52 THE HEART 



property explains the inability to tetanise it. If, however, the 

 stimulus be applied during diastole, a contraction is produced 

 which is known as an " extra-systole." Such extra -systoles are 

 followed by a longer pause than normal. This is called the " com- 

 pensatory pause." It is about equal in length to the pause fol- 

 lowing the normal beat plus the amount cut off from the previous 

 pause by the induction of the extra -systole. The ventricle there- 

 fore, owing to this property, makes but the same number of systoles 

 as usual in a given time. This is known as the " law of con- 

 servation of rhythm to physiological stimuli " (Engelmann). 

 The length of the compensatory pause is due to the refractory 

 period ; the impulse causing the normal contraction reaches the 

 ventricle while it is in a state of systole from the artificial stimulus, 

 and it therefore has no effect. The ventricle is then not stimu- 

 lated again until the next normal impulse arrives. 



If, however, the ventricle be thrown into rhythmical contrac- 

 tions by a continuous stimulus, and an extra-systole produced by 

 a strong artificial stimulus, the 'extra -systole so induced is not 

 followed by a compensatory pause, for the continuous stimulus 

 still acting produces another contraction as soon as the refractory 

 period of the extra -systole is passed. This is adduced as proof 

 that the normal physiological stimulus is not continuous, but dis- 

 continuous, inasmuch as there could be no compensatory pause 

 if it were not so. 



Another important point to be noted is that the compensatory 

 pause does not follow an extra -systole induced at the venae cavae. 

 It therefore follows that all extra -systoles observed to be followed 

 by a compensatory pause are produced by a stimulus applied to 

 some other part of the heart (e.g. auricle or ventricle). It also 

 follows that the stimulus is not conducted from outside to the 

 venae cavae, but actually arises there. 



Like other forms of muscle, the heart muscle is said to possess 

 the property of tonicity This is masked in part by the incom- 

 plete relaxation of the heart between the beats, since as Gaskell 

 points out the degree of relaxation depends not only upon the 

 tonicity of the muscle, but also upon the rate of beat, which is 

 usually such that the heart muscle probably never completely re- 

 laxes. An alteration in tone can therefore only be manifested 

 when the rate of -beat remains the same. The little but distinct 

 evidence that exists for this change is derived from the action of 



