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 mus- 

 cles 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 differ- 

 ently 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 



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