The integumentary system includes the skin, a double-layered structure covering
the surface of the body, and accessory organs such as glands, hair, and nails (see
figure 1-2). The skin is the largest organ of the body. It functions to protect inner
tissues from drying (desiccation) and underlying structures from infection and injury.
Although absorption is not one of its normal functions, the skin can absorb water and
other substances. The skin contains sensory receptors for heat, cold, touch, and pain.
Skin also helps in the regulation of body temperature and limited excretion of wastes.
Figure 1-2. Cross section of the skin.
The adult human skeleton includes 206 bones (see figure 1-3). Bones consist of
a hard outer shell (cortex) and a spongy or porous inner part (spongiosa). Within some
bones there is a cavity which contains marrow. Marrow is a soft tissue composed
chiefly of fat and blood-forming tissue (see figure 1-4). Some bones form movable
joints and, with the action of muscles, these joints bring about movement of the body
and its parts. Other bones, such as those of the skull, are joined in a fixed position.
The functions of the skeletal system are to
a. Support and give shape to the body.
b. Protect certain vital organs.
c. Provide attachment for tendons, muscles, and ligaments.