MUSCULAR SYSTEM 



31 



Involuntary muscles. — Involuntary muscle fibers have the 

 peculiar property of being able to act independently of the will 

 in contraction and relaxation. They are controlled in this action 

 by two kinds of motor nerve fibers from the sympathetic and 

 cerebrospinal nervous systems. One kind stimulates, causing 

 contraction ; the other kind inhibits, causes relaxation of the in- 

 voluntary muscle fibers. These are merely long, spindle-shaped 

 cells, which do not end in tendon fibere. They may be arranged 

 in the form of small bun- 

 dles, and are usually in the 

 form of thin sheets. Invol- 

 untary muscle tissue is 

 lighter in color than volun- 

 tarv, and usually incloses 

 hollow organs forming one 

 of the coats or layers. The 

 middle coat of the stomach 

 and intestines is composed 

 of involuntary muscle fibers. 



Function. — It is their 

 function to carry on work 

 which could not be intrusted 

 to conscious control and 

 with which the brain could 

 not well be burdened. 



Peristaltic action of the 

 stomach and intestines is 

 produced by the rhythmic 

 action of these muscle fibers. The lieart muscle fibers differ 

 from botli the typical voluntary and the typical involuntary 

 fibers. They arc striped, but operate independently of the will. 



Source of heat and power. — Muscular power comes from 

 oxidation of food material in the various tissues of the body, 

 particularly in the muscles and larger glands. During the 

 process of oxidation, carbonic gas and other materials are de- 

 veloped. Power is increased by proper nourishment, and is 

 decreased by lack of nourishment. IMuscles lose in strength by 

 overwork because they are consumed more rapidly tlian rebuilt. 



Description of voluntary muscle. — Tlie massetcr muscle is 

 located on the outer part of the cheek. Form, flat, broad, thick, 

 four-sided. Origin, on the temporal and superior maxillary 



Muscle Fibers. 



A, Bundle of voluntary fibers, side 

 view. B, C, D, Three involuntary fibers, 

 spindle-shaped cells with nuclei. 



