202 



STUDIES IN PHYSIOLOGY 



small sliver-shaped cells, each having a nucleus. Unlike the 

 voluntary muscle fibers these have no cross stripes, and hence 

 this kind of tissue is often called un- 

 striped or plain muscle. The cells are 

 usually joined in such a way that they 

 form thin sheets. They are supplied 

 with branches of blood vessels and with 

 nerve fibers from the sympathetic nerve 

 system. 



Heart Muscle. While heart muscle 

 is involuntary so far as its action is 

 concerned, in its structure it presents 

 certain peculiarities. Like other in- 

 voluntary muscles, it consists of sepa- 

 rate cells, each with a distinct nucleus. 

 FIG. 95. Two Muscle (It is impossible to distinguish the 

 Fibers of the Heart. outlines of cell bodies in vo l untary 



J = line of junction be- musc i es .) But, on the other hand, 



tween two cells. ' 



n = nucleus. heart muscle resembles voluntary mus- 



p = processes which c le in its cross-striped appearance. The 

 joined another 1,1 i ,-, ,1 



fi ber muscles that make up the wall of the 



heart contract more rapidly than do 

 other involuntary muscles, but less rapidly than other 

 cross-striped tissue. 



3. THE HYGIENE OF MUSCLE 



Necessary Conditions for Healthy Muscles. If one is to 

 have 'a well-developed and healthy muscular system, four 

 conditions must be fulfilled : the body must be supplied with 

 nutritious food ; there must be a generous amount of fresh 

 air ; the muscles must be exercised vigorously ; and this exer- 

 cise must be followed by periods of rest. We will now consider 

 in turn how each of these requirements can be met. 



Food. We have learned that 75% of muscle is com- 

 posed of water, and that proteid is the most important 



