C h a p t e r 4 2
Structure and Function of the Skeletal System
1065
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
Epiphyseal
line
Medullary
cavity
Periosteum
Nutrient
artery
Compact bone
B
A
Cancellous bone
Epiphyseal
line
Distal epiphysis
Yellow
marrow
C
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 head
Epiphyseal cartilage
Epiphysis for
greater trochanter
Epiphysis for
lesser trochanter
Diaphysis or shaft
Metaphysis
Epiphyseal cartilage
or growth plate
Epiphysis for
distal extremity
FIGURE 42-3.
A femur, showing epiphyseal cartilages for the
head, metaphysis, trochanters, and distal end of the bone.
