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.
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.
d. Serve as joint levers by which movements may be accomplished when acted upon by muscles.
e. Serve as a connective tissue in which calcium and other mineral salts are deposited.
This system includes over 350 muscles. These muscles are made up of muscle tissue. They constitute 40 to 50 percent of the body's weight. The muscular system moves and propels the body and the contents of the hollow organs. It also keeps the body erect and produces body heat. Three types of muscles are found in the body.
a. Striated Muscle. This muscle produces bodily movement. Skeletal muscle is also called voluntary muscle since it can be consciously controlled. Figures 1-5 and 1-6 illustrate some skeletal muscles.
b. Smooth Muscle. This muscle is found in various visceral organs where continuous automatic functions are necessary. Smooth muscle is also called involuntary muscle since it contracts without conscious direction by the individual. For example, the unconscious contraction of the intestinal muscle moves food through the digestive system.
c. Cardiac Muscle. This muscle is a specialized type of involuntary muscle. It is found in the muscular wall of the heart.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.