Essentials of Anatomy and Physiology

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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.










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84 Tissues and Membranes


CONNECTIVE TISSUE MEMBRANES
Table 4–5 CONNECTIVE TISSUE
MEMBRANES Many membranes are made of connective tissue.
Because these will be covered with the organ systems
Membrane Location and Function
of which they are a part, their locations and functions
Superficial • Between the skin and muscles; are summarized in Table 4–5.
fascia adipose tissue stores fat
Periosteum • Covers each bone; contains
blood vessels that enter the AGING AND TISSUES
bone
• Anchors tendons and ligaments
As mentioned in the previous chapter, aging takes
Perichondrium • Covers cartilage; contains capil- place at the cellular level, but of course is apparent in
laries, the only blood supply for the groups of cells we call tissues. In muscle tissue, for
cartilage
example, the proteins that bring about contraction
Synovial • Lines joint cavities; secretes syn- deteriorate and are not repaired or replaced. The same
ovial fluid to prevent friction
when joints move is true of collagen and elastin, the proteins of connec-
tive tissue such as the dermis of the skin. Other aspects
Deep fascia • Covers each skeletal muscle;
anchors tendons of the aging of tissues will be more meaningful to you
in the context of the functions of organs and systems,
Meninges • Cover the brain and spinal cord;
contain cerebrospinal fluid so we will save those for the following chapters.
Fibrous • Forms a sac around the heart;
pericardium lined by the serous parietal peri-
cardium 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
The mucus secreted by these membranes keeps the intermediate level with respect to the structural and
lining epithelial cells wet. Remember that these are functional complexity of the body as a whole. The fol-
living cells, and if they dry out, they will die. In the lowing chapters are concerned with the organ systems,
digestive tract, mucus also lubricates the surface to the most complex level. In the descriptions of the
permit the smooth passage of food. In the respiratory organs of these systems, you will find mention of the
tract the mucus traps dust and bacteria, which are then tissues and their contributions to each organ and
swept to the pharynx by ciliated epithelium. organ system.




STUDY OUTLINE

A tissue is a group of cells with similar struc- 1. Simple squamous—one layer of flat cells; thin and
ture and function. The four main groups of smooth. Sites: alveoli (to permit diffusion of gases);
tissues are epithelial, connective, muscle, and capillaries (to permit exchanges between blood and
nerve. tissues).
2. Stratified squamous—many layers of mostly flat
Epithelial Tissue—found on surfaces; have no cells; mitosis takes place in lowest layer. Sites: epi-
capillaries; some are capable of secretion; dermis, where surface cells are dead (a barrier to
classified as to shape of cells and number of pathogens); lining of mouth; esophagus; and vagina
layers of cells (see Table 4–1 and Figs. 4–1, (a barrier to pathogens).
4–2, and 4–3) 3. Transitional—stratified, yet surface cells are

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Tissues and Membranes 85


rounded and flatten when stretched. Site: urinary 4. Fibrous—mostly matrix, strong collagen fibers;
bladder (to permit expansion without tearing the cells are fibroblasts. Regular fibrous sites: tendons
lining). (to connect muscle to bone); ligaments (to connect
4. Simple cuboidal—one layer of cube-shaped cells. bone to bone); poor blood supply, slow healing.
Sites: thyroid gland (to secrete thyroid hormones); Irregular fibrous sites: dermis of the skin and the
salivary glands (to secrete saliva); kidney tubules (to fascia around muscles.
reabsorb useful materials back to the blood). 5. Elastic—mostly matrix, elastin fibers. Sites: walls of
5. Simple columnar—one layer of column-shaped large arteries (to maintain blood pressure); around
cells. Sites: stomach lining (to secrete gastric juice); alveoli (to promote normal exhalation).
small intestinal lining (to secrete digestive enzymes 6. Bone—cells are osteocytes; matrix is calcium salts
and absorb nutrients—microvilli increase surface and collagen, strong and not flexible; good blood
area for absorption). supply, rapid healing. Sites: bones of the skeleton
6. Ciliated—columnar cells with cilia on free surfaces. (to support the body and protect internal organs
Sites: trachea (to sweep mucus and bacteria to the from mechanical injury).
pharynx); fallopian tubes (to sweep ovum to 7. Cartilage—cells are chondrocytes; protein matrix is
uterus). firm yet flexible; no capillaries in matrix, very slow
7. Glands—epithelial tissues that produce secretions. healing. Sites: joint surfaces of bones (to prevent
• Unicellular—one-celled glands. Goblet cells friction); tip of nose and external ear (to support);
secrete mucus in the respiratory and digestive wall of trachea (to keep air passage open); discs
tracts. between vertebrae (to absorb shock).
• Multicellular—many-celled glands.
• Exocrine glands have ducts; salivary glands Muscle Tissue—specialized to contract and
secrete saliva into ducts that carry it to the oral bring about movement (see Table 4–3 and
cavity. Fig. 4–6)
• Endocrine glands secrete hormones directly 1. Skeletal—also called striated or voluntary muscle.
into capillaries (no ducts); thyroid gland se- Cells are cylindrical, have several nuclei, and have
cretes thyroxine. striations. Each cell has a motor nerve ending;
nerve impulses are essential to cause contraction.
Connective Tissue—all have a non-living Site: skeletal muscles attached to bones (to move
intercellular matrix and specialized cells (see the skeleton and produce heat).
Table 4–2 and Figs. 4–4 and 4–5) 2. Smooth—also called visceral or involuntary mus-
1. Blood—the matrix is plasma, mostly water; trans- cle. Cells have tapered ends, one nucleus each, and
ports materials in the blood. Red blood cells carry no striations. Contraction is not under voluntary
oxygen; white blood cells destroy pathogens and control. Sites: stomach and intestines (peristalsis);
provide immunity; platelets prevent blood loss, as walls of arteries and veins (to maintain blood pres-
in clotting. Blood cells are made in red bone mar- sure); iris (to constrict or dilate pupil).
row. 3. Cardiac—cells are branched, have one nucleus
2. Areolar (loose)—cells are fibroblasts, which pro- each, and faint striations. Site: walls of the four
duce protein fibers: collagen is strong, elastin is chambers of the heart (to pump blood; nerve
elastic; the matrix is collagen, elastin, and tissue impulses regulate the rate of contraction).
fluid. White blood cells and mast cells are also
present. Sites: below the dermis and below the Nerve Tissue—neurons are specialized to
epithelium of tracts that open to the environment generate and transmit impulses (see Table
(to destroy pathogens that enter the body). 4–4 and Fig. 4–7)
3. Adipose—cells are adipocytes that store fat; little 1. Cell body contains the nucleus; axon carries
matrix. Sites: between the skin and muscles (to impulses away from the cell body; dendrites carry
store energy); around the eyes and kidneys (to impulses toward the cell body.
cushion). Also involved in appetite, use of insulin, 2. A synapse is the space between two neurons; a neu-
and inflammation. rotransmitter carries the impulse across a synapse.

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86 Tissues and Membranes


3. Specialized cells in nerve tissue are neuroglia in the • Pericardial sac—parietal pericardium lines
CNS and Schwann cells in the PNS. fibrous pericardium; visceral pericardium (epi-
4. Sites: brain; spinal cord; and peripheral nerves (to cardium) covers the heart muscle.
provide sensation, movement, regulation of body • Abdominal cavity—peritoneum lines the
functions, learning, and memory). abdominal cavity; mesentery covers the
abdominal organs.
Membranes—sheets of tissue on surfaces, or • Mucous membranes—line body tracts that open
separating organs or lobes to the environment: respiratory, digestive, uri-
1. Epithelial membranes (see Fig. 4–8) nary, and reproductive. Mucus keeps the living
• Serous membranes—in closed body cavities; the epithelium wet; provides lubrication in the diges-
serous fluid prevents friction between the two tive tract; traps dust and bacteria in the respira-
layers of the serous membrane. tory tract.
• Thoracic cavity—partial pleura lines chest 2. Connective tissue membranes—see Table 4–5.
wall; visceral pleura covers the lungs.




REVIEW QUESTIONS

1. Explain the importance of each tissue in its loca- a. Peritoneum
tion: (pp. 70, 73, 79) b. Visceral pericardium
a. Simple squamous epithelium in the alveoli of c. Parietal pleura
the lungs
8. State the function of: (pp. 74, 82, 84)
b. Ciliated epithelium in the trachea
a. Serous fluid
c. Cartilage in the trachea
b. Mucus
2. Explain the importance of each tissue in its loca- c. Blood plasma
tion: (pp. 77, 79)
a. Bone tissue in bones 9. State two functions of skeletal muscles. (p. 79)
b. Cartilage on the joint surfaces of bones
c. Fibrous connective tissue in ligaments 10. Name three body tracts lined with mucous mem-
branes. (p. 82)
3. State the functions of red blood cells, white blood
cells, and platelets. (p. 75) 11. Explain how endocrine glands differ from exocrine
glands. (pp. 73–74)
4. Name two organs made primarily of nerve tissue,
and state the general functions of nerve tissue. 12. State the function of adipose tissue: (p. 76)
(p. 81) a. Around the eyes
b. Between the skin and muscles
5. State the location and function of cardiac muscle.
(p. 81) 13. State the location of: (p. 84)
6. Explain the importance of each of these tissues in a. Meninges
the small intestine: smooth muscle and columnar b. Synovial membranes
epithelium. (pp. 72, 79)
14. State the important physical characteristics of col-
7. State the precise location of each of the following lagen and elastin, and name the cells that produce
membranes: (p. 82) these protein fibers (p. 75)

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Tissues and Membranes 87


FOR FURTHER THOUGHT

1. A friend suffers a knee injury involving damage to 3. Many tissues have protective functions, but it is
bone, cartilage, and ligaments. What can you tell important to be specific about the kind of protec-
your friend about the healing of these tissues? tion provided. Name at least three tissues with a
protective function, and state what each protects
2. Stratified squamous keratinizing epithelium is an the body (or parts of the body) against.
excellent barrier to pathogens in the epidermis of
the skin. Despite the fact that it is such a good bar- 4. Why is blood classified as a connective tissue?
rier, this tissue would not be suitable for the lining What does it connect? What kinds of connections
of the trachea or small intestine. Explain why. does it make?

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CHAPTER 5





Chapter Outline Student Objectives
The Skin • Name the two major layers of the skin and the
Epidermis tissue of which each is made.
Stratum germinativum • State the locations and describe the functions of
Stratum corneum the stratum germinativum and stratum corneum.
Langerhans cells • Describe the function of Langerhans cells.
Melanocytes • Describe the function of melanocytes and melanin.
Dermis • Describe the functions of hair and nails.
Hair follicles • Name the cutaneous senses and explain their
Nail follicles importance.
Receptors • Describe the functions of the secretions of seba-
Glands ceous glands, ceruminous glands, and eccrine
Blood vessels sweat glands.
Subcutaneous Tissue • Describe how the arterioles in the dermis respond
Aging and the Integumentary System to heat, cold, and stress.
• Name the tissues that make up the subcutaneous
BOX 5–1 BURNS tissue, and describe their functions.
BOX 5–2 PREVENTING SKIN CANCER: COMMON
SENSE AND SUNSCREENS
BOX 5–3 COMMON SKIN DISORDERS
BOX 5–4 ADMINISTERING MEDICATIONS



















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The Integumentary System






New Terminology Related Clinical Terminology
Arterioles (ar-TEER-ee-ohls) Acne (AK-nee)
Ceruminous gland/Cerumen (suh-ROO-mi-nus Alopecia (AL-oh-PEE-she-ah)
GLAND/suh-ROO-men) Biopsy (BYE-op-see)
Dermis (DER-miss) Carcinoma (KAR-sin-OH-mah)
Eccrine sweat gland (EK-rin SWET GLAND) Circulatory shock (SIR-kew-lah-TOR-ee SHAHK)
Epidermis (EP-i-DER-miss) Contusion (kon-TOO-zhun)
Hair follicle (HAIR FAH-li-kull) Decubitus ulcer (dee-KEW-bi-tuss UL-ser)
Keratin (KER-uh-tin) Dehydration (DEE-high-DRAY-shun)
Melanin (MEL-uh-nin) Dermatology (DER-muh-TAH-luh-gee)
Melanocyte (muh-LAN-oh-sight) Eczema (EK-zuh-mah)
Nail follicle (NAIL FAH-li-kull) Erythema (ER-i-THEE-mah)
Papillary layer (PAP-i-LAR-ee LAY-er) Histamine (HISS-tah-meen)
Receptors (ree-SEP-turs) Hives (HIGH-VZ)
Sebaceous gland/Sebum (suh-BAY-shus Inflammation (IN-fluh-MAY-shun)
GLAND/SEE-bum) Melanoma (MEL-ah-NOH-mah)
Stratum corneum (STRA-tum KOR-nee-um) Nevus (NEE-vus)
Stratum germinativum (STRA-tum JER-min-ah- Pruritus (proo-RYE-tus)
TEE-vum)
Subcutaneous tissue (SUB-kew-TAY-nee-us TISH-
yoo)
Vasoconstriction (VAY-zoh-kon-STRIK-shun)
Vasodilation (VAY-zoh-dye-LAY-shun)



Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.











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90 The Integumentary System




The integumentary system consists of the skin, its The living keratinocytes are able to synthesize
accessory structures such as hair and sweat glands, and antimicrobial peptides called defensins; these and
the subcutaneous tissue below the skin. The skin is other chemicals are produced following any injury to
made of several different tissue types and is considered the skin, as part of the process of inflammation.
an organ. Because the skin covers the surface of the Defensins rupture the membranes of pathogens such
body, one of its functions is readily apparent: It sepa- as bacteria that may enter by way of breaks in the skin.
rates the internal environment of the body from the The living portion of the epidermis also produces a
external environment and prevents the entry of many vitamin; the cells have a form of cholesterol that, on
harmful substances. The subcutaneous tissue directly exposure to ultraviolet light, is changed to vitamin D
underneath the skin connects it to the muscles and has (then modified by the liver and kidneys to the most
other functions as well. active form, called 1,25-D, or calcitriol, which is con-
sidered a hormone). This is why vitamin D is some-
times referred to as the “sunshine vitamin.” People
THE SKIN who do not get much sunlight depend more on nutri-
tional sources of vitamin D, such as fortified milk. But
The two major layers of the skin are the outer sunlight is probably the best way to get vitamin D, and
epidermis and the inner dermis. Each of these layers 15 minutes a day a few times a week is often enough.
is made of different tissues and has very different func- Vitamin D is important for the absorption of calcium
tions. and phosphorus from food in the small intestine; this
function has been known for years. Recent research,
EPIDERMIS however, suggests that vitamin D is also involved in
maintaining muscle strength, especially in elderly peo-
The epidermis is made of stratified squamous kera- ple, in the functioning of insulin, and in some immune
tinizing epithelial tissue and is thickest on the palms responses, where it may be protective for some types
and soles. The cells that are most abundant are called of cancer.
keratinocytes, and there are no capillaries present
between them. Although the epidermis may be further Stratum Corneum
subdivided into four or five sublayers, two of these are
of greatest importance: the innermost layer, the stra- The stratum corneum, the outermost epidermal
tum germinativum, and the outermost layer, the stra- layer, consists of many layers of dead cells; all that is
tum corneum (Fig. 5–1). 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
Stratum Germinativum
does prevent most evaporation of body water. Also of
The stratum germinativum may also be called the importance, keratin prevents the entry of water. With-
stratum basale. Each name tells us something about out a waterproof stratum corneum, it would be impos-
this layer. To germinate means “to sprout” or “to sible to swim in a pool or even take a shower without
grow.” Basal means the “base” or “lowest part.” The damaging our cells.
stratum germinativum is the base of the epidermis, the The stratum corneum is also a barrier to pathogens
innermost layer in which mitosis takes place. New and chemicals. Most bacteria and other microorgan-
cells are continually being produced, pushing the isms cannot penetrate unbroken skin. The flaking of
older cells toward the skin surface. These cells pro- dead cells from the skin surface helps remove micro-
duce the protein keratin, and as they get farther away organisms, and the fatty acids in sebum help inhibit
from the capillaries in the dermis, they die. As dead their growth. Most chemicals, unless they are corro-
cells are worn off the skin’s surface, they are replaced sive, will not get through unbroken skin to the living
by cells from the lower layers. Scattered among the tissue within. One painful exception is the sap of poi-
keratinocytes of the stratum germinativum are very son ivy. This resin does penetrate the skin and initiates
different cells called Merkel cells (or Merkel discs); an allergic reaction in susceptible people. The inflam-
these are receptors for the sense of touch (Fig. 5–2). matory response that characterizes allergies causes

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The Integumentary System 91















Stratum germinativum
Pore

Stratum corneum

Epidermis
Papillary layer
with capillaries
Sebaceous gland

Dermis
Receptor for touch
(encapsulated) Pilomotor
muscle
Hair follicle


Receptor for pressure Subcutaneous
(encapsulated) tissue



Fascia of
muscle
Adipose tissue
Nerve Eccrine sweat gland
Free nerve ending
Arteriole
Venule

Figure 5–1. Skin. Structure of the skin and subcutaneous tissue.
QUESTION: Which layers of the integumentary system have blood vessels?



blisters and severe itching. The importance of the dermis, or separate the epidermis from the dermis,
stratum corneum becomes especially apparent when it and tissue fluid may collect, causing a blister. If the
is lost (see Box 5–1: Burns). skin is subjected to pressure, the rate of mitosis in the
Certain minor changes in the epidermis are stratum germinativum will increase and create a
undoubtedly familiar to you. When first wearing new thicker epidermis; we call this a callus. Although cal-
shoes, for example, the skin of the foot may be sub- luses are more common on the palms and soles, they
jected to friction. This will separate layers of the epi- may occur on any part of the skin.

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Melanocyte

Stratum corneum
Merkel cell
Langerhans cell








Figure 5–2. The epidermis, show-
ing the different kinds of cells pres-
ent and the blood supply in the
upper dermis.
QUESTION: Which type of cell
shown is capable of self-locomotion,
and what does it carry?







Dermis
Mitosis
Capillary
Sensory neuron
Stratum germinativum


BOX 5–1 BURNS
Burns of the skin may be caused by flames, hot Extensive third-degree burns are potentially life-
water or steam, sunlight, electricity, or corrosive threatening because of the loss of the stratum
chemicals. The severity of burns ranges from minor corneum. Without this natural barrier, living tissue is
to fatal, and the classification of burns is based on exposed to the environment and is susceptible to
the extent of damage. infection and dehydration.
Bacterial infection is a serious problem for burn
First-Degree Burn—only the superficial epidermis patients; the pathogens may get into the blood
is burned, and is painful but not blistered. Light- (septicemia) and quickly spread throughout
colored skin will appear red due to localized the body. Dehydration may also be fatal if medical
vasodilation in the damaged area. Vasodilation is intervention does not interrupt and correct the
part of the inflammatory response that brings more following sequence: Tissue fluid evaporates from
blood to the injured site. the burned surface, and more plasma is pulled
Second-Degree Burn—deeper layers of the epi- out of capillaries into the tissue spaces. As more
dermis are affected. Another aspect of inflam- plasma is lost, blood volume and blood pressure
mation is that damaged cells release histamine, decrease. This is called circulatory shock; eventu-
which makes capillaries more permeable. More ally the heart simply does not have enough blood
plasma leaves these capillaries and becomes tissue to pump, and heart failure is the cause of death. To
fluid, which collects at the burn site, creating blis- prevent these serious consequences, third-degree
ters. The burned skin is often very painful. burns are covered with donor skin or artificial skin
Third-Degree Burn—the entire epidermis is until skin grafts of the patient’s own skin can be put
charred or burned away, and the burn may extend in place.
into the dermis or subcutaneous tissue. Often such
a burn is not painful at first, if the receptors in the
dermis have been destroyed.


(Continued on following page)
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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, second-
degree, and third-degree burns.



Langerhans Cells production of antibodies (antibodies are proteins that
label foreign material for destruction). Because of its
Within the epidermis are Langerhans cells, which position adjacent to the external environment, the skin
are also called dendritic cells because of their is an important component of the body’s protective
branched appearance when they move (see Fig. 5–2). responses, though many of the exact aspects of this
These cells originate in the red bone marrow, and are have yet to be determined. Immunity is covered in
quite mobile. They are able to phagocytize foreign Chapter 14.
material, such as bacteria that enter the body through
breaks in the skin. With such ingested pathogens, the Melanocytes
Langerhans cells migrate to lymph nodes and present
the pathogen to lymphocytes, a type of white blood Another type of cell found in the lower epidermis is
cell. This triggers an immune response such as the the melanocyte, which is also shown in Fig. 5–2.

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94 The Integumentary System


Melanocytes produce another protein, a pigment cells as they are pushed toward the surface. This gives
called melanin. People of the same size have approxi- the skin a darker color, which prevents further expo-
mately the same number of melanocytes, though these sure of the living stratum germinativum to ultraviolet
cells may differ in their level of activity. In people with rays. People with dark skin already have good protec-
dark skin, the melanocytes continuously produce large tion against the damaging effects of ultraviolet rays;
amounts of melanin. The melanocytes of light- people with light skin do not (see Box 5–2: Preventing
skinned people produce less melanin. The activity of Skin Cancer: Common Sense and Sunscreens).
melanocytes is genetically regulated; skin color is one Melanin also gives color to hair, though its protective
of our hereditary characteristics. function is confined to the hair of the head. Two parts
In all people, melanin production is increased by of the eye obtain their color from melanin: the iris and
exposure of the skin to ultraviolet rays, which are part the interior choroid layer of the eyeball (the eye is dis-
of sunlight and are damaging to living cells. As more cussed in Chapter 9).The functions of the epidermis
melanin is produced, it is taken in by the epidermal and its cells are summarized in Table 5–1.



BOX 5–2 PREVENTING SKIN CANCER: COMMON SENSE AND SUNSCREENS

Anyone can get skin cancer, and the most impor- strongly enough to prevent a second case, for such
tant factor is exposure to sunlight. Light-skinned recurrences are often fatal.
people are, of course, more susceptible to the Although the most common forms of skin cancer
effects of ultraviolet (UV) rays, which may trigger are readily curable, prevention is a better strategy.
mutations in living epidermal cells. We cannot, and we would not want to, stay out of
Squamous cell carcinoma and basal cell carci- the sun altogether (because sunlight may be the
noma (see A in Box Fig. 5–B) are the most common best way to get sufficient vitamin D), but we may
forms of skin cancer. The lesions are visible as be able to do so when sunlight is most damaging.
changes in the normal appearance of the skin, and During the summer months, UV rays are especially
a biopsy (microscopic examination of a tissue spec- intense between 10 A.M. and 2 P.M. If we are or
imen) is used to confirm the diagnosis. These lesions must be outdoors during this time, dermatologists
usually do not metastasize rapidly, and can be recommend use of a sunscreen.
completely removed using simple procedures Sunscreens contain chemicals such as PABA
Malignant melanoma (see B in Box Fig. 5–B) is (para-amino benzoic acid) that block UV rays and
a more serious form of skin cancer, which begins in prevent them from damaging the epidermis. An
melanocytes. Any change in a pigmented spot or SPF (sun protection factor) of 15 or higher is con-
mole (nevus) should prompt a person to see a doc- sidered good protection. Use of a sunscreen on
tor. Melanoma is serious not because of its growth exposed skin not only helps prevent skin cancer but
in the skin, but because it may metastasize very also prevents sunburn and its painful effects. It is
rapidly to the lungs, liver, or other vital organ. especially important to prevent children from get-
Researchers are testing individualized vaccines for ting severely sunburned, because such burns have
people who have had melanoma. The purpose of been linked to the development of skin cancer years
the vaccine is to stimulate the immune system later.











A B
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.)

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The Integumentary System 95


epidermis. At the base of a follicle is the hair root,
Table 5–1 EPIDERMIS
which contains cells called the matrix, where mitosis
Part Function takes place (Fig. 5–3). The new cells produce keratin,
get their color from melanin, then die and become
Stratum corneum • Prevents loss or entry of water incorporated into the hair shaft, which is pushed
(keratin) • If unbroken, prevents entry of toward the surface of the skin. The hair that we comb
pathogens and most chemi-
cals and brush every day consists of dead, keratinized cells.
The rate of hair growth averages 0.3 to 0.4 in./month
Stratum germina- • Continuous mitosis produces
tivum (stratum new cells to replace worn-off (8 to 10 mm).
basale) surface cells Compared to some other mammals, humans do not
• Produces antimicrobial have very much hair. The actual functions of human
defensins hair are quite few. Eyelashes and eyebrows help to
• Cholesterol is changed to vita- keep dust and perspiration out of the eyes, and the
min D on exposure to UV rays
hairs just inside the nostrils help to keep dust out of
Langerhans cells • Phagocytize foreign material the nasal cavities. Hair of the scalp does provide insu-
and stimulate an immune
response by lymphocytes lation from cold for the head. The hair on our bodies,
however, no longer serves this function, but we have
Merkel cells • Receptors for sense of touch
the evolutionary remnants of it. Attached to each hair
Melanocytes • Produce melanin on exposure follicle is a small, smooth muscle called the pilomotor
to UV rays
Melanin • Protects living skin layers from
further exposure to UV rays


DERMIS
Hair shaft
The dermis is made of an irregular type of fibrous
connective tissue, irregular meaning that the fibers are Follicle
not parallel, but run in all directions. Fibroblasts pro-
duce both collagen and elastin fibers. Recall that col-
lagen 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 epider-
mis is called the papillary layer (see Fig. 5–1). Capil-
B
laries are abundant here to nourish not only the
Hair root
dermis but also the stratum germinativum. The epi-
dermis has no capillaries of its own, and the lower, liv-
ing cells depend on the blood supply in the dermis for
oxygen and nutrients. Capillary
Within the dermis are the accessory skin structures: Venule
hair and nail follicles, sensory receptors, and several
types of glands. Some of these project through the
Fat cells
epidermis to the skin surface, but their active portions
are in the dermis.
A
Hair Follicles
Figure 5–3. Structure of a hair follicle. (A) Longitudinal
Hair follicles are made of epidermal tissue, and the section. (B) Cross-section.
growth process of hair is very similar to growth of the QUESTION: What is the hair shaft made of?

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96 The Integumentary System


or arrector pili muscle. When stimulated by cold or Receptors
emotions such as fear, these muscles pull the hair fol-
licles upright. For an animal with fur, this would trap Most sensory receptors for the cutaneous senses
air and provide greater insulation. Because people do are found in the dermis (Merkel cells are in the stra-
not have thick fur, all this does for us is give us “goose tum germinativum, as are some nerve endings). The
bumps.” 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 partic-
Nail Follicles
ular change. For pain, heat, and cold, the receptors are
Found on the ends of fingers and toes, nail follicles free nerve endings. For touch and pressure, the
produce nails just as hair follicles produce hair. Mitosis receptors are called encapsulated nerve endings,
takes place in the nail root at the base of the nail (Fig. which means there is a cellular structure around the
5–4), and the new cells produce keratin (a stronger sensory nerve ending (see Fig. 5–1). The purpose of
form of this protein than is found in hair) and then die. these receptors and sensations is to provide the central
Although the nail itself consists of keratinized dead nervous system with information about the external
cells, the flat nail bed is living epidermis and dermis. environment and its effect on the skin. This informa-
This is why cutting a nail too short can be quite tion may stimulate responses, such as washing a
painful. Nails protect the ends of the fingers and toes painful cut finger, scratching an insect bite, or
from mechanical injury and give the fingers greater responding to a feeling of cold by putting on a sweater.
ability to pick up small objects. Fingernails are also The sensitivity of an area of skin is determined by
good for scratching. This is more important than it how many receptors are present. The skin of the fin-
may seem at first. An itch might mean the presence of gertips, for example, is very sensitive to touch because
an arthropod parasite, mosquito, tick, flea, or louse. there are many receptors per square inch. The skin of
These parasites (all but the tick are insects) feed on the upper arm, with few touch receptors per square
blood, and all are potential vectors of diseases caused inch, is less sensitive.
by bacteria, viruses, or protozoa. A quick and vigorous When receptors detect changes, they generate
scratch may kill or at least dislodge the arthropod and nerve impulses that are carried to the brain, which
prevent the transmission of the disease. Fingernails interprets the impulses as a particular sensation.
grow at the rate of about 0.12 in./month (3 mm), and Sensation, therefore, is actually a function of the brain
growth is a little faster during the summer months. (we will return to this in Chapters 8 and 9).

Glands
Glands are made of epithelial tissue. The exocrine
Free edge of nail
glands of the skin have their secretory portions in the
Nail body Lunula 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 sur-
face. Another function of sebum is to prevent drying of
skin and hair. The importance of this may not be read-
ily apparent, but skin that is dry tends to crack more
Nail bed Cuticle easily. Even very small breaks in the skin are potential
Nail root entryways for bacteria. Decreased sebum production is
another consequence of getting older, and elderly peo-
Figure 5–4. Structure of a fingernail shown in longitu- ple often have dry and more fragile skin.
dinal section. Adolescents may have the problem of overactive
QUESTION: The nail bed is which part of the skin? sebaceous glands. Too much sebum may trap bacteria

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The Integumentary System 97


within hair follicles and create small infections. Ceruminous Glands. These glands are located in the
Because sebaceous glands are more numerous around dermis of the ear canals. Their secretion is called
the nose and mouth, these are common sites of pim- cerumen or ear wax (which includes the sebum
ples in young people (see also Box 5–3: Common Skin secreted in the ear canals). Cerumen keeps the outer
Disorders). surface of the eardrum pliable and prevents drying.



BOX 5–3 COMMON SKIN DISORDERS
Impetigo—a bacterial infection often caused by skin that is often raised and has a rough or pitted
streptococci or staphylococci. The characteristic surface. Warts are probably most common on the
pustules (pus-containing lesions) crust as they heal; hands, but they may be anywhere on the skin.
the infection is contagious to others. Plantar warts on the sole of the foot may become
Eczema—a symptom of what is more properly quite painful because of the constant pressure of
called atopic dermatitis. This may be an allergic standing and walking.
reaction, and is more common in children than Fever blisters (cold sores)—caused by the herpes
adults; the rash is itchy (pruritus) and may blister simplex virus, to which most people are exposed
or ooze. Eczema may be related to foods such as as children. An active lesion, usually at the edge
fish, eggs, or milk products, or to inhaled allergens of the lip (but may be anywhere on the skin), is
such as dust, pollens, or animal dander. Prevention painful and oozes. If not destroyed by the immune
depends upon determining what the child is aller- system, the virus “hides out” and becomes dor-
gic to and eliminating or at least limiting exposure. mant in nerves of the face. Another lesion, weeks
Warts—caused by a virus that makes epidermal or months later, may be triggered by stress or
cells divide abnormally, producing a growth on the 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 per-
mission.)

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98 The Integumentary System


However, if excess cerumen accumulates in the ear in their walls permits them to constrict (close) or
canal, it may become impacted against the eardrum. dilate (open). This is important in the maintenance of
This might diminish the acuity of hearing by prevent- body temperature, because blood carries heat, which is
ing the eardrum from vibrating properly. a form of energy.
In a warm environment the arterioles dilate
Sweat Glands. There are two types of sweat glands, (vasodilation), which increases blood flow through the
apocrine and eccrine. Apocrine glands are most dermis and brings excess heat close to the body surface
numerous in the axillae (underarm) and genital areas to be radiated to the environment. In a cold environ-
and are most active in stressful and emotional situa- ment, however, body heat must be conserved if possi-
tions. Although their secretion does have an odor, it is ble, so the arterioles constrict. The vasoconstriction
barely perceptible to other people. Animals such as decreases the flow of blood through the dermis and
dogs, however, can easily distinguish among people keeps heat within the core of the body. This adjusting
because of their individual scents. If the apocrine mechanism is essential for maintaining homeostasis.
secretions are allowed to accumulate on the skin, bac- Regulation of the diameter of the arterioles in
teria metabolize the chemicals in the sweat and pro- response to external temperature changes is controlled
duce waste products that have distinct odors that by the nervous system. These changes can often be
many people find unpleasant. seen in light-skinned people. Flushing, especially in
Eccrine glands are found all over the body but are
especially numerous on the forehead, upper lip, palms, the face, may be observed in hot weather. In cold, the
skin of the extremities may become even paler as blood
and soles. The secretory portion of these glands is flow through the dermis decreases. In people with dark
simply a coiled tube in the dermis. The duct of this skin, such changes are not as readily apparent because
tube extends to the skin’s surface, where it opens into they are masked by melanin in the epidermis.
a pore. Vasoconstriction in the dermis may also occur dur-
The sweat produced by eccrine glands is important
in the maintenance of normal body temperature. In a ing stressful situations. For our ancestors, stress usu-
ally demanded a physical response: Either stand and
warm environment, or during exercise, more sweat is fight or run away to safety. This is called the “fight or
secreted onto the skin surface, where it is then evapo- flight response.” Our nervous systems are still pro-
rated by excess body heat. Recall that water has a high grammed to respond as if physical activity were neces-
heat of vaporization, which means that a great deal of sary to cope with the stress situation. Vasoconstriction
heat can be lost in the evaporation of a relatively small in the dermis will shunt, or redirect, blood to more
amount of water. Although this is a very effective vital organs such as the muscles, heart, and brain. In
mechanism of heat loss, it has a potentially serious dis- times of stress, the skin is a relatively unimportant
advantage. Loss of too much body water in sweat may organ and can function temporarily with a minimal
lead to dehydration, as in heat exhaustion or even blood flow. You have probably heard the expression
after exercise on a hot and humid day. Increased “broke out in a cold sweat,” and may even have felt it
sweating during exercise or on warm days should in a stressful situation. Such sweating feels cold
always be accompanied by increased fluid intake. because vasoconstriction in the dermis makes the skin
Those who exercise regularly know that they must
replace salt as well as water. Sodium chloride is also relatively cool.
Blood flow in the dermis may be interrupted by
lost in sweat, as are small amounts of urea (a nitroge- prolonged pressure on the skin. For example, a hospi-
nous waste product of amino acid metabolism). This tal patient who cannot turn over by herself may
excretory function of the skin is very minor, however; develop a decubitus ulcer, also called a pressure ulcer
the kidneys are primarily responsible for removing or pressure sore. The skin is compressed between the
waste products from the blood and for maintaining object outside, such as a bed, and a bony prominence
the body’s proper salt-to-water proportion.
within, such as the heel bone or the sacrum in the
lower back. Without its blood supply the skin dies,
Blood Vessels
and the dead tissue is a potential site for bacterial
Besides the capillaries in the dermis, the other blood infection.
vessels of great importance are the arterioles. The functions of dermal structures are summarized
Arterioles are small arteries, and the smooth muscle in Table 5–2.

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The Integumentary System 99


tion, the body’s response to damage (inflammation is
Table 5–2 DERMIS
described in Chapters 10 and 14).
Part Function The cells (adipocytes) of adipose tissue are special-
ized to store fat, and our subcutaneous layer of fat
Papillary layer • Contains capillaries that nourish stores excess nutrients as a potential energy source.
the stratum germinativum
This layer also cushions bony prominences, such as
Hair (follicles) • Eyelashes and nasal hair keep when sitting, and provides some insulation from cold.
dust out of eyes and nasal For people, this last function is relatively minor,
cavities
• Scalp hair provides insulation because we do not have a thick layer of fat, as do ani-
from cold for the head mals such as whales and seals. As mentioned in
Nails (follicles) • Protect ends of fingers and Chapter 4, adipose tissue is involved in the onset or
toes from mechanical injury cessation of eating and in the use of insulin by body
Receptors • Detect changes that are felt as cells, and it contributes to inflammation by produc-
the cutaneous senses: touch, ing cytokines, chemicals that activate white blood
pressure, heat, cold, and pain cells.
Sebaceous glands • Produce sebum, which prevents Just as the epidermis forms a continuous sheet that
drying of skin and hair and covers the body, the subcutaneous tissue is a continu-
inhibits growth of bacteria ous layer, though it is internal. If we consider the epi-
Ceruminous • Produce cerumen, which pre- dermis as the first line of defense against pathogens,
glands vents drying of the eardrum we can consider the subcutaneous tissue a secondary
Eccrine sweat • Produce watery sweat that is line of defense. There is, however, a significant
glands evaporated by excess body heat anatomic difference. The cells of the epidermis are
to cool the body
very closely and tightly packed, but the cells and
Arterioles • Dilate in response to warmth protein fibers of subcutaneous tissue are farther apart,
to increase heat loss
• Constrict in response to cold and there is much more tissue fluid. If we imagine the
to conserve body heat epidermis as a four-lane highway during rush hour
• Constrict in stressful situations with bumper-to-bumper traffic, the superficial fascia
to shunt blood to more vital would be that same highway at three o’clock in the
organs morning, when it is not crowded and cars can move
much faster. This is an obvious benefit for the migrat-
ing white blood cells, but may become a disadvantage
because some bacterial pathogens, once established,
can spread even more rapidly throughout subcuta-
SUBCUTANEOUS TISSUE
neous tissue.
Group A streptococcus, for example, is a cause of
The subcutaneous tissue may also be called the necrotizing fasciitis. Necrotizing means “to cause
superficial fascia, one of the connective tissue mem- death,” and fasciitis is the inflammation of a fascia, in
branes. Made of areolar connective tissue and adipose this case the superficial fascia and the deep fasciae
tissue, the superficial fascia connects the dermis to the around muscles. Necrotizing fasciitis is an extremely
underlying muscles. Its other functions are those of its serious infection and requires surgical removal of the
tissues, as you may recall from Chapter 4. infected tissue, or even amputation of an infected
Areolar connective tissue, or loose connective tis- limb, to try to stop the spread of the bacteria. The
sue, contains collagen and elastin fibers and many body’s defenses have been overwhelmed because what
white blood cells that have left capillaries to wander is usually an anatomic benefit, the “looseness” of are-
around in the tissue fluid between skin and muscles. olar connective tissue, has been turned against us and
These migrating white blood cells destroy pathogens become a detriment.
that enter the body through breaks in the skin. Mast The functions of subcutaneous tissue are summa-
cells are specialized connective tissue cells found in rized in Table 5–3. Box 5–4: Administering Medica-
areolar tissue; they produce histamine, leukotrienes, tions, describes ways in which we give medications
and other chemicals that help bring about inflamma- through the skin.

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100 The Integumentary System



Table 5–3 SUBCUTANEOUS TISSUE
Part Function
Areolar connective 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
Adipose tissue • 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 adver- is the skin behind the ear. Also available in patch
tisements for skin patches that supply nicotine, and form are medications for birth control, overactive
you know that their purpose is to help smokers give bladder, high blood pressure, and both systemic
up cigarettes. This method of supplying a medica- and localized pain relief.
tion is called transdermal administration. The name Medications may also be injected through the
is a little misleading because the most difficult part skin. Some injections are given subcutaneously, that
of cutaneous absorption of a drug is absorption is, into subcutaneous tissue (see Box Fig. 5–D).
through the stratum corneum of the epidermis. Subcutaneous adipose tissue has a moderate blood
Because such absorption is slow, skin patches are supply, so the rate of absorption of the drug will be
useful for medications needed in small but continu- moderate, but predictable. Insulin is given subcuta-
ous amounts, and over a prolonged period of time. neously. Other injections are intramuscular, and
You would expect that such patches should be absorption into the blood is rapid because muscle
worn where the epidermis is thin. These sites tissue has a very good blood supply. Most injectable
include the upper arm and the chest. The recom- vaccines are given intramuscularly, to promote
mended site for a patch to prevent motion sickness rapid absorption to stimulate antibody production.






Patch
Stratum
corneum

Dermis

Subcutaneous Adipose
tissue


Intramuscular

Skeletal
muscle


Box Figure 5–D The skin, subcutaneous tissue, and muscle sites for the delivery of
medications.

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The Integumentary System 101


harmful and to take special precautions in very hot or
AGING AND THE very cold weather.
INTEGUMENTARY SYSTEM

The effects of age on the integumentary system are
often quite visible. Both layers of skin become thinner SUMMARY
and more fragile as mitosis in the epidermis slows and
fibroblasts in the dermis die and are not replaced; The integumentary system is the outermost organ
repair of even small breaks or cuts is slower. The skin system of the body. You have probably noticed that
becomes wrinkled as collagen and elastin fibers in the many of its functions are related to this location. The
dermis deteriorate. Sebaceous glands and sweat glands skin protects the body against pathogens and chemi-
become less active; the skin becomes dry, and temper- cals, minimizes loss or entry of water, blocks the
ature regulation in hot weather becomes more diffi- harmful effects of sunlight, and produces vitamin D.
cult. Hair follicles become inactive and hair on the Sensory receptors in the skin provide information
scalp and body thins. Melanocytes die and are not about the external environment, and the skin helps
replaced; the hair that remains becomes white. There regulate body temperature in response to environ-
is often less fat in the subcutaneous tissue, which may mental changes. The subcutaneous tissue is a second-
make an elderly person more sensitive to cold. It is ary line of defense against pathogens, a site of fat
important for elderly people (and those who care for storage and of the other metabolic functions of adi-
them) to realize that extremes of temperature may be pose tissue.



STUDY OUTLINE


The integumentary system consists of the Dermis—made of irregular fibrous connec-
skin and its accessory structures and the tive tissue; collagen provides strength, and
subcutaneous tissue. The two major layers of elastin provides elasticity; capillaries in the
the skin are the outer epidermis and the papillary layer nourish the stratum germina-
inner dermis. tivum (see Fig. 5–1 and Table 5–2)
1. Hair follicles—mitosis takes place in the hair root;
Epidermis—made of stratified squamous new cells produce keratin, die, and become the hair
epithelium; no capillaries; cells called ker- shaft. Hair of the scalp provides insulation from
atinocytes (see Figs. 5–1 and 5–2 and Table cold for the head; eyelashes keep dust out of eyes;
5–1) nostril hairs keep dust out of nasal cavities (see
1. Stratum germinativum—the innermost layer Figs. 5–1 and 5–3).
where mitosis takes place; new cells produce ker- 2. Nail follicles—at the ends of fingers and toes; mito-
atin and die as they are pushed toward the surface. sis takes place in the nail root; the nail itself is dead,
Defensins are antimicrobial peptides produced keratinized cells. Nails protect the ends of the fin-
when the skin is injured. Vitamin D is formed from gers and toes, enable the fingers to pick up small
cholesterol on exposure to the UV rays of sunlight. objects, and provide for efficient scratching (see
2. Stratum corneum—the outermost layers of dead Fig. 5–4).
cells; keratin prevents loss and entry of water and 3. Receptors—detect changes in the skin: touch, pres-
resists entry of pathogens and chemicals. sure, heat, cold, and pain; provide information
3. Langerhans cells—phagocytize foreign material, about the external environment that initiates appro-
take it to lymph nodes, and stimulate an immune priate responses; sensitivity of the skin depends on
response by lymphocytes. the number of receptors present.
4. Melanocytes—in the lower epidermis, produce 4. Sebaceous glands—secrete sebum into hair follicles
melanin. UV rays stimulate melanin production; or to the skin surface; sebum inhibits the growth of
melanin prevents further exposure of the stratum bacteria and prevents drying of skin and hair.
germinativum to UV rays by darkening the 5. Ceruminous glands—secrete cerumen in the ear
skin. canals; cerumen prevents drying of the eardrum.

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102 The Integumentary System


6. Apocrine sweat glands—modified scent glands in organs, such as muscles, to permit a physical
axillae and genital area; activated by stress and response, if necessary.
emotions.
7. Eccrine sweat glands—most numerous on face, Subcutaneous Tissue—also called the superfi-
palms, soles. Activated by high external tempera- cial fascia; connects skin to muscles (see Fig.
ture or exercise; sweat on skin surface is evaporated 5–1 and Table 5–3)
by excess body heat; potential disadvantage is dehy- 1. Areolar tissue—also called loose connective tissue;
dration. Excretion of small amounts of NaCl and the matrix contains tissue fluid and WBCs that
urea is a very minor function. destroy pathogens that get through breaks in the
8. Arterioles—smooth muscle permits constriction skin; mast cells produce chemicals that bring about
or dilation. Vasoconstriction in cold temperatures inflammation.
decreases dermal blood flow to conserve heat in 2. Adipose tissue—stores fat as potential energy;
the body core. Vasodilation in warm temperatures cushions bony prominences; provides some insula-
increases dermal blood flow to bring heat to the tion from cold. Other functions: contributes to
surface to be lost. Vasoconstriction during stress appetite, the use of insulin, and the activation of
shunts blood away from the skin to more vital WBCs.




REVIEW QUESTIONS

1. Name the parts of the integumentary system. 7. Describe the functions of nails. (p. 96)
(p. 90)
8. Name the cutaneous senses. Describe the impor-
2. Name the two major layers of skin, the location tance of these senses. (p. 96)
of each, and the tissue of which each is made.
(pp. 90, 95) 9. Explain the functions of sebum and cerumen.
(pp. 96–97)
3. In the epidermis: (pp. 90, 93)
a. Where does mitosis take place? 10. Explain how sweating helps maintain normal
b. What protein do the new cells produce? body temperature. (p. 98)
c. What happens to these cells? 11. Explain how the arterioles in the dermis respond
d. What is the function of Langerhans cells? to cold or warm external temperatures and to
4. Describe the functions of the stratum corneum. stress situations. (p. 98)
(p. 90) 12. What vitamin is produced in the skin? What is the
5. Name the cells that produce melanin. What is stimulus for the production of this vitamin?
the stimulus? Describe the function of melanin (p. 90)
(p. 94)
13. Name the tissues of which the superficial fascia is
6. Where, on the body, does human hair have impor- made. Describe the functions of these tissues.
tant functions? Describe these functions. (p. 95) (p. 99)

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The Integumentary System 103


FOR FURTHER THOUGHT

1. The epidermis has no capillaries of its own; the scaly red patches, often circular or oval in shape. It
stratum corneum is made of dead cells and doesn’t is not caused by a worm, but rather by certain
need a blood supply at all. Explain why the epider- fungi. What is the food of these fungi; that is, what
mis is affected by a decubitus ulcer. Name another are they digesting?
group of people, besides hospital or nursing home
4. There are several kinds of cells in the epidermis.
patients, that is especially susceptible to developing
Which cells exert their functions when they are
pressure ulcers.
dead? Which cells must be living in order to func-
2. Going out in the sun stimulates quite a bit of activ- tion?
ity in the skin, especially on a hot summer day.
5. Wearing a hat in winter is a good idea. What hap-
Describe what is happening in the skin in response
pens to the arterioles in the dermis in a cold envi-
to sunlight.
ronment? How does this affect heat loss? Is the
3. Ringworm is a skin condition characterized by head an exception? Explain.

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CHAPTER 6





Chapter Outline Student Objectives
Functions of the Skeleton • Describe the functions of the skeleton.
Types of Bone Tissue • Explain how bones are classified, and give an
Classification of Bones example of each type.
Embryonic Growth of Bone • Describe how the embryonic skeleton model is
Factors That Affect Bone Growth and replaced by bone.
Maintenance • Name the nutrients necessary for bone growth,
The Skeleton and explain their functions.
Skull • Name the hormones involved in bone growth and
Vertebral Column maintenance, and explain their functions.
Rib Cage • Explain what is meant by “exercise” for bones, and
The Shoulder and Arm explain its importance.
The Hip and Leg • Name all the bones of the human skeleton (be able
Joints—Articulations to point to each on diagrams, skeleton models, or
The Classification of Joints yourself).
Synovial Joints • Describe the functions of the skull, vertebral col-
Aging and the Skeletal System umn, rib cage, scapula, and pelvic bone.
• Explain how joints are classified. For each type,
BOX 6–1 FRACTURES AND THEIR REPAIR give an example, and describe the movement pos-
BOX 6–2 OSTEOPOROSIS sible.
BOX 6–3 HERNIATED DISC • Describe the parts of a synovial joint, and explain
BOX 6–4 ABNORMALITIES OF THE CURVES their functions.
OF THE SPINE
BOX 6–5 ARTHRITIS














104

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The Skeletal System






New Terminology Related Clinical Terminology
Appendicular (AP-en-DIK-yoo-lar) Autoimmune disease
Articulation (ar-TIK-yoo-LAY-shun) (AW-toh-im-YOON di-ZEEZ)
Axial (AK-see-uhl) Bursitis (burr-SIGH-tiss)
Bursa (BURR-sah) Cleft palate (KLEFT PAL-uht)
Diaphysis (dye-AFF-i-sis) Fracture (FRAK-chur)
Epiphyseal disc (e-PIFF-i-SEE-al DISK) Herniated disc (HER-nee-ay-ted DISK)
Epiphysis (e-PIFF-i-sis) Kyphosis (kye-FOH-sis)
Fontanel (FON-tah-NELL) Lordosis (lor-DOH-sis)
Haversian system (ha-VER-zhun SIS-tem) Osteoarthritis (AHS-tee-oh-ar-THRY-tiss)
Ligament (LIG-uh-ment) Osteomyelitis (AHS-tee-oh-my-uh-LYE-tiss)
Ossification (AHS-i-fi-KAY-shun) Osteoporosis (AHS-tee-oh-por-OH-sis)
Osteoblast (AHS-tee-oh-BLAST) Rheumatoid arthritis
Osteoclast (AHS-tee-oh-KLAST) (ROO-muh-toyd ar-THRY-tiss)
Paranasal sinus (PAR-uh-NAY-zuhl SIGH-nus) Rickets (RIK-ets)
Periosteum (PER-ee-AHS-tee-um) Scoliosis (SKOH-lee-OH-sis)
Suture (SOO-cher)
Symphysis (SIM-fi-sis)
Synovial fluid (sin-OH-vee-al FLOO-id)



Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.

















105

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106 The Skeletal System




Imagine for a moment that people did not have late the amount of calcium that is deposited in, or
skeletons. What comes to mind? Probably that each of removed from, the bone matrix.
us would be a little heap on the floor, much like a jel- In bone as an organ, two types of bone tissue are
lyfish out of water. Such an image is accurate and present (Fig. 6–1). Compact bone looks solid but is
reflects the most obvious function of the skeleton: to very precisely structured. Compact bone is made of
support the body. Although it is a framework for the osteons or haversian systems, microscopic cylinders
body, the skeleton is not at all like the wooden beams of bone matrix with osteocytes in concentric rings
that support a house. Bones are living organs that around central haversian canals. In the haversian
actively contribute to the maintenance of the internal canals are blood vessels; the osteocytes are in contact
environment of the body. with these blood vessels and with one another through
The skeletal system consists of bones and other microscopic channels (canaliculi) in the matrix.
structures that make up the joints of the skeleton. The The second type of bone tissue is spongy bone,
types of tissue present are bone tissue, cartilage, and which does look rather like a sponge with its visible
fibrous connective tissue, which forms the ligaments holes or cavities. Osteocytes, matrix, and blood vessels
that connect bone to bone. 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,
FUNCTIONS OF THE SKELETON and the five kinds of white blood cells.

1. Provides a framework that supports the body; the
muscles that are attached to bones move the skele- CLASSIFICATION OF BONES
ton.
2. Protects some internal organs from mechanical 1. Long bones—the bones of the arms, legs, hands,
injury; the rib cage protects the heart and lungs, for and feet (but not the wrists and ankles). The shaft
example. of a long bone is the diaphysis, and the ends are
3. Contains and protects the red bone marrow, the called epiphyses (see Fig. 6–1). The diaphysis is
primary hemopoietic (blood-forming) tissue. made of compact bone and is hollow, forming a
4. Provides a storage site for excess calcium. Calcium canal within the shaft. This marrow canal (or
may be removed from bone to maintain a normal medullary cavity) contains yellow bone marrow,
blood calcium level, which is essential for blood which is mostly adipose tissue. The epiphyses are
clotting and proper functioning of muscles and made of spongy bone covered with a thin layer of
nerves. compact bone. Although red bone marrow is pres-
ent in the epiphyses of children’s bones, it is largely
replaced by yellow bone marrow in adult bones.
TYPES OF BONE TISSUE 2. Short bones—the bones of the wrists and ankles.
3. Flat bones—the ribs, shoulder blades, hip bones,
Bone was described as a tissue in Chapter 4. Recall and cranial bones.
that bone cells are called osteocytes, and the matrix 4. Irregular bones—the vertebrae and facial bones.
of bone is made of calcium salts and collagen. The
calcium salts are calcium carbonate (CaCO ) and cal- Short, flat, and irregular bones are all made of
3
cium phosphate (Ca (PO ) ), which give bone the spongy bone covered with a thin layer of compact
3 4 2
strength required to perform its supportive and pro- bone. Red bone marrow is found within the spongy
tective functions. Bone matrix is non-living, but it bone.
changes constantly, with calcium that is taken from The joint surfaces of bones are covered with artic-
bone into the blood replaced by calcium from the diet. ular cartilage, which provides a smooth surface. Cov-
In normal circumstances, the amount of calcium that ering the rest of the bone is the periosteum, a fibrous
is removed is replaced by an equal amount of calcium connective tissue membrane whose collagen fibers
deposited. This is the function of osteocytes, to regu- merge with those of the tendons and ligaments that

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Proximal
epiphysis







Osteon Haversian canal
(haversian system)
Concentric rings
of osteocytes





Marrow



Diaphysis



Yellow bone
marrow











Marrow (medullary)
cavity

Periosteum
Distal
epiphysis Venule
Compact bone
Arteriole
Spongy bone Canaliculi Fibrous layer
Periosteum
Osteocyte Osteogenic layer
(osteoblasts)
A B


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?







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108 The Skeletal System


are attached to the bone. The periosteum anchors junction of the diaphysis with each epiphysis. An epi-
these structures and contains both the blood vessels physeal disc is still cartilage, and the bone grows in
that enter the bone itself and osteoblasts that will length as more cartilage is produced on the epiphysis
become active if the bone is damaged. 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
EMBRYONIC GROWTH OF BONE testosterone), all of the cartilage of the epiphyseal
discs is replaced by bone. This is called closure of the
During embryonic development, the skeleton is first epiphyseal discs (or we say the discs are closed), and
made of cartilage and fibrous connective tissue, which the bone lengthening process stops.
are gradually replaced by bone. Bone matrix is pro- Also in bones are specialized cells called osteo-
duced by cells called osteoblasts (a blast cell is a “grow- clasts (a clast cell is a “destroying” cell), which are able
ing” or “producing” cell, and osteo means “bone”). In to dissolve and reabsorb the minerals of bone matrix,
the embryonic model of the skeleton, osteoblasts dif- a process called resorption. Osteoclasts are very
ferentiate from the fibroblasts that are present. The active in embryonic long bones, and they reabsorb
production of bone matrix, called ossification, begins bone matrix in the center of the diaphysis to form the
in a center of ossification in each bone. marrow canal. Blood vessels grow into the marrow
The cranial and facial bones are first made of canals of embryonic long bones, and red bone marrow
fibrous connective tissue. In the third month of fetal is established. After birth, the red bone marrow is
development, fibroblasts (spindle-shaped connective replaced by yellow bone marrow. Red bone marrow
tissue cells) become more specialized and differentiate remains in the spongy bone of short, flat, and irregu-
into osteoblasts, which produce bone matrix. From lar bones. For other functions of osteoclasts and
each center of ossification, bone growth radiates out- osteoblasts, see Box 6–1: Fractures and Their Repair.
ward 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 FACTORS THAT AFFECT BONE
remaining between the bones of the skull. These GROWTH AND MAINTENANCE
are called fontanels (Fig. 6–2), which permit com-
pression of the baby’s head during birth without 1. Heredity—each person has a genetic potential for
breaking the still thin cranial bones. The fontanels height, that is, a maximum height, with genes
also permit the growth of the brain after birth. You inherited from both parents. Many genes are
may have heard fontanels referred to as “soft spots,” involved, and their interactions are not well under-
and indeed they are. A baby’s skull is quite fragile and stood. Some of these genes are probably those for
must be protected from trauma. By the age of 2 years, the enzymes involved in cartilage and bone pro-
all the fontanels have become ossified, and the skull duction, for this is how bones grow.
becomes a more effective protective covering for the 2. Nutrition—nutrients are the raw materials of
brain. which bones are made. Calcium, phosphorus, and
The rest of the embryonic skeleton is first made of protein become part of the bone matrix itself.
cartilage, and ossification begins in the third month of Vitamin D is needed for the efficient absorption of
gestation in the long bones. Osteoblasts produce bone calcium and phosphorus by the small intestine.
matrix in the center of the diaphyses of the long bones Vitamins A and C do not become part of bone but
and in the center of short, flat, and irregular bones. are necessary for the process of bone matrix forma-
Bone matrix gradually replaces the original cartilage tion (ossification). Without these and other nutri-
(Fig. 6–3). ents, bones cannot grow properly. Children who
The long bones also develop centers of ossification are malnourished grow very slowly and may not
in their epiphyses. At birth, ossification is not yet com- reach their genetic potential for height.
plete and continues throughout childhood. In long 3. Hormones—endocrine glands produce hormones
bones, growth occurs in the epiphyseal discs at the that stimulate specific effects in certain cells.

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Anterior fontanel Parietal bone
Frontal bone Parietal bone
Posterior fontanel Posterior fontanel
Frontal bone















Occipital bone
Mastoid fontanel
Occipital bone
Anterior fontanel
Temporal bone
Sphenoid fontanel
Mandible
Sphenoid bone
Zygomatic bone
Maxilla 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 pho-
tographs by Dan Kaufman.)
QUESTION: What is the difference between the frontal bone of the infant skull and that of
the adult skull?












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Chondrocytes producing cartilage




B Epiphyseal disc
Osteoblasts producing bone


Bone







Epiphyseal
disc




Cartilage


Medullary cavity
containing
marrow
Compact bone





Secondary
ossification
Cartilaginous center
model
Bone collar Compact bone
and calcifying
cartilage in
ossification center


Medullary cavity and development
A of secondary ossification centers


Spongy bone

Articular cartilage



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?
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BOX 6–1 FRACTURES AND THEIR REPAIR
A fracture means that a bone has been broken. nutrition. The elderly and those in poor health often
There are different types of fractures classified as to have slow healing of fractures. A diet with sufficient
extent of damage. calcium, phosphorus, vitamin D, and protein is also
important. If any of these nutrients is lacking, bone
Simple (closed)—the broken parts are still in nor-
repair will be a slower process.
mal 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 with-
out apparent trauma; may accompany bone disor-
ders 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 perios-
teum 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 sim-
ple 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. Box Figure 6–A Types of fractures. Several types of
Other factors that influence repair include the fractures are depicted in the right arm.
age of the person, general state of health, and










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112 The Skeletal System


Several hormones make important contributions to strong cords or sheets of fibrous connective tissue. The
bone growth and maintenance. These include importance of ligaments becomes readily apparent
growth hormone, thyroxine, parathyroid hormone, when a joint is sprained. A sprain is the stretching or
and insulin, which help regulate cell division, pro- even tearing of the ligaments of a joint, and though the
tein synthesis, calcium metabolism, and energy bones are not broken, the joint is weak and unsteady.
production. The sex hormones estrogen or testos- We do not often think of our ligaments, but they are
terone help bring about the cessation of bone necessary to keep our bones in the proper positions to
growth. The hormones and their specific functions keep us upright or to bear weight.
are listed in Table 6–1. There are 206 bones in total, and the complete
4. Exercise or “stress”—for bones, exercise means skeleton is shown in Fig. 6–4.
bearing weight, which is just what bones are spe-
cialized to do. Without this stress (which is nor- SKULL
mal), bones will lose calcium faster than it is
replaced. Exercise need not be strenuous; it can be The skull consists of 8 cranial bones and 14 facial
as simple as the walking involved in everyday activ- bones. Also in the head are three small bones in each
ities. Bones that do not get this exercise, such as middle ear cavity and the hyoid bone that supports the
those of patients confined to bed, will become thin- base of the tongue. The cranial bones form the brain-
ner and more fragile. This condition is discussed case (lined with the meninges) that encloses and pro-
further in Box 6–2: Osteoporosis. tects 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
THE SKELETON areas of the head. These are the frontal bone, parietal
bones (two), temporal bones (two), and occipital bone.
The human skeleton has two divisions: the axial skele- The sphenoid bone and ethmoid bone are part of the
ton, which forms the axis of the body, and the appen- floor of the braincase and the orbits (sockets) for
dicular skeleton, which supports the appendages or the eyes. The frontal bone forms the forehead and
limbs. The axial skeleton consists of the skull, vertebral the anterior part of the top of the skull. Parietal means
column, and rib cage. The bones of the arms and “wall,” and the two large parietal bones form the pos-
legs and the shoulder and pelvic girdles make up the terior top and much of the side walls of the skull. Each
appendicular skeleton. Many bones are connected to temporal bone on the side of the skull contains an
other bones across joints by ligaments, which are external auditory meatus (ear canal), a middle ear cav-



Table 6–1 HORMONES INVOLVED IN BONE GROWTH AND MAINTENANCE

Growth hormone (anterior pituitary gland) • 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)
Thyroxine (thyroid gland) • Increases the rate of protein synthesis
• Increases energy production from all food types
Insulin (pancreas) • Increases energy production from glucose
Parathyroid hormone (parathyroid glands) • Increases the reabsorption of calcium from bones to the blood
(raises blood calcium level)
• Increases the absorption of calcium by the small intestine and kid-
neys (to the blood)
Calcitonin (thyroid gland) • Decreases the reabsorption of calcium from bones (lowers blood
calcium level)
Estrogen (ovaries) or • Promotes closure of the epiphyses of long bones (growth stops)
Testosterone (testes) • Helps retain calcium in bones to maintain a strong bone matrix

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The Skeletal System 113


BOX 6–2 OSTEOPOROSIS

Bone is an active tissue; calcium is constantly being As bones lose calcium and become thin and brit-
removed to maintain normal blood calcium levels. tle, fractures are much more likely to occur. Among
Usually, however, calcium is replaced in bones at a elderly women, a fractured hip (the neck of the
rate equal to its removal, and the bone matrix femur) is an all-too-common consequence of this
remains strong. degenerative bone disorder. Such a serious injury is
Osteoporosis is characterized by excessive loss not inevitable, however, and neither is the thinning
of calcium from bones without sufficient replace- of the vertebrae that bows the spines of some eld-
ment. Research has suggested that a certain gene erly people. After menopause, women may wish to
for bone buildup in youth is an important factor; have a bone density test to determine the strength
less buildup would mean earlier bone thinning. of their bone matrix. Several medications are avail-
Contributing environmental factors include smok- able that diminish the rate of bone loss. A diet high
ing, insufficient dietary intake of calcium, inactivity, in calcium and vitamin D is essential for both men
and lack of the sex hormones. Osteoporosis is most and women, as is moderate exercise. Young women
common among elderly women, because estrogen and teenagers should make sure they get adequate
secretion decreases sharply at menopause (in older dietary calcium to form strong bone matrix,
men, testosterone is still secreted in significant because this will delay the serious effects of osteo-
amounts). Factors such as bed rest or inability to porosis later in life.
get even minimal exercise will make calcium loss
even more rapid.

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.



ity, and an inner ear labyrinth. The occipital bone bone has a vertical projection called the crista galli
forms the lower, posterior part of the braincase. Its (“rooster’s comb”) that anchors the cranial meninges.
foramen magnum is a large opening for the spinal The rest of the ethmoid bone forms the roof and
cord, and the two condyles (rounded projections) on upper walls of the nasal cavities, and the upper part of
either side articulate with the atlas, the first cervical the nasal septum.
vertebra. The sphenoid bone is said to be shaped like All of the joints between cranial bones are immov-
a bat, and the greater wing is visible on the side of the able joints called sutures. It may seem strange to refer
skull between the frontal and temporal bones. The to a joint without movement, but the term joint (or
body of the bat has a depression called the sella tur- articulation) is used for any junction of two bones.
cica, which encloses the pituitary gland. The ethmoid (The classification of joints will be covered later in

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114 The Skeletal System


Skull (cranium)
Zygomatic arch


Maxilla Cervical vertebrae
Mandible Thoracic vertebrae
Clavicle
Sternum Scapula

Humerus Ribs



Lumbar vertebrae


Radius
Ulna
Ilium
Sacrum Figure 6–4. Skeleton. Anterior
Carpals
view.
Metacarpals Coccyx QUESTION: Which of the bones
Phalanges shown here would be classified
Pubis as irregular bones?
Ischium
Femur



Patella




Tibia
Fibula

Tarsals
Metatarsals
Phalanges






this chapter.) In a suture, the serrated, or sawtooth, the lambdoidal suture between the occipital and pari-
edges of adjacent bones fit into each other. These etal bones. Not visible is the sagittal suture, where the
interlocking projections prevent sliding or shifting of two parietal bones articulate along the midline of the
the bones if the skull is subjected to a blow or pres- top of the skull. All the bones of the skull, as well as
sure. In Fig. 6–5 you can see the coronal suture the large sutures, are shown in Figs. 6–5 through 6–8.
between the frontal and parietal bones, the squamosal Their anatomically important parts are described in
suture between the parietal and temporal bones, and Table 6–2.

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Parietal bone Coronal suture

Frontal bone

Squamosal suture Sphenoid bone
Ethmoid bone

Lacrimal bone
Temporal bone
Lacrimal canal

Nasal bone
Lambdoidal suture
Zygomatic bone
Occipital bone
Zygomatic process Condyloid process
Mandibular fossa
Mastoid process Maxilla
External auditory
meatus Mental foramen
Occipital bone Coronoid process
Mandible

Frontal bone Body

Zygomatic bone
Frontal bone
Maxillary bone Coronal suture
Parietal bone
Mandible
Squamosal suture
Parietal bone
Temporal bone
Temporal bone Nasal bone
Sphenoid bone
Sphenoid bone
Perpendicular plate (ethmoid)
Ethmoid bone
Zygomatic bone
Lacrimal bone Middle nasal concha (ethmoid)
Vomer
Nasal bone Inferior nasal concha
Maxilla
Inferior nasal concha
Mandible
Vomer
Mental foramen
Palatine bone



Figure 6–5. Skull. Lateral view of right side. Figure 6–6. Skull. Anterior view.
QUESTION: What might be the purpose of the openings at the back of the eye sockets?


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116 The Skeletal System






Palatine process (maxilla)
Palatine bone

Zygomatic bone





Vomer

Zygomatic process
Temporal bone
Styloid process
External auditory meatus

Mastoid process
Foramen magnum
Occipital condyles


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 The plow-shaped vomer forms the lower part of the
jaw) is movable; it forms a condyloid joint with each nasal septum; it articulates with the ethmoid bone. On
temporal bone. The other joints between facial bones either side of the vomer are the conchae, six scroll-
are all sutures. The maxillae are the two upper jaw like bones that curl downward from the sides of the
bones, which also form the anterior portion of the nasal cavities; they help increase the surface area of the
hard palate (roof of the mouth). Sockets for the roots nasal mucosa. These facial bones are included in Table
of the teeth are found in the maxillae and the 6–2.
mandible. The two nasal bones form the bridge of Paranasal sinuses are air cavities located in the
the nose where they articulate with the frontal bone maxillae and frontal, sphenoid, and ethmoid bones
(the rest of the nose is supported by cartilage). There (Fig. 6–9). As the name paranasal suggests, they open
is a lacrimal bone at the medial side of each orbit; the into the nasal cavities and are lined with ciliated
lacrimal canal contains the lacrimal sac, a passageway epithelium continuous with the mucosa of the nasal
for tears. Each of the two zygomatic bones forms the cavities. We are aware of our sinuses only when they
point of a cheek, and articulates with the maxilla, become “stuffed up,” which means that the mucus
frontal bone, and temporal bone. The two palatine they produce cannot drain into the nasal cavities. This
bones are the posterior portion of the hard palate. may happen during upper respiratory infections such

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Crista galli



Ethmoid bone Cribriform plate Frontal bone

Olfactory foramina
Sphenoid bone


Greater wing Sella turcica

Squamosal suture






Temporal bone

Parietal bone


Lambdoidal suture Foramen magnum


Occipital bone


A

C
Crista galli

B

















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?

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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 • Frontal sinus • Air cavity that opens into nasal cavity
• Coronal suture • Joint between frontal and parietal bones
Parietal (2) • Sagittal suture • Joint between the 2 parietal bones
Temporal (2) • Squamosal suture • Joint between temporal and parietal bone
• External auditory meatus • The tunnel-like ear canal
• Mastoid process • Oval projection behind the ear canal
• Mastoid sinus • Air cavity that opens into middle ear
• Mandibular fossa • Oval depression anterior to the ear canal; artic-
ulates with mandible
• Zygomatic process • Anterior projection that articulates with the
zygomatic bone
Occipital • Foramen magnum • Large opening for the spinal cord
• Condyles • Oval projections on either side of the foramen
magnum; articulate with the atlas
• Lambdoidal suture • Joint between occipital and parietal bones
Sphenoid • Greater wing • Flat, lateral portion between the frontal and
temporal bones
• Sella turcica • Central depression that encloses the pituitary
gland
• Sphenoid sinus • Air cavity that opens into nasal cavity
Ethmoid • Ethmoid sinus • Air cavity that opens into nasal cavity
• Crista galli • Superior projection for attachment of
meninges
• Cribriform plate and • On either side of base of crista galli; olfactory
olfactory foramina nerves pass through foramina
• Perpendicular plate • Upper part of nasal septum
• Conchae (4 are part of • Shelf-like projections into nasal cavities that
ethmoid; 2 inferior are increase surface area of nasal mucosa
separate bones)
Mandible • Body • U-shaped portion with lower teeth
• Condyles • Oval projections that articulate with the tem-
poral bones
• Sockets • Conical depressions that hold roots of lower
teeth
Maxilla (2) • Maxillary sinus • Air cavity that opens into nasal cavity
• Palatine process • Projection that forms anterior part of hard
palate
• Sockets • Conical depressions that hold roots of upper
teeth
Nasal (2) — • Form the bridge of the nose
Lacrimal (2) • Lacrimal canal • Opening for nasolacrimal duct to take tears to
nasal cavity
Zygomatic (2) — • Form point of cheek; articulate with frontal,
temporal, and maxillae
Palatine (2) — • Form the posterior part of hard palate
Vomer — • Lower part of nasal septum


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The Skeletal System 119


Ethmoid sinus




Frontal sinus



















Sphenoid sinus




Ethmoid sinus
Maxillary 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?


as colds, or with allergies such as hay fever. These of vertebrae indicate their location along the length of
sinuses, however, do have functions: They make the the spinal column. There are 7 cervical vertebrae, 12
skull lighter in weight, because air is lighter than bone, thoracic, 5 lumbar, 5 sacral fused into 1 sacrum, and
and they provide resonance for the voice, meaning 4 to 5 small coccygeal vertebrae fused into 1 coccyx
more air to vibrate and thus deepen the pitch of the (Fig. 6–10).
voice. The seven cervical vertebrae are those within the
The mastoid sinuses are air cavities in the mastoid neck. The first vertebra is called the atlas, which artic-
process of each temporal bone; they open into the ulates with the occipital bone to support the skull and
middle ear. Before the availability of antibiotics, mid- forms a pivot joint with the odontoid process of the
dle ear infections often caused mastoiditis, infection of axis, the second cervical vertebra. This pivot joint
these sinuses. allows us to turn our heads from side to side. The
Within each middle ear cavity are three auditory remaining five cervical vertebrae do not have individ-
bones: the malleus, incus, and stapes. As part of the ual names.
hearing process (discussed in Chapter 9), these bones The thoracic vertebrae articulate (form joints)
transmit vibrations from the eardrum to the receptors with the ribs on the posterior side of the trunk. The
in the inner ear (see Fig. 9–7). 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:
VERTEBRAL COLUMN
the sacroiliac joints. The coccyx is the remnant of
The vertebral column (spinal column or backbone) is tail vertebrae, and some muscles of the perineum
made of individual bones called vertebrae. The names (pelvic floor) are anchored to it.

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120 The Skeletal System


B
Atlas/Axis
Odontoid process
•
1
2
• Vertebral canal
3
Cervical
4
vertebrae Spinous process
5 •
6 of axis
7
1
2 • Facets of atlas for occipital condyles
3 Intervertebral
4 • discs
5 C
6 7th Lumbar
Thoracic 7 •
vertebrae 8 •
9 Facet for rib Transverse process
10
11 Spinous process •
12
Vertebral body
1
2
Lumbar 3 • D
vertebrae • 1st Lumbar
4
5
Vertebral
• Articular surface canal
for ilium
Sacrum


Coccyx
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 lat-
eral view.
QUESTION: Compare the size of the individual thoracic
A
and lumbar vertebrae. What is the reason for this differ-
ence?


All of the vertebrae articulate with one another in The supporting part of a vertebra is its body; the
sequence, connected by ligaments, to form a flexible bodies of adjacent vertebrae are separated by discs
backbone that supports the trunk and head. They also of fibrous cartilage. These discs cushion and absorb
form the vertebral canal, a continuous tunnel (lined shock and permit some movement between vertebrae
with the meninges) within the bones that contains the (symphysis joints). Since there are so many joints,
spinal cord and protects it from mechanical injury. the backbone as a whole is quite flexible (see also Box
The spinous and transverse processes are projections 6–3: Herniated Disc).
for the attachment of the muscles that bend the verte- The normal spine in anatomic position has four
bral column. The facets of some vertebrae are small natural curves, which are named after the vertebrae
flat surfaces for articulation with other bones, such as that form them. Refer to Fig. 6–10, and notice that the
the ribs with the facets of the thoracic vertebrae. cervical curve is forward, the thoracic curve backward,

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BOX 6–3 HERNIATED DISC
The vertebrae are separated by discs of fibrous car- The terms herniated disc or ruptured disc more
tilage that act as cushions to absorb shock. An inter- accurately describe what happens. The nucleus pul-
vertebral disc has a tough outer covering and a soft posus is forced out, often posteriorly, where it puts
center called the nucleus pulposus. Extreme pres- pressure on a spinal nerve. For this reason a herni-
sure on a disc may rupture the outer layer and force ated disc may be very painful or impair function in
the nucleus pulposus out. This may occur when a the muscles supplied by the nerve.
person lifts a heavy object improperly, that is, using Healing of a herniated disc may occur naturally if
the back rather than the legs and jerking upward, the damage is not severe and the person rests and
which puts sudden, intense pressure on the spine. avoids activities that would further compress the
Most often this affects discs in the lumbar region. disc. Surgery may be required, however, to remove
Although often called a “slipped disc,” the the portion of the nucleus pulposus that is out of
affected disc is usually not moved out of position. place and disrupting nerve functioning.
















































Box Figure 6–C Herniated disc. As a result of compression, a ruptured intervertebral disc puts pres-
sure on a spinal nerve.







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122 The Skeletal System


lages join the 7th rib cartilage. The last two pairs are
BOX 6–4 ABNORMALITIES OF THE
CURVES OF THE SPINE 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 en-
Scoliosis—an abnormal lateral curvature, which closes and protects the heart and lungs. Keep in mind,
may be congenital, the result of having one leg
longer than the other, or the result of chronic though, that the rib cage also protects organs in the
poor posture during childhood while the verte- upper abdominal cavity, such as the liver and spleen.
brae are still growing. Usually the thoracic verte- The other important function of the rib cage depends
brae are affected, which displaces the rib cage to upon its flexibility: The ribs are pulled upward and
one side. In severe cases, the abdominal organs outward by the external intercostal muscles. This
may be compressed, and the expansion of the enlarges the chest cavity, which expands the lungs and
rib cage during inhalation may be impaired. contributes to inhalation.
Kyphosis*—an exaggerated thoracic curve;
sometimes referred to as hunchback. THE SHOULDER AND ARM
Lordosis*—an exaggerated lumbar curve;
sometimes referred to as swayback. The shoulder girdles attach the arms to the axial skele-
These abnormal curves are usually the result ton. Each consists of a scapula (shoulder blade) and
of degenerative bone diseases such as osteo- clavicle (collarbone). The scapula is a large, flat bone
porosis or tuberculosis of the spine. If osteoporo- with several projections (the spine of the scapula, the
sis, for example, causes the bodies of the thoracic coracoid process) that anchor some of the muscles that
vertebrae to collapse, the normal thoracic curve move the upper arm and the forearm. A shallow
will be increased. Most often the vertebral body depression called the glenoid fossa forms a ball-and-
“settles” slowly (rather than collapses suddenly) socket joint with the humerus, the bone of the upper
and there is little, if any, damage to the spinal arm (Fig. 6–12).
nerves. The damage to the vertebrae, however,
cannot be easily corrected, so these conditions Each clavicle articulates laterally with a scapula
should be thought of in terms of prevention and medially with the manubrium of the sternum. In
rather than cure. this position the clavicles act as braces for the scapulae
and prevent the shoulders from coming too far for-
ward. Although the shoulder joint is capable of a wide
*Although descriptive of normal anatomy, the terms
kyphosis and lordosis, respectively, are commonly used to range of movement, the shoulder itself must be rela-
describe the abnormal condition associated with each. tively 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
the lumbar curve forward, and the sacral curve back-
ward. These curves center the skull over the rest of the joint is anchored here. Proximally, the humerus forms
body, which enables a person to more easily walk a ball-and-socket joint with the scapula. Distally, the
upright (see Box 6–4: Abnormalities of the Curves of humerus forms a hinge joint with the ulna of the
the Spine). forearm. This hinge joint, the elbow, permits move-
ment in one plane, that is, back and forth with no lat-
eral movement.
RIB CAGE
The forearm bones are the ulna on the little finger
The rib cage consists of the 12 pairs of ribs and the side and the radius on the thumb side. The semilunar
sternum, or breastbone. The three parts of the ster- notch of the ulna is part of the hinge joint of the
num are the upper manubrium, the central body, and elbow; it articulates with the trochlea of the humerus.
the lower xiphoid process (Fig. 6–11). The radius and ulna articulate proximally to form a
All of the ribs articulate posteriorly with the tho- pivot joint, which permits turning the hand palm up
racic vertebrae. The first seven pairs of ribs are called to palm down. You can demonstrate this yourself by
true ribs; they articulate directly with the manubrium holding your arm palm up in front of you, and noting
and body of the sternum by means of costal cartilages. that the radius and ulna are parallel to each other.
The next three pairs are called false ribs; their carti- Then turn your hand palm down, and notice that your

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The Skeletal System 123




1st rib
1st thoracic vertebra
2nd

Manubrium 3rd


4th
Body of sternum

Costal 5th
cartilages
6th

7th
8th

Xiphoid process 9th
10th
11th
12th thoracic vertebra
12th


Figure 6–11. Rib cage. Anterior view.
QUESTION: With what bones do all of the ribs articulate?



upper arm does not move. The radius crosses over the THE HIP AND LEG
ulna, which permits the hand to perform a great vari-
ety of movements without moving the entire arm. The pelvic girdle (or pelvic bone) consists of the two
The carpals are eight small bones in the wrist; hip bones (coxae or innominate bones), which articu-
gliding joints between them permit a sliding move- late with the axial skeleton at the sacrum. Each hip
ment. The carpals also articulate with the distal ends bone has three major parts: the ilium, ischium, and
of the ulna and radius, and with the proximal ends of pubis, and these are shown in Fig. 6–13, which depicts
the metacarpals, the five bones of the palm of the both a male and a female pelvis. The ilium is the
hand. All of the joints formed by the carpals and flared, upper portion that forms the sacroiliac joint.
metacarpals make the hand very flexible at the wrist The ischium is the lower, posterior part that we sit
(try this yourself: flexion to extension should be almost on. The pubis is the lower, most anterior part. The
180 degrees), but the thumb is more movable than the two pubic bones articulate with one another at the
fingers because of its carpometacarpal joint. This is a pubic symphysis, with a disc of fibrous cartilage
saddle joint, which enables the thumb to cross over between them. Notice the pubic angle of both the
the palm, and permits gripping. male and female pelvises in Fig. 6–13. The wider
The phalanges are the bones of the fingers. There female angle is an adaptation for childbirth, in that it
are two phalanges in each thumb and three in each helps make the pelvic outlet larger.
of the fingers. Between phalanges are hinge joints, The acetabulum is the socket in the hip bone that
which permit movement in one plane. Important forms a ball-and-socket joint with the femur.
parts of the shoulder and arm bones are described Compared to the glenoid fossa of the scapula, the
in Table 6–3. acetabulum is a much deeper socket. This has great

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Acromial end
Clavicle
Acromicon process
Coracoid process • • • •

•
Head • •
Sternal end


Glenoid fossa
•
Deltoid tubercle
Scapula
•





Humerus •





Capitulum
• Trochlea
•
Head • • Semilunar notch On posterior side
Olecranon process
Radial tuberosity •



Radius • • Ulna








Scaphoid
Lunate
Trapezium •
Carpals • • • Triquetrum
Trapezoid • • • • Carpals
Capitate Pisiform
Hamate
Metacarpals




Phalanges





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.
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The Skeletal System 125



Table 6–3 BONES OF THE SHOULDER AND ARM—IMPORTANT PARTS
Bone Part Description
Scapula • Glenoid fossa • Depression that articulates with humerus
• Spine • Long, posterior process for muscle attachment
• Acromion process • Articulates with clavicle
Clavicle • Acromial end • Articulates with scapula
• Sternal end • Articulates with manubrium of sternum
Humerus • Head • Round process that articulates with scapula
• Deltoid tubercle • Round process for the deltoid muscle
• Olecranon fossa • Posterior, oval depression for the olecranon process
of the ulna
• Capitulum • Round process superior to radius
• Trochlea • Concave surface that articulates with ulna
Radius • Head • Articulates with the ulna
Ulna • Olecranon process • Fits into olecranon fossa of humerus
• Semilunar notch • “Half-moon” depression that articulates with the
trochlea of ulna
Carpals (8) • Scaphoid • Lunate • Proximal row
• Triquetrum • Pisiform
• Trapezium • Trapezoid • Distal row
• Capitate • Hamate



functional importance because the hip is a weight- fibula is important in that leg muscles are attached and
bearing joint, whereas the shoulder is not. Because the anchored to it, and it helps stabilize the ankle. Two
acetabulum is deep, the hip joint is not easily dislo- bones on one is a much more stable arrangement than
cated, even by activities such as running and jumping one bone on one, and you can see that the malleoli of
(landing), which put great stress on the joint. the tibia and fibula overlap the sides of the talus. The
The femur is the long bone of the thigh. As men- tibia and fibula do not form a pivot joint as do the
tioned, the femur forms a very movable ball-and- radius and ulna in the forearm; this also contributes to
socket joint with the hip bone. At the proximal end of the stability of the lower leg and foot and the support
the femur are the greater and lesser trochanters, large of the entire body.
projections that are anchors for muscles. At its distal The tarsals are the seven bones in the ankle. As
end, the femur forms a hinge joint, the knee, with the you would expect, they are larger and stronger than
tibia of the lower leg. Notice in Fig. 6–14 that each the carpals of the wrist, and their gliding joints do not
bone has condyles, which are the rounded projections provide nearly as much movement. The largest is the
that actually form the joint. The patella, or kneecap, calcaneus, or heel bone; the talus transmits weight
is anterior to the knee joint, enclosed in the tendon of between the calcaneus and the tibia. Metatarsals are
the quadriceps femoris, a large muscle group of the the five long bones of each foot, and phalanges are
thigh. the bones of the toes. There are two phalanges in the
The tibia is the weight-bearing bone of the lower big toe and three in each of the other toes. The pha-
leg. You can feel the tibial tuberosity (a bump) and langes of the toes form hinge joints with each other.
anterior crest (a ridge) on the front of your own leg. Because there is no saddle joint in the foot, the big toe
The medial malleolus, what we may call the “inner is not as movable as the thumb. The foot has two
ankle bone,” is at the distal end. Notice in Fig. 6–14 major arches, longitudinal and transverse, that are
that the fibula is not part of the knee joint and does supported by ligaments. These are adaptations for
not bear much weight. The lateral malleolus of the walking completely upright, in that arches provide for
fibula is the “outer ankle bone” you can find just above spring or bounce in our steps. Important parts of hip
your foot. Though not a weight-bearing bone, the and leg bones are described in Table 6–4.

(text continued on page 128)

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Figure 6–13. Hip bones and sacrum. (A) Male pelvis, anterior view. (B) Male pelvis, lat-
eral 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 differ-
ence?






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Acetabulum
Pubis
Head
Ischium Greater trochanter
Neck

Lesser trochanter







Femur








Patella
Medial condyle
Lateral condyle
Medial condyle
Lateral condyle
Head
Tibial tuberosity

Anterior crest
Tibia
Fibula
Tibia Fibula





Talus
Medial malleolus
Talus Lateral malleolus
Navicular Calcaneus Tarsals Calcaneus
Cuneiforms Cuboid
Tarsals First Metatarsals
Second
Third Phalanges Metatarsals Cuboid
Phalanges
B
A




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?
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128 The Skeletal System



Table 6–4 BONES OF THE HIP AND LEG—IMPORTANT PARTS
Bone Part Description
Pelvic (2 hip bones) • Ilium • Flared, upper portion
• Iliac crest • Upper edge of ilium
• Posterior superior iliac spine • Posterior continuation of iliac crest
• Ischium • Lower, posterior portion
• Pubis • Anterior, medial portion
• Pubic symphysis • Joint between the 2 pubic bones
• Acetabulum • Deep depression that articulates with femur
Femur • Head • Round process that articulates with hip bone
• Neck • Constricted portion distal to head
• Greater trochanter • Large lateral process for muscle attachment
• Lesser trochanter • Medial process for muscle attachment
• Condyles • Rounded processes that articulate with tibia
Tibia • Condyles • Articulate with the femur
• Tibial tuberosity • Round process for the patellar ligament
• Anterior crest • Vertical ridge
• Medial malleolus • Distal process; medial “ankle bone”
Fibula • Head • Articulates with tibia
• Lateral malleolus • Distal process; lateral “ankle bone”
Tarsals (7) • Calcaneus • Heel bone
• Talus • Articulates with calcaneus and tibia
• Cuboid, navicular —
• Cuneiform: 1st, 2nd, 3rd —


a strong sheath, like a sleeve. Lining the joint capsule
JOINTS—ARTICULATIONS
is the synovial membrane, which secretes synovial
fluid into the joint cavity. Synovial fluid is thick and
A joint is where two bones meet, or articulate.
slippery and prevents friction as the bones move.
Many synovial joints also have bursae (or bursas),
THE CLASSIFICATION OF JOINTS
which are small sacs of synovial fluid between the joint
The classification of joints is based on the amount and the tendons that cross over the joint. Bursae per-
of movement possible. A synarthrosis is an immovable mit the tendons to slide easily as the bones are moved.
joint, such as a suture between two cranial bones. If a joint is used excessively, the bursae may become
An amphiarthrosis is a slightly movable joint, such inflamed and painful; this condition is called bursitis.
as the symphysis joint between adjacent vertebrae. A Some other disorders of joints are described in Box
diarthrosis is a freely movable joint. This is the largest 6–5: Arthritis.
category of joints and includes the ball-and-socket
joint, the pivot, hinge, and others. Examples of each AGING AND THE SKELETAL SYSTEM
type of joint are described in Table 6–5, and many of
these are illustrated in Fig. 6–15.
With age, bone tissue tends to lose more calcium than
is replaced. Bone matrix becomes thinner, the bones
SYNOVIAL JOINTS
themselves more brittle, and fractures are more likely
All diarthroses, or freely movable joints, are synovial to occur with mild trauma.
joints because they share similarities of structure. A Erosion of the articular cartilages of joints is also a
typical synovial joint is shown in Fig. 6–16. On the common consequence of aging. Joints affected include
joint surface of each bone is the articular cartilage, weight-bearing joints such as the knees, and active,
which provides a smooth surface. The joint capsule, small joints such as those of the fingers.
made of fibrous connective tissue, encloses the joint in (text continued on page 131)

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A Ball and socket B Hinge C Pivot
Acetabulum of
hip bone
Odontoid process
Head of of axis
• femur
•
Atlas
Trochlea
of humerus •
•
•

•
Semilunar notch
of ulna
Bodies of
vertebra
•
Intervertebral
• disc
•
• Carpals
•
•
Metacarpal
•
of thumb
Trapezium
D Gliding F Saddle (carpal)
•
E Symphysis

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) Suture—fibrous connective tissue • Between cranial bones; between
between bone surfaces facial bones
Amphiarthrosis (slightly Symphysis—disc of fibrous cartilage • Between vertebrae; between pubic
movable) between bones bones
Diarthrosis (freely movable) Ball and socket—movement in all • Scapula and humerus; pelvic bone
planes and femur
Hinge—movement in one plane • Humerus and ulna; femur and tibia;
between phalanges
Condyloid—movement in one plane • Temporal bone and mandible
with some lateral movement
Pivot—rotation • Atlas and axis; radius and ulna
Gliding—side-to-side movement • Between carpals
Saddle—movement in several planes • Carpometacarpal of thumb

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Bone

Bursa
Synovial
membrane
Figure 6–16. Structure of a synovial joint. See
Joint Articular text for description.
capsule cartilage QUESTION: How can you tell that this is a joint
between two long bones? Give an example.
Joint cavity
(synovial fluid)
Tendon


Bone










BOX 6–5 ARTHRITIS

The term arthritis means “inflammation of a bacterial and viral infections have been suggested
joint.” Of the many types of arthritis, we will con- as possibilities.
sider two: osteoarthritis and rheumatoid arthritis. Rheumatoid arthritis often begins in joints of the
Osteoarthritis is a natural consequence of get- extremities, such as those of the fingers. The
ting older. In joints that have borne weight for autoimmune activity seems to affect the synovial
many years, the articular cartilage is gradually worn membrane, and joints become painful and stiff.
away. The once smooth joint surface becomes Sometimes the disease progresses to total destruc-
rough, and the affected joint is stiff and painful. As tion of the synovial membrane and calcification of
you might guess, the large, weight-bearing joints the joint. Such a joint is then fused and has no
are most often subjected to this form of arthritis. If mobility at all. Autoimmune damage may also
we live long enough, most of us can expect some occur in the heart and blood vessels, and those with
osteoarthritis in knees, hips, or ankles. RA are more prone to heart attacks and strokes (RA
Rheumatoid arthritis (RA) can be a truly crip- is a systemic, not a localized, disease).
pling disease that may begin in early middle age Treatment of rheumatoid arthritis is directed at
or, less commonly, during adolescence. It is an reducing inflammation as much as possible, for it is
autoimmune disease, which means that the the inflammatory process that causes the damage.
immune system mistakenly directs its destructive Therapies being investigated involve selectively
capability against part of the body. Exactly what blocking specific aspects of the immune response,
triggers this abnormal response by the immune such as antibody production. At present there is no
system is not known with certainty, but certain cure for autoimmune diseases.











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The Skeletal System 131


Although the normal wear and tear of joints cannot cles that move the skeleton. It is important to remem-
be prevented, elderly people can preserve their bone ber, however, that bones have other functions as well.
matrix with exercise (dancing counts) and diets high in As a storage site for excess calcium, bones contribute
calcium and vitamin D. 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
SUMMARY blood cells, and platelets. Some bones protect vital
organs such as the brain, heart, and lungs. As you can
Your knowledge of the bones and joints will be useful see, bones themselves may also be considered vital
in the next chapter as you learn the actions of the mus- organs.




STUDY OUTLINE

The skeleton is made of bone and cartilage Embryonic Growth of Bone
and has these functions: 1. The embryonic skeleton is first made of other tis-
1. Is a framework for support, connected by liga- sues that are gradually replaced by bone. Ossifica-
ments, moved by muscles. tion begins in the third month of gestation;
2. Protects internal organs from mechanical injury. osteoblasts differentiate from fibroblasts and pro-
3. Contains and protects red bone marrow. duce bone matrix.
4. Stores excess calcium; important to regulate blood 2. Cranial and facial bones are first made of fibrous
calcium level. connective tissue; osteoblasts produce bone matrix
in a center of ossification in each bone; bone growth
Bone Tissue (see Fig. 6–1) radiates outward; fontanels remain at birth, permit
1. Osteocytes (cells) are found in the matrix of cal- compression of infant skull during birth; fontanels
cium phosphate, calcium carbonate, and collagen. are calcified by age 2 (see Fig. 6–2).
2. Compact bone—haversian systems are present. 3. All other bones are first made of cartilage; in a long
3. Spongy bone—no haversian systems; red bone bone the first center of ossification is in the diaph-
marrow present. ysis, other centers develop in the epiphyses. After
4. Articular cartilage—smooth, on joint surfaces. birth a long bone grows at the epiphyseal discs:
5. Periosteum—fibrous connective tissue membrane; Cartilage is produced on the epiphysis side, and
anchors tendons and ligaments; has blood vessels bone replaces cartilage on the diaphysis side.
that enter the bone. Osteoclasts form the marrow cavity by reabsorb-
ing bone matrix in the center of the diaphysis (see
Classification of Bones Fig. 6–3).
1. Long—arms, legs; shaft is the diaphysis (compact
bone) with a marrow cavity containing yellow bone Factors That Affect Bone Growth and
marrow (fat); ends are epiphyses (spongy bone) (see Maintenance
Fig. 6–1). 1. Heredity—many pairs of genes contribute to
2. Short—wrists, ankles (spongy bone covered with genetic potential for height.
compact bone). 2. Nutrition—calcium, phosphorus, and protein be-
3. Flat—ribs, pelvic bone, cranial bones (spongy bone come part of the bone matrix; vitamin D is needed
covered with compact bone). for absorption of calcium in the small intestine;
4. Irregular—vertebrae, facial bones (spongy bone vitamins C and A are needed for bone matrix pro-
covered with compact bone). duction (calcification).

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132 The Skeletal System


3. Hormones—produced by endocrine glands; con- 2. Appendicular—bones of the arms and legs and the
cerned with cell division, protein synthesis, calcium shoulder and pelvic girdles.
metabolism, and energy production (see Table 6–1). • Shoulder and arm—see Fig. 6–12 and Table 6–3.
4. Exercise or stress—weight-bearing bones must • Scapula—shoulder muscles are attached; gle-
bear weight or they will lose calcium and become noid fossa articulates with humerus.
brittle. • Clavicle—braces the scapula.
• Humerus—upper arm; articulates with the
The Skeleton—206 bones (see Fig. 6–4); scapula and the ulna (elbow).
bones are connected by ligaments • Radius and ulna—forearm—articulate with
1. Axial—skull, vertebrae, rib cage. one another and with carpals.
• Skull—see Figs. 6–5 through 6–8 and Table 6–2. • Carpals—eight—wrist; metacarpals—five—
• Eight cranial bones form the braincase, which hand; phalanges—14—fingers (for joints, see
also protects the eyes and ears; 14 facial bones Table 6–5).
make up the face; the immovable joints • Hip and leg—see Figs. 6–13 and 6–14 and Table
between these bones are called sutures. 6–4.
• Paranasal sinuses are air cavities in the maxil- • Pelvic bone—two hip bones; ilium, ischium,
lae, frontal, sphenoid, and ethmoid bones; they pubis; acetabulum articulates with femur.
lighten the skull and provide resonance for • Femur—thigh; articulates with pelvic bone
voice (see Fig. 6–9). and tibia (knee).
• Three auditory bones in each middle ear cav- • Patella—kneecap; in tendon of quadriceps
ity transmit vibrations for the hearing process. femoris muscle.
• Vertebral column—see Fig. 6–10. • Tibia and fibula—lower leg; tibia bears weight;
• Individual bones are called vertebrae: 7 cervi- fibula does not bear weight, but does anchor
cal, 12 thoracic, 5 lumbar, 5 sacral (fused into muscles and stabilizes ankle.
one sacrum), 4 to 5 coccygeal (fused into one • Tarsals—seven—ankle; calcaneus is heel bone.
coccyx). Supports trunk and head, encloses • Metatarsals—five—foot; phalanges—14—toes
and protects the spinal cord in the vertebral (see Table 6–5 for joints).
canal. Discs of fibrous cartilage absorb shock
between the bodies of adjacent vertebrae, also Joints—Articulations
permit slight movement. Four natural curves 1. Classification based on amount of movement:
center head over body for walking upright (see • Synarthrosis—immovable.
Table 6–5 for joints). • Amphiarthrosis—slightly movable.
• Rib cage—see Fig. 6–11. • Diarthrosis—freely movable (see Table 6–5 for
• Sternum and 12 pairs of ribs; protects thoracic examples; see also Fig. 6–15).
and upper abdominal organs from mechanical 2. Synovial joints—all diarthroses have similar struc-
injury and is expanded to contribute to inhala- ture (see Fig. 6–16):
tion. Sternum consists of manubrium, body, • Articular cartilage—smooth on joint surfaces.
and xiphoid process. All ribs articulate with • Joint capsule—strong fibrous connective tissue
thoracic vertebrae; true ribs (first seven pairs) sheath that encloses the joint.
articulate directly with sternum by means of • Synovial membrane—lines the joint capsule;
costal cartilages; false ribs (next three pairs) secretes synovial fluid that prevents friction.
articulate with 7th costal cartilage; floating • Bursae—sacs of synovial fluid that permit ten-
ribs (last two pairs) do not articulate with the dons to slide easily across joints.
sternum.