THE MUSCULAR SYSTEM 59 



in being composed of relatively minute, spindle-shaped cells, each 

 with a single nucleus (see Fig. 1). The nerve supply comes in most 

 cases directly from the sympathetic system instead of the central 

 nervous system, as is the case with the voluntary muscles, so the 

 contractions are not under the direct control of the will. All 

 movements of internal organs — that is, those having to do with 

 the vegetative life of the body — are governed by these muscles. 

 They are, therefore, more important than the voluntary muscles, 

 for life would cease at once if the visceral organs failed to act. 



The heart muscle is between the voluntary and involuntary 

 muscles in both structure and physiologic properties. Although 

 it is not directly under the control of the will, it is composed of 

 muscle-fibers striated or striped in a peculiar fashion. It contracts 

 slower than voluntary, but faster than smooth muscle tissue. The 

 power of rhythmic contractility is developed to a wonderful degree 

 in heart muscle. 



