66 CONSCIOUS NERVOUS OPERATIONS 



elasticity disappears, and a hard, rigid condition sets in, 

 accompanied by more or less contraction. This is due to 

 a coagulation of the protoplasm of the muscle cells, simi- 

 lar to the clotting of blood. Rigor mortis passes away 

 after a time, and the dead body becomes soft and flabby 

 a sign of approaching decay. 



80. Plain or Involuntary Muscles. Though the plain 

 muscles are not under the control of the will, they still 

 have nervous connection with the central nervous system. 

 Most of the nerves supplying the organs having plain or 

 umtriped muscular tissue come from the sympathetic nerv- 

 ous system; but from every ganglion of the sympathetic 

 chain nerve fibers communicate with the brain and spinal 

 cord. The muscles of the blood vessels, lymphatics, 

 glands, and other internal organs, are of unstriped struc- 

 ture, and carry on their work without affecting conscious- 

 ness. Their action under stimulus is similar to that of 

 the skeletal muscles, but takes place much more slowly. 



81. Plain Muscle Fibers. Plain muscles are made up, 

 like skeletal muscles, of bundles of fibers, and these of 

 muscle cells. The cells, however, differ from those of the 

 voluntary muscles. They are long, spindle-shaped fibers, 

 having a rod-shaped nucleus in the center (Fig. 41). The 

 nerves of plain muscle fibers do not end in end plates, but 

 form plexuses, or networks, which ramify between and 

 around the muscle fibers. The nerves of the heart mus- 

 cles end as do those of the unstriped muscles. 



82. Rhythmic and Peristaltic Movements of Involuntary Mus- 

 cle. One of the characteristics of involuntary muscles is 

 a tendency to alternate regular periods of activity and 

 rest. The heart is the most familiar illustration of this 

 rhythmical tendency, but it is seen in some other organs, 

 and especially in some of the lower animals. 



