282 



ADVANCED PHYSIOLOGY 



larity with striped and unstriped muscles. The cells com- 

 posing them are an elongate rectangular in form, and the 

 fibres made up of these cells show irregular striping; Fig. 141. 

 In its action cardiac muscle is unique in several ways; it 

 acts quickly, though not as rapidly as 

 the ordinary striped variety. Unlike 

 either striped or unstriped muscle, it 

 always contracts to its shortest possible 

 length, no matter how weak the stimulus 

 applied to it. It can "beat" when un- 

 connected with the brain, but even then 

 it apparently depends for its impulse 

 upon nerve cells in its own tissue. 



Cardiac tissue is unique in that it can- 

 not be thrown into tetanus. As has 

 already been noted, this condition is 

 usually provoked by the rapid recurrence 

 of some stimulus. In heart muscle, how- 

 ever > ^ wo succeeding stimuli do not pro- 

 duce any larger contraction than one, 

 and if another is applied just before 

 the muscle begins to relax, it has no effect. As soon as 

 the muscle really begins to relax, it becomes open to stimu- 

 lation and if irritated will begin a second contraction before 

 it has fully relaxed from the first. Hence, no number of 

 repeated stimuli can induce tetanus in cardiac muscle; it 

 must begin to relax before it becomes responsive to any out- 

 side influence. 



USE OF MUSCLES 



Effect of Use. It is a fact almost too trite for mention that 

 the activities of both work and play tend to strengthen the 

 body. But increased strength is only one of the benefits 

 derived from the use of the muscles. When muscles are 

 Active, all the other organs of the body are affected, and 



FIG 141 CARDIAC 

 MUSCLE CELLS 



