THE HEART 119 



full effect, strong ones are unable to produce an excessive 

 contraction. 



The second peculiarity of the contraction of the heart 

 is that it abolishes for a time (the refractory period) all 

 the other properties of the heart muscle, including the 

 power of further contraction. This ensures a period of 

 rest to the heart, and makes it impossible for it to pass 

 into a tetanic condition (Fig. 3). 



The property of conductivity is exercised through 

 the muscle fibres themselves, and not through nerves. 

 The advantage of this is that, even although the heart 

 be extensively diseased, conduction can still go on so 

 long as the muscle cells are connected to each other, 

 which would hardly be possible if conduction were 

 dependent on a comparatively small number of nerve 

 fibres. 



It should be pointed out, further, that the physio- 

 logical properties of the heart muscle are all independent 

 of each other, so that one may be diminished or even 

 abolished altogether without affecting the other. 



The question must now be discussed, Does the heart 

 wall possess, in addition to these properties, the quality 

 of tonicity ? By this one means does the volume of the 

 heart and the capacity of its chambers tend to be kept 

 always somewhat reduced in consequence of a slight 

 tonic contraction of its muscle fibres, or, what comes to 

 much the same thing, as a result of their elastic 

 resilience ? To this question physiology now furnishes 

 an affirmative reply, and various clinical facts render 

 the existence of tonicity undoubted. It will be obvious 

 that tonicity must oppose dilatation of the heart from 



