THE PACE-MAKER 



61 



When all three chambers are beating independently, the 

 rhythm is quickest at the sinus, slowest at the ventricle, 

 and intermediate at the auricle. Normally therefore, the 

 inherent rhythm of the auricle and ventricle is not called 

 into play, for its effect is anticipated by contraction of 

 these chambers, due to the arrival of a wave from the 

 sinus. The auricle and ventricle, therefore, contract at 



Fig. 7. — Heart of tortoise prepared to show partial heart-block, 

 auricular tissue is cut between As and Av (Gaskell). 



The 



the rate set by the sinus. For this reason the sinus is called 

 the pace-maker of the heart. To confirm this view, Gaskell 

 cooled the sinus and warmed the ventricle. By so doing 

 he lowered the rhythmicity of the former and raised the 

 rhythmicity of the latter. The result was that the beat 

 passed from ventricle to sinus. The progression of the 

 beat is therefore due to the fact that different parts of 

 the heart possess different degrees of rhythmicity. 



It will be noticed that the rhythmicity is greatest in that 



