Tracing shows the contractions excited in an isolated quiescent ventricle by stimu- 

 lating it with, a rapidly interrupted current (50 interruptions per second). 



Tracing shows the result of stimulating the same ventricle with a slowly interrupted 

 current (5 interruptions per second) of precisely the same strength. 



is no evidence whatever of its having the slightest effect in weakening 

 the contractions— a resnlt which has been shown to follow the appli- 

 cation of a weak interrupted current to certain cardiac tissues, e.g., 

 the auricle of the tortoise and the ventricle of the frog. 



In the case of the intact heart with the normal circulation going 

 on, the passage of a constant current through the apex of the ventricle 

 has been seen to be accompanied by a reversed order of contraction 

 of the parts of the heart, the apex of the ventricle — through which 

 the current is passing — being the portion which initiates the systole of 

 the whole organ. 



IV. Action of the Cardiac Nerves. 



All the nerve impulses affecting the heart appear to pass along the 

 vagus nerve trunks. 



Stimulation of the peripheral end of a cut vagus nerve exerts upon 

 the heart an inhibitory influence of a very powerful nature. The 

 inhibitory phase is often followed by a phase during which the heart's 

 rhythm is accelerated. Such an accelerating after effect of vagus 

 stimulation is, however, slight in degree and variable in occurrence. 



Stimulation of either the right or the left vagus is effective. 



When continued stimulation of one vagus fails at length to keep 

 the heart quiescent any longer, stimulation of the other vagus usually 

 causes a further inhibition of the cardiac action. 



The latent period of vagus stimulation appears to be short. 



The vagus has a very low minimal stimulation ; a very weak inter- 

 rupted current such as is insufficient (when applied to the peripheral 



