100 Canadian Record of Science. 



well to name this part " sinus extension," and consider it a 

 separate part of the heart. It is in reality more allied func- 

 tionally to the sinus than to the auricle proper. 



Very frequently in the Chelonians and especially in the 

 marine turtles, stimulation of the vagus with a weak cur- 

 rent suffices to arrest the auricles proper ; in that case the 

 contraction wave of the sinus is conducted along the sinus 

 extension to the ventricle. The same occurs in the alliga- 

 tor and the fish. 



The Law of Inverse Proportion. — My work on the various 

 genera of Chelonians and the alligator has shown conclus- 

 ively that, whether the vagus or the sympathetic cause the 

 cardiac acceleration and augmentation, one law invariably 

 applies, viz., that the increase in the rate and force of the 

 heart-beat, after stimulation of the vagus or accelerating 

 sympathetic, is always inversely as the rate and force at the 

 time of stimulation, i. e., the slower and weaker the heart, 

 the greater the increase. The vagus seems to be the most 

 constant and powerful cardiac augmentor known to us. 



Faradisation of the Heart directly has given for the sea 

 turtle results analagous to those obtained in other Cheloni- 

 ans and the fish, but in the sea turtle there seems to be less 

 effect, especially as regards dilation around the area stimu- 

 lated. 



The fact that arrest of the sinus by direct stimulation is 

 not possible when the heart nutrition has much suffered (and 

 especially, therefore, its nerves), and that the dilating effects 

 are like those produced by vagus stimulation, etc., favors the 

 view that the results of faradisation are not due to direct 

 stimulation of the heart muscle but accomplished mediately 

 through its nerves. The light-colored areas, which, I have 

 pointed out, are seen in all cases at the exact points of contact 

 of the electrodes, are due to direct effects of thernuscle (con- 

 traction) . 



Spontaneous Rhythm. — The order in which spontaneous 

 rhythm most readily arises and is best maintained is : sinus, 

 sinus extension, auricle, ventricle. A large number of 

 experiments on the ventricle, especially, of the sea-turtle, 

 show that in the latter, as in all other Chelonians thus far 



