbonate (CaCO3) and calcium
phosphate (Ca3(PO4)2), which give bone the
strength required to perform its supportive and protective
functions. Bone matrix is non-living, but it
changes constantly, with calcium that is taken from
bone into the blood replaced by calcium from the diet.
In normal circumstances, the amount of calcium that
is removed is replaced by an equal amount of calcium
deposited. This is the function of osteocytes, to regulate
the amount of calcium that is deposited in, or
removed from, the bone matrix.
In bone as an organ, two types of bone tissue are
present (Fig. 6–1). Compact bone looks solid but is
very precisely structured. Compact bone is made of
osteons or haversian systems, microscopic cylinders
of bone matrix with osteocytes in concentric rings
around central haversian canals. In the haversian
canals are blood vessels; the osteocytes are in contact
with these blood vessels and with one another through
microscopic channels (canaliculi) in the matrix.
The second type of bone tissue is spongy bone,
which does look rather like a sponge with its visible
holes or cavities. Osteocytes, matrix, and blood vessels
are present but are not arranged in haversian systems.
The cavities in spongy bone often contain red bone
marrow, which produces red blood cells, platelets,
and the five kinds of white blood cells.
CLASSIFICATION OF BONES
1. Long bones—the bones of the arms, legs, hands,
and feet (but not the wrists and ankles). The shaft
of a long bone is the diaphysis, and the ends are
called epiphyses (see Fig. 6–1). The diaphysis is
made of compact bone and is hollow, forming a
canal within the shaft. This marrow canal (or
medullary cavity) contains yellow bone marrow,
which is mostly adipose tissue. The epiphyses are
made of spongy bone covered with a thin layer of
compact bone. Although red bone marrow is present
in the epiphyses of children’s bones, it is largely
replaced by yellow bone marrow in adult bones.
2. Short bones—the bones of the wrists and ankles.
3. Flat bones—the ribs, shoulder blades, hip bones,
and cranial bones.
4. Irregular bones—the vertebrae and facial bones.
Short, flat, and irregular bones are all made of
spongy bone covered with a thin layer of compact
bone. Red bone marrow is found within the spongy
bone.
The joint surfaces of bones are covered with articular
cartilage, which provides a smooth surface. Covering
the rest of the bone is the periosteum, a fibrous
connective tissue membrane whose collagen fibers
merge with those of the tendons and ligaments that
106 The Skeletal System
107
Diaphysis
Proximal
epiphysis
Distal
epiphysis
Yellow bone
marrow
Marrow (medullary)
cavity
Periosteum
Compact bone
Spongy bone
Marrow
Canaliculi
Venule
Osteocyte
Fibrous layer
Osteogenic layer
(osteoblasts)
Periosteum
Osteon
(haversian system)
Haversian canal
Concentric rings
of osteocytes
A B
Arteriole
Figure 6–1. Bone tissue. (A) Femur with distal end cut in longitudinal section.
(B) Compact bone showing haversian systems (osteons).
QUESTION: What is the purpose of the blood vessels in bone tissue?
are attached to the bone. The periosteum anchors
these structures and contains both the blood vessels
that enter the bone itself and osteoblasts that will
become active if the bone is damaged.
EMBRYONIC GROWTH OF BONE
During embryonic development, the skeleton is first
made of cartilage and fibrous connective tissue, which
are gradually replaced by bone. Bone matrix is produced
by cells called osteoblasts (a blast cell is a “growing”
or “producing” cell, and osteo means “bone”). In
the embryonic model of the skeleton, osteoblasts differentiate
from the fibroblasts that are present. The
production of bone matrix, called ossification, begins
in a center of ossification in each bone.
The cranial and facial bones are first made of
fibrous connective tissue. In the third month of fetal
development, fibroblasts (spindle-shaped connective
tissue cells) become more specialized and differentiate
into osteoblasts, which produce bone matrix. From
each center of ossification, bone growth radiates outward
as calcium salts are deposited in the collagen of
the model of the bone. This process is not complete
at birth; a baby has areas of fibrous connective tissue
remaining between the bones of the skull. These
are called fontanels (Fig. 6–2), which permit compression
of the baby’s head during birth without
breaking the still thin cranial bones. The fontanels
also permit the growth of the brain after birth. You
may have heard fontanels referred to as “soft spots,”
and indeed they are. A baby’s skull is quite fragile and
must be protected from trauma. By the age of 2 years,
all the fontanels have become ossified, and the skull
becomes a more effective protective covering for the
brain.
The rest of the embryonic skeleton is first made of
cartilage, and ossification begins in the third month of
gestation in the long bones. Osteoblasts produce bone
matrix in the center of the diaphyses of the long bones
and in the center of short, flat, and irregular bones.
Bone matrix gradually replaces the original cartilage
(Fig. 6–3).
The long bones also develop centers of ossification
in their epiphyses. At birth, ossification is not yet complete
and continues throughout childhood. In long
bones, growth occurs in the epiphyseal discs at the
junction of the diaphysis with each epiphysis. An epiphyseal
disc is still cartilage, and the bone grows in
length as more cartilage is produced on the epiphysis
side (see Fig. 6–3). On the diaphysis side, osteoblasts
produce bone matrix to replace the cartilage. Between
the ages of 16 and 25 years (influenced by estrogen or
testosterone), all of the cartilage of the epiphyseal
discs is replaced by bone. This is called closure of the
epiphyseal discs (or we say the discs are closed), and
the bone lengthening process stops.
Also in bones are specialized cells called osteoclasts
(a clast cell is a “destroying” cell), which are able
to dissolve and reabsorb the minerals of bone matrix,
a process called resorption. Osteoclasts are very
active in embryonic long bones, and they reabsorb
bone matrix in the center of the diaphysis to form the
marrow canal. Blood vessels grow into the marrow
canals of embryonic long bones, and red bone marrow
is established. After birth, the red bone marrow is
replaced by yellow bone marrow. Red bone marrow
remains in the spongy bone of short, flat, and irregular
bones. For other functions of osteoclasts and
osteoblasts, see Box 6–1: Fractures and Their Repair.
FACTORS THAT AFFECT BONE
GROWTH AND MAINTENANCE
1. Heredity—each person has a genetic potential for
height, that is, a maximum height, with genes
inherited from both parents. Many genes are
involved, and their interactions are not well understood.
Some of these genes are probably those for
the enzymes involved in cartilage and bone production,
for this is how bones grow.
2. Nutrition—nutrients are the raw materials of
which bones are made. Calcium, phosphorus, and
protein become part of the bone matrix itself.
Vitamin D is needed for the efficient absorption of
calcium and phosphorus by the small intestine.
Vitamins A and C do not become part of bone but
are necessary for the process of bone matrix formation
(ossification). Without these and other nutrients,
bones cannot grow properly. Children who
are malnourished grow very slowly and may not
reach their genetic potential for height.
3. Hormones—endocrine glands produce hormones
that stimulate specific effects in certain cells.
108 The Skeletal System
(text continued on page 112)
Frontal bone
Anterior fontanel Parietal bone
Posterior fontanel Posterior fontanel
Mastoid fontanel
Temporal bone
Sphenoid fontanel
Mandible
Sphenoid bone
Zygomatic bone
Maxilla
Occipital bone
Occipital bone
Frontal bone
Parietal bone
Anterior fontanel
A B
C D
Figure 6–2. Infant skull with fontanels. (A) Lateral view of left side. (B) Superior view.
(C) Fetal skull in anterior superior view. (D) Fetal skull in left lateral view. Try to name the
bones; use part A as a guide. The fontanels are translucent connective tissue. (C and D photographs
by Dan Kaufman.)
QUESTION: What is the difference between the frontal bone of the infant skull and that of
the adult skull?
109
B Epiphyseal disc
Chondrocytes producing cartilage
Bone
Cartilage
Epiphyseal
disc
Cartilaginous
model
Medullary cavity
containing
marrow
Bone collar
and calcifying
cartilage in
ossification center
Secondary
ossification
center
Compact bone
Compact bone
Spongy bone
Articular cartilage
Medullary cavity and development
A of secondary ossification centers
Osteoblasts producing bone
Figure 6–3. The ossification process in a long bone. (A) Progression of ossification from
the cartilage model of the embryo to the bone of a young adult. (B) Microscopic view of
an epiphyseal disc showing cartilage production and bone replacement.
QUESTION: The epiphyseal discs of the bone on the far right are closed. What does that
mean?
110
111
BOX 6–1 FRACTURES AND THEIR REPAIR
Box Figure 6–A Types of fractures. Several types of
fractures are depicted in the right arm.
A fracture means that a bone has been broken.
There are different types of fractures classified as to
extent of damage.
Simple (closed)—the broken parts are still in normal
anatomic position; surrounding tissue damage
is minimal (skin is not pierced).
Compound (open)—the broken end of a bone
has been moved, and it pierces the skin; there may
be extensive damage to surrounding blood vessels,
nerves, and muscles.
Greenstick—the bone splits longitudinally. The
bones of children contain more collagen than do
adult bones and tend to splinter rather than break
completely.
Comminuted—two or more intersecting breaks
create several bone fragments.
Impacted—the broken ends of a bone are forced
into one another; many bone fragments may be
created.
Pathologic (spontaneous)—a bone breaks without
apparent trauma; may accompany bone disorders
such as osteoporosis.
The Repair Process
Even a simple fracture involves significant bone
damage that must be repaired if the bone is to
resume its normal function. Fragments of dead or
damaged bone must first be removed. This is
accomplished by osteoclasts, which dissolve and
reabsorb the calcium salts of bone matrix. Imagine
a building that has just collapsed; the rubble must
be removed before reconstruction can take place.
This is what the osteoclasts do. Then, new bone
must be produced. The inner layer of the periosteum
contains osteoblasts that are activated when
bone is damaged. The osteoblasts produce bone
matrix to knit the broken ends of the bone
together.
Because most bone has a good blood supply, the
repair process is usually relatively rapid, and a simple
fracture often heals within 6 weeks. Some parts
of bones, however, have a poor blood supply, and
repair of fractures takes longer. These areas are the
neck of the femur (the site of a “fractured hip”) and
the lower third of the tibia.
Other factors that influence repair include the
age of the person, general state of health, and
nutrition. The elderly and those in poor health often
have slow healing of fractures. A diet with sufficient
calcium, phosphorus, vitamin D, and protein is also
important. If any of these nutrients is lacking, bone
repair will be a slower process.
Several hormones make important contributions to
bone growth and maintenance. These include
growth hormone, thyroxine, parathyroid hormone,
and insulin, which help regulate cell division, protein
synthesis, calcium metabolism, and energy
production. The sex hormones estrogen or testosterone
help bring about the cessation of bone
growth. The hormones and their specific functions
are listed in Table 6–1.
4. Exercise or “stress”—for bones, exercise means
bearing weight, which is just what bones are specialized
to do. Without this stress (which is normal),
bones will lose calcium faster than it is
replaced. Exercise need not be strenuous; it can be
as simple as the walking involved in everyday activities.
Bones that do not get this exercise, such as
those of patients confined to bed, will become thinner
and more fragile. This condition is discussed
further in Box 6–2: Osteoporosis.
THE SKELETON
The human skeleton has two divisions: the axial skeleton,
which forms the axis of the body, and the appendicular
skeleton, which supports the appendages or
limbs. The axial skeleton consists of the skull, vertebral
column, and rib cage. The bones of the arms and
legs and the shoulder and pelvic girdles make up the
appendicular skeleton. Many bones are connected to
other bones across joints by ligaments, which are
strong cords or sheets of fibrous connective tissue. The
importance of ligaments becomes readily apparent
when a joint is sprained. A sprain is the stretching or
even tearing of the ligaments of a joint, and though the
bones are not broken, the joint is weak and unsteady.
We do not often think of our ligaments, but they are
necessary to keep our bones in the proper positions to
keep us upright or to bear weight.
There are 206 bones in total, and the complete
skeleton is shown in Fig. 6–4.
SKULL
The skull consists of 8 cranial bones and 14 facial
bones. Also in the head are three small bones in each
middle ear cavity and the hyoid bone that supports the
base of the tongue. The cranial bones form the braincase
(lined with the meninges) that encloses and protects
the brain, eyes, and ears. The names of some of
these bones will be familiar to you; they are the same
as the terminology used (see Chapter 1) to describe
areas of the head. These are the frontal bone, parietal
bones (two), temporal bones (two), and occipital bone.
The sphenoid bone and ethmoid bone are part of the
floor of the braincase and the orbits (sockets) for
the eyes. The frontal bone forms the forehead and
the anterior part of the top of the skull. Parietal means
“wall,” and the two large parietal bones form the posterior
top and much of the side walls of the skull. Each
temporal bone on the side of the skull contains an
external auditory meatus (ear canal), a middle ear cav-
112 The Skeletal System
Table 6–1 HORMONES INVOLVED IN BONE GROWTH AND MAINTENANCE
Growth hormone (anterior pituitary gland)
Thyroxine (thyroid gland)
Insulin (pancreas)
Parathyroid hormone (parathyroid glands)
Calcitonin (thyroid gland)
Estrogen (ovaries) or
Testosterone (testes)
• Increases the rate of mitosis of chondrocytes and osteoblasts
• Increases the rate of protein synthesis (collagen, cartilage matrix,
and enzymes for cartilage and bone formation)
• Increases the rate of protein synthesis
• Increases energy production from all food types
• Increases energy production from glucose
• Increases the reabsorption of calcium from bones to the blood
(raises blood calcium level)
• Increases the absorption of calcium by the small intestine and kidneys
(to the blood)
• Decreases the reabsorption of calcium from bones (lowers blood
calcium level)
• Promotes closure of the epiphyses of long bones (growth stops)
• Helps retain calcium in bones to maintain a strong bone matrix
ity, and an inner ear labyrinth. The occipital bone
forms the lower, posterior part of the braincase. Its
foramen magnum is a large opening for the spinal
cord, and the two condyles (rounded projections) on
either side articulate with the atlas, the first cervical
vertebra. The sphenoid bone is said to be shaped like
a bat, and the greater wing is visible on the side of the
skull between the frontal and temporal bones. The
body of the bat has a depression called the sella turcica,
which encloses the pituitary gland. The ethmoid
bone has a vertical projection called the crista galli
(“rooster’s comb”) that anchors the cranial meninges.
The rest of the ethmoid bone forms the roof and
upper walls of the nasal cavities, and the upper part of
the nasal septum.
All of the joints between cranial bones are immovable
joints called sutures. It may seem strange to refer
to a joint without movement, but the term joint (or
articulation) is used for any junction of two bones.
(The classification of joints will be covered later in
The Skeletal System 113
BOX 6–2 OSTEOPOROSIS
Normal Bone Osteoporosis
A B
Box Figure 6–B (A) Normal spongy bone, as in the body of a vertebra. (B) Spongy bone thinned
by osteoporosis.
Bone is an active tissue; calcium is constantly being
removed to maintain normal blood calcium levels.
Usually, however, calcium is replaced in bones at a
rate equal to its removal, and the bone matrix
remains strong.
Osteoporosis is characterized by excessive loss
of calcium from bones without sufficient replacement.
Research has suggested that a certain gene
for bone buildup in youth is an important factor;
less buildup would mean earlier bone thinning.
Contributing environmental factors include smoking,
insufficient dietary intake of calcium, inactivity,
and lack of the sex hormones. Osteoporosis is most
common among elderly women, because estrogen
secretion decreases sharply at menopause (in older
men, testosterone is still secreted in significant
amounts). Factors such as bed rest or inability to
get even minimal exercise will make calcium loss
even more rapid.
As bones lose calcium and become thin and brittle,
fractures are much more likely to occur. Among
elderly women, a fractured hip (the neck of the
femur) is an all-too-common consequence of this
degenerative bone disorder. Such a serious injury is
not inevitable, however, and neither is the thinning
of the vertebrae that bows the spines of some elderly
people. After menopause, women may wish to
have a bone density test to determine the strength
of their bone matrix. Several medications are available
that diminish the rate of bone loss. A diet high
in calcium and vitamin D is essential for both men
and women, as is moderate exercise. Young women
and teenagers should make sure they get adequate
dietary calcium to form strong bone matrix,
because this will delay the serious effects of osteoporosis
later in life.
this chapter.) In a suture, the serrated, or sawtooth,
edges of adjacent bones fit into each other. These
interlocking projections prevent sliding or shifting of
the bones if the skull is subjected to a blow or pressure.
In Fig. 6–5 you can see the coronal suture
between the frontal and parietal bones, the squamosal
suture between the parietal and temporal bones, and
the lambdoidal suture between the occipital and parietal
bones. Not visible is the sagittal suture, where the
two parietal bones articulate along the midline of the
top of the skull. All the bones of the skull, as well as
the large sutures, are shown in Figs. 6–5 through 6–8.
Their anatomically important parts are described in
Table 6–2.
114 The Skeletal System
Phalanges
Phalanges
Tarsals
Metatarsals
Humerus
Radius
Ulna
Carpals
Metacarpals
Maxilla
Mandible
Skull (cranium)
Sternum
Clavicle
Scapula
Sacrum
Zygomatic arch
Cervical vertebrae
Thoracic vertebrae
Ribs
Lumbar vertebrae
Ilium
Coccyx
Pubis
Ischium
Femur
Patella
Tibia
Fibula
Figure 6–4. Skeleton. Anterior
view.
QUESTION: Which of the bones
shown here would be classified
as irregular bones?
115
Parietal bone
Parietal bone
Maxillary bone
Squamosal suture
Squamosal suture
Temporal bone
Mandible
Occipital bone
Occipital bone
Zygomatic process
Mandibular fossa
Mastoid process
External auditory
meatus
Coronoid process
Body
Mandible
Mandible
Mental foramen
Mental foramen
Maxilla
Maxilla
Condyloid process
Zygomatic bone
Zygomatic bone
Zygomatic bone
Lambdoidal suture
Nasal bone
Sphenoid bone
Lacrimal canal
Lacrimal bone
Lacrimal bone
Temporal bone
Sphenoid bone
Sphenoid bone
Parietal bone
Frontal bone
Frontal bone
Frontal bone
Coronal suture
Coronal suture
Ethmoid bone
Ethmoid bone
Nasal bone
Inferior nasal concha
Vomer
Inferior nasal concha
Middle nasal concha (ethmoid)
Vomer
Palatine bone
Temporal bone
Nasal bone
Perpendicular plate (ethmoid)
QUESTION: What might be the purpose of the openings at the back of the eye sockets?
Figure 6–5. Skull. Lateral view of right side. Figure 6–6. Skull. Anterior view.
116 The Skeletal System
Palatine process (maxilla)
Palatine bone
Zygomatic bone
Vomer
Zygomatic process
Temporal bone
Styloid process
External auditory meatus
Mastoid process
Occipital condyles
Foramen magnum
Occipital bone
Figure 6–7. Skull. Inferior view with mandible removed.
QUESTION: What is the purpose of the foramen magnum?
Of the 14 facial bones, only the mandible (lower
jaw) is movable; it forms a condyloid joint with each
temporal bone. The other joints between facial bones
are all sutures. The maxillae are the two upper jaw
bones, which also form the anterior portion of the
hard palate (roof of the mouth). Sockets for the roots
of the teeth are found in the maxillae and the
mandible. The two nasal bones form the bridge of
the nose where they articulate with the frontal bone
(the rest of the nose is supported by cartilage). There
is a lacrimal bone at the medial side of each orbit; the
lacrimal canal contains the lacrimal sac, a passageway
for tears. Each of the two zygomatic bones forms the
point of a cheek, and articulates with the maxilla,
frontal bone, and temporal bone. The two palatine
bones are the posterior portion of the hard palate.
The plow-shaped vomer forms the lower part of the
nasal septum; it articulates with the ethmoid bone. On
either side of the vomer are the conchae, six scrolllike
bones that curl downward from the sides of the
nasal cavities; they help increase the surface area of the
nasal mucosa. These facial bones are included in Table
6–2.
Paranasal sinuses are air cavities located in the
maxillae and frontal, sphenoid, and ethmoid bones
(Fig. 6–9). As the name paranasal suggests, they open
into the nasal cavities and are lined with ciliated
epithelium continuous with the mucosa of the nasal
cavities. We are aware of our sinuses only when they
become “stuffed up,” which means that the mucus
they produce cannot drain into the nasal cavities. This
may happen during upper respiratory infections such
(text continued on page 119)
117
Crista galli
Cribriform plate
Olfactory foramina
Greater wing
Lambdoidal suture
Frontal bone
Sphenoid bone
Sella turcica
Squamosal suture
Temporal bone
Parietal bone
Foramen magnum
Occipital bone
Ethmoid bone
A
C
B
Crista galli
Sella turcica
Figure 6–8. (A) Skull. Superior view with the top of cranium removed. (B) Sphenoid
bone in superior view. (C) Ethmoid bone in superior view. (B and C photographs by Dan
Kaufman.)
QUESTION: What are the olfactory foramina of the ethmoid bone for?
Table 6–2 BONES OF THE SKULL—IMPORTANT PARTS
Terminology of Bone Markings
Foramen—a hole or opening Meatus—a tunnel-like cavity Condyle—a rounded projection
Fossa—a depression Process—a projection Plate—a flat projection
Crest—a ridge or edge Facet—a flat projection Tubercle—a round projection
Bone Part Description
Frontal
Parietal (2)
Temporal (2)
Occipital
Sphenoid
Ethmoid
Mandible
Maxilla (2)
Nasal (2)
Lacrimal (2)
Zygomatic (2)
Palatine (2)
Vomer
• Frontal sinus
• Coronal suture
• Sagittal suture
• Squamosal suture
• External auditory meatus
• Mastoid process
• Mastoid sinus
• Mandibular fossa
• Zygomatic process
• Foramen magnum
• Condyles
• Lambdoidal suture
• Greater wing
• Sella turcica
• Sphenoid sinus
• Ethmoid sinus
• Crista galli
• Cribriform plate and
olfactory foramina
• Perpendicular plate
• Conchae (4 are part of
ethmoid; 2 inferior are
separate bones)
• Body
• Condyles
• Sockets
• Maxillary sinus
• Palatine process
• Sockets
—
• Lacrimal canal
—
—
—
• Air cavity that opens into nasal cavity
• Joint between frontal and parietal bones
• Joint between the 2 parietal bones
• Joint between temporal and parietal bone
• The tunnel-like ear canal
• Oval projection behind the ear canal
• Air cavity that opens into middle ear
• Oval depression anterior to the ear canal; articulates
with mandible
• Anterior projection that articulates with the
zygomatic bone
• Large opening for the spinal cord
• Oval projections on either side of the foramen
magnum; articulate with the atlas
• Joint between occipital and parietal bones
• Flat, lateral portion between the frontal and
temporal bones
• Central depression that encloses the pituitary
gland
• Air cavity that opens into nasal cavity
• Air cavity that opens into nasal cavity
• Superior projection for attachment of
meninges
• On either side of base of crista galli; olfactory
nerves pass through foramina
• Upper part of nasal septum
• Shelf-like projections into nasal cavities that
increase surface area of nasal mucosa
• U-shaped portion with lower teeth
• Oval projections that articulate with the temporal
bones
• Conical depressions that hold roots of lower
teeth
• Air cavity that opens into nasal cavity
• Projection that forms anterior part of hard
palate
• Conical depressions that hold roots of upper
teeth
• Form the bridge of the nose
• Opening for nasolacrimal duct to take tears to
nasal cavity
• Form point of cheek; articulate with frontal,
temporal, and maxillae
• Form the posterior part of hard palate
• Lower part of nasal septum
118
as colds, or with allergies such as hay fever. These
sinuses, however, do have functions: They make the
skull lighter in weight, because air is lighter than bone,
and they provide resonance for the voice, meaning
more air to vibrate and thus deepen the pitch of the
voice.
The mastoid sinuses are air cavities in the mastoid
process of each temporal bone; they open into the
middle ear. Before the availability of antibiotics, middle
ear infections often caused mastoiditis, infection of
these sinuses.
Within each middle ear cavity are three auditory
bones: the malleus, incus, and stapes. As part of the
hearing process (discussed in Chapter 9), these bones
transmit vibrations from the eardrum to the receptors
in the inner ear (see Fig. 9–7).
VERTEBRAL COLUMN
The vertebral column (spinal column or backbone) is
made of individual bones called vertebrae. The names
of vertebrae indicate their location along the length of
the spinal column. There are 7 cervical vertebrae, 12
thoracic, 5 lumbar, 5 sacral fused into 1 sacrum, and
4 to 5 small coccygeal vertebrae fused into 1 coccyx
(Fig. 6–10).
The seven cervical vertebrae are those within the
neck. The first vertebra is called the atlas, which articulates
with the occipital bone to support the skull and
forms a pivot joint with the odontoid process of the
axis, the second cervical vertebra. This pivot joint
allows us to turn our heads from side to side. The
remaining five cervical vertebrae do not have individual
names.
The thoracic vertebrae articulate (form joints)
with the ribs on the posterior side of the trunk. The
lumbar vertebrae, the largest and strongest bones of
the spine, are found in the small of the back. The
sacrum permits the articulation of the two hip bones:
the sacroiliac joints. The coccyx is the remnant of
tail vertebrae, and some muscles of the perineum
(pelvic floor) are anchored to it.
The Skeletal System 119
Frontal sinus
Ethmoid sinus
Maxillary sinus
Sphenoid sinus
Ethmoid sinus
A B
Figure 6–9. Paranasal sinuses. (A) Anterior view of the skull. (B) Left lateral view of skull.
QUESTION: Which of these sinuses often cause the pain of a sinus headache?
120 The Skeletal System
1
2
3
4
5
6
7
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
4
5
Coccyx
Sacrum
Lumbar
vertebrae
Thoracic
vertebrae
Cervical
vertebrae
•
•
Intervertebral
discs
•
Articular surface
for ilium
•
•
Vertebral body
Vertebral
canal
•
•
Spinous process
Facet for rib
•
Transverse process
•
•
Odontoid process
•
Vertebral canal
Spinous process
of axis
Facets of atlas for occipital condyles
D
1st Lumbar
C
7th Lumbar
B
Atlas/Axis
A
Figure 6–10. Vertebral column. (A) Lateral view of left
side. (B) Atlas and axis, superior view. (C) 7th thoracic
vertebra, left lateral view. (D) 1st lumbar vertebra, left lateral
view.
QUESTION: Compare the size of the individual thoracic
and lumbar vertebrae. What is the reason for this difference?
All of the vertebrae articulate with one another in
sequence, connected by ligaments, to form a flexible
backbone that supports the trunk and head. They also
form the vertebral canal, a continuous tunnel (lined
with the meninges) within the bones that contains the
spinal cord and protects it from mechanical injury.
The spinous and transverse processes are projections
for the attachment of the muscles that bend the vertebral
column. The facets of some vertebrae are small
flat surfaces for articulation with other bones, such as
the ribs with the facets of the thoracic vertebrae.
The supporting part of a vertebra is its body; the
bodies of adjacent vertebrae are separated by discs
of fibrous cartilage. These discs cushion and absorb
shock and permit some movement between vertebrae
(symphysis joints). Since there are so many joints,
the backbone as a whole is quite flexible (see also Box
6–3: Herniated Disc).
The normal spine in anatomic position has four
natural curves, which are named after the vertebrae
that form them. Refer to Fig. 6–10, and notice that the
cervical curve is forward, the thoracic curve backward,
BOX 6–3 HERNIATED DISC
The vertebrae are separated by discs of fibrous cartilage
that act as cushions to absorb shock. An intervertebral
disc has a tough outer covering and a soft
center called the nucleus pulposus. Extreme pressure
on a disc may rupture the outer layer and force
the nucleus pulposus out. This may occur when a
person lifts a heavy object improperly, that is, using
the back rather than the legs and jerking upward,
which puts sudden, intense pressure on the spine.
Most often this affects discs in the lumbar region.
Although often called a “slipped disc,” the
affected disc is usually not moved out of position.
Box Figure 6–C Herniated disc. As a result of compression, a ruptured intervertebral disc puts pressure
on a spinal nerve.
The terms herniated disc or ruptured disc more
accurately describe what happens. The nucleus pulposus
is forced out, often posteriorly, where it puts
pressure on a spinal nerve. For this reason a herniated
disc may be very painful or impair function in
the muscles supplied by the nerve.
Healing of a herniated disc may occur naturally if
the damage is not severe and the person rests and
avoids activities that would further compress the
disc. Surgery may be required, however, to remove
the portion of the nucleus pulposus that is out of
place and disrupting nerve functioning.
121
the lumbar curve forward, and the sacral curve backward.
These curves center the skull over the rest of the
body, which enables a person to more easily walk
upright (see Box 6–4: Abnormalities of the Curves of
the Spine).
RIB CAGE
The rib cage consists of the 12 pairs of ribs and the
sternum, or breastbone. The three parts of the sternum
are the upper manubrium, the central body, and
the lower xiphoid process (Fig. 6–11).
All of the ribs articulate posteriorly with the thoracic
vertebrae. The first seven pairs of ribs are called
true ribs; they articulate directly with the manubrium
and body of the sternum by means of costal cartilages.
The next three pairs are called false ribs; their cartilages
join the 7th rib cartilage. The last two pairs are
called floating ribs because they do not articulate
with the sternum at all (see Fig. 6–10).
An obvious function of the rib cage is that it encloses
and protects the heart and lungs. Keep in mind,
though, that the rib cage also protects organs in the
upper abdominal cavity, such as the liver and spleen.
The other important function of the rib cage depends
upon its flexibility: The ribs are pulled upward and
outward by the external intercostal muscles. This
enlarges the chest cavity, which expands the lungs and
contributes to inhalation.
THE SHOULDER AND ARM
The shoulder girdles attach the arms to the axial skeleton.
Each consists of a scapula (shoulder blade) and
clavicle (collarbone). The scapula is a large, flat bone
with several projections (the spine of the scapula, the
coracoid process) that anchor some of the muscles that
move the upper arm and the forearm. A shallow
depression called the glenoid fossa forms a ball-andsocket
joint with the humerus, the bone of the upper
arm (Fig. 6–12).
Each clavicle articulates laterally with a scapula
and medially with the manubrium of the sternum. In
this position the clavicles act as braces for the scapulae
and prevent the shoulders from coming too far forward.
Although the shoulder joint is capable of a wide
range of movement, the shoulder itself must be relatively
stable if these movements are to be effective.
The humerus is the long bone of the upper arm. In
Fig. 6–12, notice the deltoid tubercle (or tuberosity);
the triangular deltoid muscle that caps the shoulder
joint is anchored here. Proximally, the humerus forms
a ball-and-socket joint with the scapula. Distally, the
humerus forms a hinge joint with the ulna of the
forearm. This hinge joint, the elbow, permits movement
in one plane, that is, back and forth with no lateral
movement.
The forearm bones are the ulna on the little finger
side and the radius on the thumb side. The semilunar
notch of the ulna is part of the hinge joint of the
elbow; it articulates with the trochlea of the humerus.
The radius and ulna articulate proximally to form a
pivot joint, which permits turning the hand palm up
to palm down. You can demonstrate this yourself by
holding your arm palm up in front of you, and noting
that the radius and ulna are parallel to each other.
Then turn your hand palm down, and notice that your
122 The Skeletal System
BOX 6–4 ABNORMALITIES OF THE
CURVES OF THE SPINE
Scoliosis—an abnormal lateral curvature, which
may be congenital, the result of having one leg
longer than the other, or the result of chronic
poor posture during childhood while the vertebrae
are still growing. Usually the thoracic vertebrae
are affected, which displaces the rib cage to
one side. In severe cases, the abdominal organs
may be compressed, and the expansion of the
rib cage during inhalation may be impaired.
Kyphosis*—an exaggerated thoracic curve;
sometimes referred to as hunchback.
Lordosis*—an exaggerated lumbar curve;
sometimes referred to as swayback.
These abnormal curves are usually the result
of degenerative bone diseases such as osteoporosis
or tuberculosis of the spine. If osteoporosis,
for example, causes the bodies of the thoracic
vertebrae to collapse, the normal thoracic curve
will be increased. Most often the vertebral body
“settles” slowly (rather than collapses suddenly)
and there is little, if any, damage to the spinal
nerves. The damage to the vertebrae, however,
cannot be easily corrected, so these conditions
should be thought of in terms of prevention
rather than cure.
*Although descriptive of normal anatomy, the terms
kyphosis and lordosis, respectively, are commonly used to
describe the abnormal condition associated with each.
upper arm does not move. The radius crosses over the
ulna, which permits the hand to perform a great variety
of movements without moving the entire arm.
The carpals are eight small bones in the wrist;
gliding joints between them permit a sliding movement.
The carpals also articulate with the distal ends
of the ulna and radius, and with the proximal ends of
the metacarpals, the five bones of the palm of the
hand. All of the joints formed by the carpals and
metacarpals make the hand very flexible at the wrist
(try this yourself: flexion to extension should be almost
180 degrees), but the thumb is more movable than the
fingers because of its carpometacarpal joint. This is a
saddle joint, which enables the thumb to cross over
the palm, and permits gripping.
The phalanges are the bones of the fingers. There
are two phalanges in each thumb and three in each
of the fingers. Between phalanges are hinge joints,
which permit movement in one plane. Important
parts of the shoulder and arm bones are described
in Table 6–3.
THE HIP AND LEG
The pelvic girdle (or pelvic bone) consists of the two
hip bones (coxae or innominate bones), which articulate
with the axial skeleton at the sacrum. Each hip
bone has three major parts: the ilium, ischium, and
pubis, and these are shown in Fig. 6–13, which depicts
both a male and a female pelvis. The ilium is the
flared, upper portion that forms the sacroiliac joint.
The ischium is the lower, posterior part that we sit
on. The pubis is the lower, most anterior part. The
two pubic bones articulate with one another at the
pubic symphysis, with a disc of fibrous cartilage
between them. Notice the pubic angle of both the
male and female pelvises in Fig. 6–13. The wider
female angle is an adaptation for childbirth, in that it
helps make the pelvic outlet larger.
The acetabulum is the socket in the hip bone that
forms a ball-and-socket joint with the femur.
Compared to the glenoid fossa of the scapula, the
acetabulum is a much deeper socket. This has great
The Skeletal System 123
Manubrium
Body of sternum
Costal
cartilages
Xiphoid process
12th thoracic vertebra
1st rib
1st thoracic vertebra 2nd
3rd
4th
5th
6th
7th
8th
9th
10th
11th
12th
Figure 6–11. Rib cage. Anterior view.
QUESTION: With what bones do all of the ribs articulate?
•
•
•
•
•
•
•
•
•
•
• • •
• •
•
•
•
•
•
•
•
•
• •
Clavicle
Sternal end
Glenoid fossa
Scapula
Trochlea
Semilunar notch
Olecranon process
On posterior side
Ulna
Lunate
Triquetrum
Pisiform
Hamate
Carpals
Metacarpals
Phalanges
Capitate
Trapezoid
Trapezium
Scaphoid
Carpals
Radius
Radial tuberosity
Head
Capitulum
Humerus
Deltoid tubercle
Head
Coracoid process
Acromicon process
Acromial end
Figure 6–12. Bones of arm and shoulder girdle. Anterior view of right arm.
QUESTION: What types of joints are found in the arm? Begin at the shoulder and work
downward.
124
functional importance because the hip is a weightbearing
joint, whereas the shoulder is not. Because the
acetabulum is deep, the hip joint is not easily dislocated,
even by activities such as running and jumping
(landing), which put great stress on the joint.
The femur is the long bone of the thigh. As mentioned,
the femur forms a very movable ball-andsocket
joint with the hip bone. At the proximal end of
the femur are the greater and lesser trochanters, large
projections that are anchors for muscles. At its distal
end, the femur forms a hinge joint, the knee, with the
tibia of the lower leg. Notice in Fig. 6–14 that each
bone has condyles, which are the rounded projections
that actually form the joint. The patella, or kneecap,
is anterior to the knee joint, enclosed in the tendon of
the quadriceps femoris, a large muscle group of the
thigh.
The tibia is the weight-bearing bone of the lower
leg. You can feel the tibial tuberosity (a bump) and
anterior crest (a ridge) on the front of your own leg.
The medial malleolus, what we may call the “inner
ankle bone,” is at the distal end. Notice in Fig. 6–14
that the fibula is not part of the knee joint and does
not bear much weight. The lateral malleolus of the
fibula is the “outer ankle bone” you can find just above
your foot. Though not a weight-bearing bone, the
fibula is important in that leg muscles are attached and
anchored to it, and it helps stabilize the ankle. Two
bones on one is a much more stable arrangement than
one bone on one, and you can see that the malleoli of
the tibia and fibula overlap the sides of the talus. The
tibia and fibula do not form a pivot joint as do the
radius and ulna in the forearm; this also contributes to
the stability of the lower leg and foot and the support
of the entire body.
The tarsals are the seven bones in the ankle. As
you would expect, they are larger and stronger than
the carpals of the wrist, and their gliding joints do not
provide nearly as much movement. The largest is the
calcaneus, or heel bone; the talus transmits weight
between the calcaneus and the tibia. Metatarsals are
the five long bones of each foot, and phalanges are
the bones of the toes. There are two phalanges in the
big toe and three in each of the other toes. The phalanges
of the toes form hinge joints with each other.
Because there is no saddle joint in the foot, the big toe
is not as movable as the thumb. The foot has two
major arches, longitudinal and transverse, that are
supported by ligaments. These are adaptations for
walking completely upright, in that arches provide for
spring or bounce in our steps. Important parts of hip
and leg bones are described in Table 6–4.
The Skeletal System 125
Table 6–3 BONES OF THE SHOULDER AND ARM—IMPORTANT PARTS
Bone Part Description
Scapula
Clavicle
Humerus
Radius
Ulna
Carpals (8)
• Glenoid fossa
• Spine
• Acromion process
• Acromial end
• Sternal end
• Head
• Deltoid tubercle
• Olecranon fossa
• Capitulum
• Trochlea
• Head
• Olecranon process
• Semilunar notch
• Scaphoid • Lunate
• Triquetrum • Pisiform
• Trapezium • Trapezoid
• Capitate • Hamate
• Depression that articulates with humerus
• Long, posterior process for muscle attachment
• Articulates with clavicle
• Articulates with scapula
• Articulates with manubrium of sternum
• Round process that articulates with scapula
• Round process for the deltoid muscle
• Posterior, oval depression for the olecranon process
of the ulna
• Round process superior to radius
• Concave surface that articulates with ulna
• Articulates with the ulna
• Fits into olecranon fossa of humerus
• “Half-moon” depression that articulates with the
trochlea of ulna
• Proximal row
• Distal row
(text continued on page 128)
Figure 6–13. Hip bones and sacrum. (A) Male pelvis, anterior view. (B) Male pelvis, lateral
view of right side. (C) Female pelvis, anterior view. (D) Female pelvis, lateral view of
right side.
QUESTION: Compare the male and female pelvic inlets. What is the reason for this difference?
126
Ischium
Medial condyle
Medial condyle
Tibial tuberosity
Pubis
Tibia
Medial malleolus
Talus
Navicular
Tarsals
Cuneiforms
First
Second
Third
Acetabulum
Femur
Patella
Lateral condyle
Lateral condyle
Head
Greater trochanter
Neck
Lesser trochanter
Fibula
Lateral malleolus
Calcaneus
Cuboid
Tarsals
Metatarsals
Phalanges
A
Fibula
Talus
Calcaneus
Cuboid
Tibia
Phalanges Metatarsals
B
Anterior crest
Head
Figure 6–14. (A) Bones of the leg and portion of hip bone, anterior view of left leg.
(B) Lateral view of left foot.
QUESTION: What types of joints found in the arm do not have counterparts in the leg?
127
128 The Skeletal System
Table 6–4 BONES OF THE HIP AND LEG—IMPORTANT PARTS
Bone Part Description
Pelvic (2 hip bones)
Femur
Tibia
Fibula
Tarsals (7)
• Ilium
• Iliac crest
• Posterior superior iliac spine
• Ischium
• Pubis
• Pubic symphysis
• Acetabulum
• Head
• Neck
• Greater trochanter
• Lesser trochanter
• Condyles
• Condyles
• Tibial tuberosity
• Anterior crest
• Medial malleolus
• Head
• Lateral malleolus
• Calcaneus
• Talus
• Cuboid, navicular
• Cuneiform: 1st, 2nd, 3rd
• Flared, upper portion
• Upper edge of ilium
• Posterior continuation of iliac crest
• Lower, posterior portion
• Anterior, medial portion
• Joint between the 2 pubic bones
• Deep depression that articulates with femur
• Round process that articulates with hip bone
• Constricted portion distal to head
• Large lateral process for muscle attachment
• Medial process for muscle attachment
• Rounded processes that articulate with tibia
• Articulate with the femur
• Round process for the patellar ligament
• Vertical ridge
• Distal process; medial “ankle bone”
• Articulates with tibia
• Distal process; lateral “ankle bone”
• Heel bone
• Articulates with calcaneus and tibia
—— JOINTS—ARTICULATIONS
A joint is where two bones meet, or articulate.
THE CLASSIFICATION OF JOINTS
The classification of joints is based on the amount
of movement possible. A synarthrosis is an immovable
joint, such as a suture between two cranial bones.
An amphiarthrosis is a slightly movable joint, such
as the symphysis joint between adjacent vertebrae. A
diarthrosis is a freely movable joint. This is the largest
category of joints and includes the ball-and-socket
joint, the pivot, hinge, and others. Examples of each
type of joint are described in Table 6–5, and many of
these are illustrated in Fig. 6–15.
SYNOVIAL JOINTS
All diarthroses, or freely movable joints, are synovial
joints because they share similarities of structure. A
typical synovial joint is shown in Fig. 6–16. On the
joint surface of each bone is the articular cartilage,
which provides a smooth surface. The joint capsule,
made of fibrous connective tissue, encloses the joint in
a strong sheath, like a sleeve. Lining the joint capsule
is the synovial membrane, which secretes synovial
fluid into the joint cavity. Synovial fluid is thick and
slippery and prevents friction as the bones move.
Many synovial joints also have bursae (or bursas),
which are small sacs of synovial fluid between the joint
and the tendons that cross over the joint. Bursae permit
the tendons to slide easily as the bones are moved.
If a joint is used excessively, the bursae may become
inflamed and painful; this condition is called bursitis.
Some other disorders of joints are described in Box
6–5: Arthritis.
AGING AND THE SKELETAL SYSTEM
With age, bone tissue tends to lose more calcium than
is replaced. Bone matrix becomes thinner, the bones
themselves more brittle, and fractures are more likely
to occur with mild trauma.
Erosion of the articular cartilages of joints is also a
common consequence of aging. Joints affected include
weight-bearing joints such as the knees, and active,
small joints such as those of the fingers.
(text continued on page 131)
•
•
•
•
•
•
Carpals
•
•
Head of
femur
Acetabulum of
hip bone
•
•
Trochlea
of humerus
Semilunar notch
of ulna
Bodies of
vertebra
Intervertebral
disc
•
•
Metacarpal
of thumb
Trapezium
(carpal)
•
•
Atlas
Odontoid process
of axis
D Gliding
E Symphysis
F Saddle
A Ball and socket B Hinge C Pivot
Figure 6–15. Types of joints. For each type, a specific joint is depicted, and a simple
diagram shows the position of the joint surfaces. (A) Ball and socket. (B) Hinge. (C) Pivot.
(D) Gliding. (E) Symphysis. (F) Saddle.
QUESTION: Which of these types of joints is most movable? Which is least movable?
Table 6–5 TYPES OF JOINTS
Category Type and Description Examples
Synarthrosis (immovable)
Amphiarthrosis (slightly
movable)
Diarthrosis (freely movable)
Suture—fibrous connective tissue
between bone surfaces
Symphysis—disc of fibrous cartilage
between bones
Ball and socket—movement in all
planes
Hinge—movement in one plane
Condyloid—movement in one plane
with some lateral movement
Pivot—rotation
Gliding—side-to-side movement
Saddle—movement in several planes
• Between cranial bones; between
facial bones
• Between vertebrae; between pubic
bones
• Scapula and humerus; pelvic bone
and femur
• Humerus and ulna; femur and tibia;
between phalanges
• Temporal bone and mandible
• Atlas and axis; radius and ulna
• Between carpals
• Carpometacarpal of thumb
129
130
Bone
Synovial
membrane
Joint
capsule
Joint cavity
(synovial fluid)
Bursa
Articular
cartilage
Tendon
Bone
Figure 6–16. Structure of a synovial joint. See
text for description.
QUESTION: How can you tell that this is a joint
between two long bones? Give an example.
BOX 6–5 ARTHRITIS
bacterial and viral infections have been suggested
as possibilities.
Rheumatoid arthritis often begins in joints of the
extremities, such as those of the fingers. The
autoimmune activity seems to affect the synovial
membrane, and joints become painful and stiff.
Sometimes the disease progresses to total destruction
of the synovial membrane and calcification of
the joint. Such a joint is then fused and has no
mobility at all. Autoimmune damage may also
occur in the heart and blood vessels, and those with
RA are more prone to heart attacks and strokes (RA
is a systemic, not a localized, disease).
Treatment of rheumatoid arthritis is directed at
reducing inflammation as much as possible, for it is
the inflammatory process that causes the damage.
Therapies being investigated involve selectively
blocking specific aspects of the immune response,
such as antibody production. At present there is no
cure for autoimmune diseases.
The term arthritis means “inflammation of a
joint.” Of the many types of arthritis, we will consider
two: osteoarthritis and rheumatoid arthritis.
Osteoarthritis is a natural consequence of getting
older. In joints that have borne weight for
many years, the articular cartilage is gradually worn
away. The once smooth joint surface becomes
rough, and the affected joint is stiff and painful. As
you might guess, the large, weight-bearing joints
are most often subjected to this form of arthritis. If
we live long enough, most of us can expect some
osteoarthritis in knees, hips, or ankles.
Rheumatoid arthritis (RA) can be a truly crippling
disease that may begin in early middle age
or, less commonly, during adolescence. It is an
autoimmune disease, which means that the
immune system mistakenly directs its destructive
capability against part of the body. Exactly what
triggers this abnormal response by the immune
system is not known with certainty, but certain
Although the normal wear and tear of joints cannot
be prevented, elderly people can preserve their bone
matrix with exercise (dancing counts) and diets high in
calcium and vitamin D.
SUMMARY
Your knowledge of the bones and joints will be useful
in the next chapter as you learn the actions of the muscles
that move the skeleton. It is important to remember,
however, that bones have other functions as well.
As a storage site for excess calcium, bones contribute
to the maintenance of a normal blood calcium level.
The red bone marrow found in flat and irregular
bones produces the blood cells: red blood cells, white
blood cells, and platelets. Some bones protect vital
organs such as the brain, heart, and lungs. As you can
see, bones themselves may also be considered vital
organs.
The Skeletal System 131
STUDY OUTLINE
The skeleton is made of bone and cartilage
and has these functions:
1. Is a framework for support, connected by ligaments,
moved by muscles.
2. Protects internal organs from mechanical injury.
3. Contains and protects red bone marrow.
4. Stores excess calcium; important to regulate blood
calcium level.
Bone Tissue (see Fig. 6–1)
1. Osteocytes (cells) are found in the matrix of calcium
phosphate, calcium carbonate, and collagen.
2. Compact bone—haversian systems are present.
3. Spongy bone—no haversian systems; red bone
marrow present.
4. Articular cartilage—smooth, on joint surfaces.
5. Periosteum—fibrous connective tissue membrane;
anchors tendons and ligaments; has blood vessels
that enter the bone.
Classification of Bones
1. Long—arms, legs; shaft is the diaphysis (compact
bone) with a marrow cavity containing yellow bone
marrow (fat); ends are epiphyses (spongy bone) (see
Fig. 6–1).
2. Short—wrists, ankles (spongy bone covered with
compact bone).
3. Flat—ribs, pelvic bone, cranial bones (spongy bone
covered with compact bone).
4. Irregular—vertebrae, facial bones (spongy bone
covered with compact bone).
Embryonic Growth of Bone
1. The embryonic skeleton is first made of other tissues
that are gradually replaced by bone. Ossification
begins in the third month of gestation;
osteoblasts differentiate from fibroblasts and produce
bone matrix.
2. Cranial and facial bones are first made of fibrous
connective tissue; osteoblasts produce bone matrix
in a center of ossification in each bone; bone growth
radiates outward; fontanels remain at birth, permit
compression of infant skull during birth; fontanels
are calcified by age 2 (see Fig. 6–2).
3. All other bones are first made of cartilage; in a long
bone the first center of ossification is in the diaphysis,
other centers develop in the epiphyses. After
birth a long bone grows at the epiphyseal discs:
Cartilage is produced on the epiphysis side, and
bone replaces cartilage on the diaphysis side.
Osteoclasts form the marrow cavity by reabsorbing
bone matrix in the center of the diaphysis (see
Fig. 6–3).
Factors That Affect Bone Growth and
Maintenance
1. Heredity—many pairs of genes contribute to
genetic potential for height.
2. Nutrition—calcium, phosphorus, and protein become
part of the bone matrix; vitamin D is needed
for absorption of calcium in the small intestine;
vitamins C and A are needed for bone matrix production
(calcification).
3. Hormones—produced by endocrine glands; concerned
with cell division, protein synthesis, calcium
metabolism, and energy production (see Table 6–1).
4. Exercise or stress—weight-bearing bones must
bear weight or they will lose calcium and become
brittle.
The Skeleton—206 bones (see Fig. 6–4);
bones are connected by ligaments
1. Axial—skull, vertebrae, rib cage.
• Skull—see Figs. 6–5 through 6–8 and Table 6–2.
Eight cranial bones form the braincase, which
also protects the eyes and ears; 14 facial bones
make up the face; the immovable joints
between these bones are called sutures.
Paranasal sinuses are air cavities in the maxillae,
frontal, sphenoid, and ethmoid bones; they
lighten the skull and provide resonance for
voice (see Fig. 6–9).
Three auditory bones in each middle ear cavity
transmit vibrations for the hearing process.
• Vertebral column—see Fig. 6–10.
Individual bones are called vertebrae: 7 cervical,
12 thoracic, 5 lumbar, 5 sacral (fused into
one sacrum), 4 to 5 coccygeal (fused into one
coccyx). Supports trunk and head, encloses
and protects the spinal cord in the vertebral
canal. Discs of fibrous cartilage absorb shock
between the bodies of adjacent vertebrae, also
permit slight movement. Four natural curves
center head over body for walking upright (see
Table 6–5 for joints).
• Rib cage—see Fig. 6–11.
Sternum and 12 pairs of ribs; protects thoracic
and upper abdominal organs from mechanical
injury and is expanded to contribute to inhalation.
Sternum consists of manubrium, body,
and xiphoid process. All ribs articulate with
thoracic vertebrae; true ribs (first seven pairs)
articulate directly with sternum by means of
costal cartilages; false ribs (next three pairs)
articulate with 7th costal cartilage; floating
ribs (last two pairs) do not articulate with the
sternum.
2. Appendicular—bones of the arms and legs and the
shoulder and pelvic girdles.
• Shoulder and arm—see Fig. 6–12 and Table 6–3.
Scapula—shoulder muscles are attached; glenoid
fossa articulates with humerus.
Clavicle—braces the scapula.
Humerus—upper arm; articulates with the
scapula and the ulna (elbow).
Radius and ulna—forearm—articulate with
one another and with carpals.
Carpals—eight—wrist; metacarpals—five—
hand; phalanges—14—fingers (for joints, see
Table 6–5).
• Hip and leg—see Figs. 6–13 and 6–14 and Table
6–4.
Pelvic bone—two hip bones; ilium, ischium,
pubis; acetabulum articulates with femur.
Femur—thigh; articulates with pelvic bone
and tibia (knee).
Patella—kneecap; in tendon of quadriceps
femoris muscle.
Tibia and fibula—lower leg; tibia bears weight;
fibula does not bear weight, but does anchor
muscles and stabilizes ankle.
Tarsals—seven—ankle; calcaneus is heel bone.
Metatarsals—five—foot; phalanges—14—toes
(see Table 6–5 for joints).
Joints—Articulations
1. Classification based on amount of movement:
• Synarthrosis—immovable.
• Amphiarthrosis—slightly movable.
• Diarthrosis—freely movable (see Table 6–5 for
examples; see also Fig. 6–15).
2. Synovial joints—all diarthroses have similar structure
(see Fig. 6–16):
• Articular cartilage—smooth on joint surfaces.
• Joint capsule—strong fibrous connective tissue
sheath that encloses the joint.
• Synovial membrane—lines the joint capsule;
secretes synovial fluid that prevents friction.
• Bursae—sacs of synovial fluid that permit tendons
to slide easily across joints.
132 The Skeletal System
The Skeletal System 133
REVIEW QUESTIONS
1. Explain the differences between compact bone
and spongy bone, and state where each type is
found. (p. 106)
2. State the locations of red bone marrow, and name
the blood cells it produces. (p. 106)
3. Name the tissue of which the embryonic skull is
first made. Explain how ossification of cranial
bones occurs. (p. 108)
4. State what fontanels are, and explain their function.
(p. 108)
5. Name the tissue of which the embryonic femur
is first made. Explain how ossification of this
bone occurs. Describe what happens in epiphyseal
discs to produce growth of long bones.
(p. 108)
6. Explain what is meant by “genetic potential” for
height, and name the nutrients a child must have
in order to attain genetic potential. (p. 108)
7. Explain the functions of calcitonin and parathyroid
hormone with respect to bone matrix and to
blood calcium level. (p. 112)
8. Explain how estrogen or testosterone affects bone
growth, and when. (p. 112)
9. State one way each of the following hormones
helps promote bone growth: insulin, thyroxine,
growth hormone. (p. 112)
10. Name the bones that make up the braincase.
(p. 112)
11. Name the bones that contain paranasal sinuses
and explain the functions of these sinuses.
(pp. 116, 119)
12. Name the bones that make up the rib cage, and
describe two functions of the rib cage. (p. 122)
13. Describe the functions of the vertebral column.
State the number of each type of vertebra.
(pp. 119–120)
14. Explain how the shoulder and hip joints are similar
and how they differ. (pp. 122, 125)
15. Give a specific example (name two bones) for each
of the following types of joints: (p. 129)
a. Hinge
b. Symphysis
c. Pivot
d. Saddle
e. Suture
f. Ball and socket
16. Name the part of a synovial joint with each of the
following functions: (p. 128)
a. Fluid within the joint cavity that prevents friction
b. Encloses the joint in a strong sheath
c. Provides a smooth surface on bone surfaces
d. Lines the joint capsule and secretes synovial
fluid
17. Refer to the diagram (Fig. 6–4) of the full skeleton,
and point to each bone on yourself. (p. 114)
FOR FURTHER THOUGHT
1. Following a severe spinal cord injury in the lumbar
region, the voluntary muscles of the legs and hips
will be paralyzed. Describe the effects of paralysis
on the skeleton.
2. The sutures of the adult skull are joints that do not
allow movement. Why have joints at all if no movement
is permitted? Explain.
3. Without looking at any of the illustrations, try to
name all the bones that form the orbits, the sockets
for the eyes. Check your list with Figs. 6–5 and
6–6.
4. In an effort to prevent sudden infant death syndrome
(SIDS), parents were advised to put their
infants to sleep lying on their backs, not their
stomachs. Since then (1994), the number of SIDS
deaths has decreased markedly. What do you think
has happened to the skulls of many of those
infants? Explain.
5. A 5-month-old infant is brought to a clinic after
having diarrhea for 2 days. The nurse checks the
baby’s anterior fontanel and notices that it appears
sunken. What has caused this?
6. Look at the lateral view of the adult skull in Fig.
6–5 and notice the size of the face part in proportion
to the braincase part. Compare the infant skull
in Fig. 6–2, and notice how small the infant face is,
relative to the size of the braincase. There is a very
good reason for this. What do you think it is?
7. Look at the photograph here, Question Fig. 6–A.
Name the bone and the type of section in which it
is cut. What are the large arrows indicating? What
are the smaller arrows indicating? Look carefully
at this site, describe it, and explain possible consequences.
134 The Skeletal System
Question Figure 6–A (Photograph by Dan Kaufman.