The Heart of the Sea-Turtle. 315 



caretta and imbricata seems to reach its maximum sooner 

 than in the Terrapin. This does not apply equally to C. 

 mydas, I think. Further, when the heart is enfeebled in 

 all kinds of Chelonians, the maximum is more rapidly at- 

 tained, and this remark applies with especial force to the 

 marine turtles. 



The beat may recommence after standstill from vagus 

 stimulation, in the order : sinus (always), sinus extension^ 

 ventricle, auricles ; or in the order : sinus, sinus extension 

 and auricles, ventricle ; and the same holds for the Alliga- 

 tor and the Fish. 



Unilateral Effects of Vagus Stimulation. — These have 

 been referred to in my paper on the Terrapin (pp. 249, 250), 

 and relate especially to greater dilation of one auricle, than 

 the other during stimulation of its corresponding nerve. 

 While such dilating effects have been noticed for the sea- 

 turtle, arrest of an auricle answering to the vagus stimu- 

 lated, has been more frequently observed than in the Ter- 

 rapin; in several cases this phenomenon has been very 

 pronounced, and has followed on every stimulation of the 

 nerve with a sufficiently weak current. 



Stimulation of the Central End of one Vagus, the Medulla 

 and the other Vagus being intact. — The results may be stated 

 briefly as follows : — 



1. In all the specimens of the sea-turtle examined in this 

 way (with one exception, in which there was doubt as to 

 the soundness of the medulla) either arrest or slowing of the 

 rhythm has followed. 



2. In most cases this could be repeated 3 to 6 times at 

 short intervals, but with less and less effect on each occa- 

 sion. Considering the great vital tenacity of the nerves in 

 the Chelonians, this seems to point to exhaustion of the 

 inhibitory centre. 



3. In a certain proportion of cases there is decided after- 

 acceleration (e.g., from 33 to 38 beats). 



4. As was seen with the Slider Serrapin, there are great 

 differences in capacity for this form of inhibition in differ- 

 ent specimens of the same species. 



