ACTION OF THE INHIBITOR Y NER VES. 



207 



the auricular, and therefore also of the ventricular, contractions is not 

 due to a slowing of the rate of the heart-beat, for, as is seen, the sinus 

 continues beating at the same rate as before ; but is due to a partial 

 blocking at the sino-auricular junction, caused by the stimulation of the 

 vagus nerve. In the majority of instances the vagus of the tortoise 



FIG. 114. Heart of tortoise. Suspension method. Stimulation of the peripheral 

 end of the cut coronary nerve. 



FIG. 115. Heart of tortoise. 



Suspension method, 

 nerve in the neck. 



Stimulation of right vagus 



(especially the right vagus) causes a slowing or stoppage of the whole 

 heart-beat, just as in other animals. It is further noticeable in these 

 two curves that when, as in Fig. 114, the rate of the auricular contrac- 

 tions is unaltered, the contractions of the ventricle remain absolutely 

 unaffected, while in Fig. 115 the contractions of the ventricle, instead of 

 being diminished by the stimulation of the nerve, are increased in strict 

 accordance with the amount of slowing of the auricular contractions. 



