THE HEART 51 



and other forms of muscle as regards its response to a single 

 stimulus. But there is this remarkable difference to be noted, 

 that whereas in other forms the response varies in force according 

 to the intensity of the stimulus, cardiac muscle responds with a 

 maximal contraction to all efficient stimuli, be they minimal or 

 maximal. This is sometimes termed " the all or nothing law " ; 

 therefore once a heart responds to a stimulus it is of no avail to 

 increase the intensity of that stimulus. It should be noted, how- 

 ever, that heart muscle may give in response to the first few stimu- 

 lations gradually increasing contractions, thereby manifesting the 

 so-called " Treppe or staircase phenomenon." This is explained 

 on the supposition that the first few contractions render the tissue 

 more excitable to that form of stimulation. 



Another peculiarity manifested by the heart muscle is in re- 

 sponse to rhythmic stimulation. If a ventricle preparation, giving 

 but one contraction to one stimulation, be treated in this way by 

 single induction shocks, it starts to pulsate in regular fashion, 

 but the frequency of beat is always less than the stimulation fre- 

 quency. The number of contractions obtained by this method 

 can be increased either by increasing the freque'ncy of stimula- 

 tion or the intensity of current with the same rate of stimulation. 

 The same phenomenon can be produced upon the ventricle by 

 a constant current of appropriate intensity, the normal response 

 being twenty to thirty regular beats, after which it ceases. The 

 ventricle can also be made to give a similar rhythmic response 

 by injecting blood or normal safine into it at a suitable pressure. 

 Certain drugs, such as delphinin, are said to cause a series of 

 rhythmic contractions. 



A further peculiarity of cardiac muscle is that it cannot' be 

 tctanised. It is impossible to bring about a summation of stimuli 

 in the normal heart. This, however, is said to be possible on the 

 muscarine-poisoned heart, and on the heart stopped by excitation 

 of the vagus. 



Thus far we have discussed the effect of stimulation upon 

 the heart at rest. When we come to study the effect upon the 

 beating heart, certain other remarkable differences from skeletal 

 muscle are revealed. For instance, if a stimulus be applied to 

 the rhythmically beating ventricle just before or during systole 

 it is without any visible effect. The heart muscle is therefore 

 said to possess "a refractory period," and the possession of this 



