Imagine for a moment that people did not have

skeletons. What comes to mind? Probably that each of
us would be a little heap on the floor, much like a jel-
lyfish out of water. Such an image is accurate and
reflects the most obvious function of the skeleton: to
support the body. Although it is a framework for the
body, the skeleton is not at all like the wooden beams
that support a house. Bones are living organs that
actively contribute to the maintenance of the internal
environment of the body.
The skeletal systemconsists of bones and other
structures that make up the joints of the skeleton. The
types of tissue present are bone tissue, cartilage, and
fibrous connective tissue, which forms the ligaments
that connect bone to bone.

FUNCTIONS OF THE SKELETON

1. Provides a framework that supports the body; the
   muscles that are attached to bones move the skele-
   ton.


2. Protects some internal organs from mechanical
   injury; the rib cage protects the heart and lungs, for
   example.


3. Contains and protects the red bone marrow, the
   primary hemopoietic (blood-forming) tissue.


4. Provides a storage site for excess calcium. Calcium
   may be removed from bone to maintain a normal
   blood calcium level, which is essential for blood
   clotting and proper functioning of muscles and
   nerves.

TYPES OF BONE TISSUE

Bonewas described as a tissue in Chapter 4. Recall
that bone cells are called osteocytes, and the matrix
of bone is made of calcium saltsand collagen. The
calcium salts are calcium carbonate (CaCO 3 ) and cal-
cium phosphate (Ca 3 (PO 4 ) 2 ), which give bone the
strength required to perform its supportive and pro-
tective functions. Bone matrix is non-living, but it
changes constantly, with calcium that is taken from
bone into the blood replaced by calcium from the diet.
In normal circumstances, the amount of calcium that
is removed is replaced by an equal amount of calcium
deposited. This is the function of osteocytes, to regu-

late the amount of calcium that is deposited in, or

removed from, the bone matrix.

In bone as an organ, two types of bone tissue are

present (Fig. 6–1). Compact bonelooks solid but is

very precisely structured. Compact bone is made of

osteonsor haversian systems, microscopic cylinders

of bone matrix with osteocytes in concentric rings

around central haversian canals. In the haversian

canals are blood vessels; the osteocytes are in contact

with these blood vessels and with one another through

microscopic channels (canaliculi) in the matrix.

The second type of bone tissue is spongy bone,

which does look rather like a sponge with its visible

holes or cavities. Osteocytes, matrix, and blood vessels

are present but are not arranged in haversian systems.

The cavities in spongy bone often contain red bone

marrow, which produces red blood cells, platelets,

and the five kinds of white blood cells.

CLASSIFICATION OF BONES

1.Long bones—the bones of the arms, legs, hands,

and feet (but not the wrists and ankles). The shaft

of a long bone is the diaphysis, and the ends are

called epiphyses(see Fig. 6–1). The diaphysis is

made of compact bone and is hollow, forming a

canal within the shaft. This marrow canal (or

medullary cavity) contains yellow bone marrow,

which is mostly adipose tissue. The epiphyses are

made of spongy bone covered with a thin layer of

compact bone. Although red bone marrow is pres-

ent in the epiphyses of children’s bones, it is largely

replaced by yellow bone marrow in adult bones.

2.Short bones—the bones of the wrists and ankles.

3.Flat bones—the ribs, shoulder blades, hip bones,

and cranial bones.

4.Irregular bones—the vertebrae and facial bones.

Short, flat, and irregular bones are all made of

spongy bone covered with a thin layer of compact

bone. Red bone marrow is found within the spongy

bone.

The joint surfaces of bones are covered with artic-

ular cartilage, which provides a smooth surface. Cov-

ering the rest of the bone is the periosteum, a fibrous

connective tissue membrane whose collagen fibers

merge with those of the tendons and ligaments that

106 The Skeletal System