The Heart of the Snake. 491 



ventricular contraction independently of a wave of contrac- 

 tion over both parts of the auricle. 



Inexcitability of the Sinus and Auricles under vagus stimula- 

 tion. — One of most interesting results of the recent cardiac 

 studies has been the unexpected demonstration that certain 

 parts of the heart in some animals (the particular region 

 being variable) are, to a greater or less degree, inexcitable 

 to direct stimulation when the heart is arrested by vagus 

 influence. 



*Mc William has stated that the excitability of the auricle 

 is temporarily abolished in the Eel's heart under vagus 

 stimulation. But this investigator is entirely in error when 

 he affirms that such is not observed in the heart of the 

 Snake, though he is correct as regards the other animals he 

 instances. I have established, by experiments, that during 

 vagus stimulation, the sinus and auricles of the heart of the 

 Snake do not respond to direct, stimulation. If the heart be at 

 its best, and the stimulating currrent sufficiently strong, 

 the excitability may be wholly abolished ; but with a weaker 

 current, or a less vigorous heart, the effect may be only 

 partial. I have shown that in 2 Menobranchus the ventricle 

 is the part of the heart most readily and most profoundly 

 affected by stimulation of the vagus, and that during such 

 stimulation the ventricle is inexcitable. According to 

 3 Mc William, in the Newt, the sinus, auricles and ventricle, 

 are all inexcitable to direct stimulation during strong 

 inhibition. 



These peculiarities and differences show how dangerous 

 it is to assume the applicability of the same physiological 

 generalization to animals, even closely related morphologi- 

 cally. This conviction on my part had much to do with 

 my beginning these studies and continuing them on ani- 

 mals of related groups ; for I felt satisfied that a systematic 

 comparison would establish differences unsuspected by those 

 accustomed to extend conclusions derived from experiments 



1 Op. cit., p. 226. 

 1 Journal of Physiology, Vol. vii. 



3 Proceedings of the British Physiological Society, Journal of 

 Physiology, Vol. vi. 



