THE MUSCULAR SYSTEM
The function of muscular tissue is to produce movement of parts by contraction. Contraction may or may not be under the control of the will. The action of voluntary muscles is consciously controlled; involuntary muscle is not so controlled, and a person is usually not cognizant of its activities. These two types of muscle tissue differ also in microscopic structure, form, and distribution. They are alike in that the structural unit in each case is the muscle fiber, which is a cell highly differentiated for the function of contraction.
Involuntary, or nonstriated, muscles are found in the walls of the digestive tract, arteries and veins, bronchial tubes, uterus, bladder, in the glands, etc. The fibers are either more or less scattered or disposed in sheets. Each fiber is an elongated, spindle-shaped cell, containing a single, centrally located nucleus, and a longitudinally striated cytoplasm. They are under the control of the sympathetic nervous system, and they contract much less rapidly than voluntary muscles.
The striated muscle tissue comprises the skeletal muscles for the movement of the arms, legs, trunk, jaw, and head, the eye muscles for rotating the eyeballs, the muscles of the face, etc. With the exception of the striated muscles of the esophagus, the somewhat differently constructed muscles of the heart, and a few others, striated muscles are voluntary. The fibers are very long as compared with their diameter. The sarcolemma, or thin outer wall of the fiber, incloses the sarcoplasm, in
