Porth's Essentials of Pathophysiology, 4e

1065

Structure and Function of the Skeletal System

C h a p t e r 4 2

the fused bones of the skull provide protection for the brain. Skeletal muscles, which are attached to bones by tendons, use bones as levers to move the body and its parts. Bones also serve as reservoirs for storage of min- erals such as calcium and phosphate; and the bulk of blood cells are formed within the marrow cavities of certain bones. Classification of Bones Bones can be classified according to type: compact (dense) or spongy (cancellous). They can also be clas- sified according to their shape, and the effect that bone shape has on the location of compact and spongy bone. Compact and Spongy Bone. If a bone is cut, two dis- tinct structural arrangements of bone tissue can be rec- ognized—a dense layer of compact bone that forms the outside of the bone and a spongelike meshwork consisting of trabeculae (thin, anastomosing spicules of bone tis- sue) that forms the interior of bone (Fig. 42-2). Spongy bone is relatively light, but its structure is such that it has considerable tensile strength and weight-bearing properties. Although bones contain both compact and spongy elements, their proportions vary in different bones throughout the body and in different parts of the same bone, depending on the relative needs for strength and lightness.

Classification of Bones by Shape. Bones are classi- fied by shape as long, short, flat, and irregular. Long bones are found in the upper and lower extremities. Short bones are irregularly shaped bones located in the ankle and the wrist. Except for their surface, which is compact bone, these bones are spongy throughout. Flat bones are composed of a layer of spongy bone between two layers of compact bone. They are found in areas where extensive protection of underlying structures is needed such as the skull and rib cage, or where a broad surface for muscle attachment is needed, as in the scap- ula. Irregular bones , because of their shapes, cannot be classified in any of the previous groups. This group includes bones such as the vertebrae and the bones of the jaw. Parts of a Long Bone A typical long bone has a shaft, or diaphysis , and two ends, called epiphyses (Fig. 42-3). Long bones usually are narrow in the midportion and broad at the ends so that the weight they bear can be distributed over a wider surface. The shaft of a long bone is formed mainly of compact bone roughly hollowed out to form a marrow- filled medullary canal. The ends of long bones are cov- ered with articular cartilage. In growing bones, the part of the bone shaft that funnels out as it approaches the epiphysis is called the metaphysis. It is composed of bony trabeculae that have cores of cartilage. In the child, the epiphysis is separated from the metaphysis by the cartilaginous growth plate. After puberty, the metaphysis and epiphysis merge, and the growth plate is obliterated. Bones are covered, except at their articular ends, by a membrane called the periosteum (see Fig. 42-2A). The periosteum consists of an outer layer of connective tissue

Compact bone Proximal epiphysis

Yellow marrow

Epiphyseal line

Medullary cavity

Periosteum

Epiphysis for head

Epiphyseal cartilage

Epiphysis for greater trochanter

A

Nutrient artery

Epiphysis for lesser trochanter

Compact bone

Cancellous bone

Diaphysis or shaft

B

C

Metaphysis

Epiphyseal line Distal epiphysis

Epiphyseal cartilage or growth plate

FIGURE 42-2. Diagram of bone structures. (A) Periosteum and bone marrow, (B) compact and spongy bone, and (C) a typical long bone showing the epiphysis, diaphysis, metaphysis, epiphyseal line, and sources of blood supply from the nutrient arteries.

Epiphysis for distal extremity

FIGURE 42-3. A femur, showing epiphyseal cartilages for the head, metaphysis, trochanters, and distal end of the bone.