Essentials of Anatomy and Physiology

Although the normal wear and tear of joints cannot
be prevented, elderly people can preserve their bone
matrix with exercise (dancing counts) and diets high in
calcium and vitamin D.

SUMMARY

Your knowledge of the bones and joints will be useful
in the next chapter as you learn the actions of the mus-

cles that move the skeleton. It is important to remem-

ber, however, that bones have other functions as well.

As a storage site for excess calcium, bones contribute

to the maintenance of a normal blood calcium level.

The red bone marrow found in flat and irregular

bones produces the blood cells: red blood cells, white

blood cells, and platelets. Some bones protect vital

organs such as the brain, heart, and lungs. As you can

see, bones themselves may also be considered vital

organs.

The Skeletal System 131

STUDY OUTLINE

The skeleton is made of bone and cartilage
and has these functions:

1. Is a framework for support, connected by liga-
   ments, moved by muscles.


2. Protects internal organs from mechanical injury.


3. Contains and protects red bone marrow.


4. Stores excess calcium; important to regulate blood
   calcium level.

Bone Tissue (see Fig. 6–1)

1. Osteocytes (cells) are found in the matrix of cal-
   cium phosphate, calcium carbonate, and collagen.


2. Compact bone—haversian systems are present.


3. Spongy bone—no haversian systems; red bone
   marrow present.


4. Articular cartilage—smooth, on joint surfaces.


5. Periosteum—fibrous connective tissue membrane;
   anchors tendons and ligaments; has blood vessels
   that enter the bone.

Classification of Bones

1. Long—arms, legs; shaft is the diaphysis (compact
   bone) with a marrow cavity containing yellow bone
   marrow (fat); ends are epiphyses (spongy bone) (see
   Fig. 6–1).


2. Short—wrists, ankles (spongy bone covered with
   compact bone).


3. Flat—ribs, pelvic bone, cranial bones (spongy bone
   covered with compact bone).


4. Irregular—vertebrae, facial bones (spongy bone
   covered with compact bone).

Embryonic Growth of Bone

1. The embryonic skeleton is first made of other tis-
   sues that are gradually replaced by bone. Ossifica-
   tion begins in the third month of gestation;
   osteoblasts differentiate from fibroblasts and pro-
   duce bone matrix.


2. Cranial and facial bones are first made of fibrous
   connective tissue; osteoblasts produce bone matrix
   in a center of ossification in each bone; bone growth
   radiates outward; fontanels remain at birth, permit
   compression of infant skull during birth; fontanels
   are calcified by age 2 (see Fig. 6–2).


3. All other bones are first made of cartilage; in a long
   bone the first center of ossification is in the diaph-
   ysis, other centers develop in the epiphyses. After
   birth a long bone grows at the epiphyseal discs:
   Cartilage is produced on the epiphysis side, and
   bone replaces cartilage on the diaphysis side.
   Osteoclasts form the marrow cavity by reabsorb-
   ing bone matrix in the center of the diaphysis (see
   Fig. 6–3).

Factors That Affect Bone Growth and

Maintenance

1. Heredity—many pairs of genes contribute to
   genetic potential for height.


2. Nutrition—calcium, phosphorus, and protein be-
   come part of the bone matrix; vitamin D is needed
   for absorption of calcium in the small intestine;
   vitamins C and A are needed for bone matrix pro-
   duction (calcification).