48 



MUSCLES AND TENDONS 



as, for example, those which cause the movements of 

 the chest in respiration. These muscles seem thoroughly 

 under the control of the will, since 

 we can prolong, shorten, or even 

 stop breathing for a time. There is, 

 however, a point where the volun- 

 tary control is mastered by the in- 

 voluntary and we are forced to breathe 

 and not suffocate. 



FIG. 40. Isolated 

 smooth muscle 

 fibres. 



FIG. 41. Smooth or involuntary muscle fibres, 

 showing their arrangement. 



Heart muscle. In addition to these two types of 

 muscle structure, there is a third found only in the 

 heart, which somewhat resembles both. Its cells are 

 striped like those of the voluntary muscles and yet its 

 action is like that of the involuntary muscles, since it is 

 not under the control of the will. It contracts more 

 quickly than the involuntary and yet not so quickly as 

 the voluntary. 



Effect of work. The muscles do an amount of work 

 far beyond our comprehension, as we shall see in our 

 study of motion and locomotion. They thrive upon hard 

 work and grow larger and stronger as the demands upon 

 them increase, provided only that they are not ex- 

 hausted by overwork. Conversely, when the demands 

 upon them are slight, they become smaller and weaker. 

 Contrary to popular belief, however, they are never 

 completely lost through disuse, because, even when they 

 are idle, there is yet sufficient tonic contraction to main- 



