The Heart of the Snake. 495 



at its best, the heart may be arrested on placing the elec- 

 trodes over either auricle or sinus. "When somewhat 

 exhausted, the auricles alone, or but one of them, may be 

 arrested on placing the electrodes over one auricle. Arrest 

 of the sinus is, of course, always followed by stoppage of 

 the rest of the heart. 



The behaviour of the ventricle, when thus directly stimu- 

 lated, differs from that pointed out in the heart of the 

 Sea Turtle. 1 In the Snake, stimulation of the ventricle is 

 never followed by that " intervermiform " action so common 

 in the Sea Turtle, and less frequently seen in the Terrapin. 

 One of the first effects, if the current be not too strong, 

 may be accelerated action ; and I believe the ventricle is 

 only arrested by the escape of current on the rest of the 

 heart, so that the ventricular pause is really due to the 

 arrest of the sinus. As after arrest from vagus stimulation, 

 the sinus and ventricle often beat for some time before the 

 auricles begin. The usual paralysis and light colored points, 

 to which allusion has been made in my other papers, are 

 evident in the Snake, though not so marked as in the 

 Chelonians. I see no reason to change the opinion expressed 

 in former papers as to the meaning of these phenomena, 

 nor to doubt that arrest of the heart on direct faridisa- 

 tion is owing to stimulation of the fine terminals of the vagi 

 nerves within the heart's substance. 



Independent Cardiac Rhythm. — As in former instances, 

 ligatures have been used to separate one part of the heart 

 from another. They are unquestionably much more 

 reliable than clamps or other apparatus. When the ven- 

 tricle is ligatured off from the rest of the heart, in no 

 case does an independant rhythm arise in it. Notwith- 

 standing the increase in pressure, the parts above continue 

 to beat well, even more vigorously than before. A ligature 

 between the sinus and auricles drawn tightly enough to 

 prevent any wave of contraction passing down over it 

 completely arrests all parts below ; and I have in no case 

 seen an independant rhythm arise in these regions of the 

 heart. In short, my experiments have given negative re- 



1 Op. cit. 



