> both at 4 cm. 



The Heart of the Sea-Turtle. 311 



Specimen III. Left vagus, secondary coil at 4 cm. from primary. 

 Right " " 1 



« IV. Right « l equalinpower . 

 Left " J H 

 Specimen V. Right vagus ) equal in power . 

 Left " J M F 



" VI. Left " inhibits with 2 c. at 3 cm. 



Right " - " . "5 '• 



" VII. Right " " " 6 " 



" Left " requires the strongest current. 



" VIII. Left 

 " Right 



A comparison of these results with those reported for the Ter- 

 rapin (pp. 247, 248, 249) will show a great resemblance. In by far 

 the larger number of cases, the right vagus has greater inhibitory 

 power than the left, 'exceptions to this, though few, sometimes 

 occurring. In the marine turtle, no case in which the left vagus 

 was wholly without effect on the rhythm was found. 



This difference between the two vagi, which does not seem 

 to be confined to the Chelonians, but is seen also, as I have 

 shown, in the Alligator, calls for explanation. Meyer's 1 

 explanation, that there were certain cases in which there 

 were no inhibitory fibres in the left vagus, does not agree 

 with facts ; for in all cases, the left vagus has some effect 

 either on the force or the rate of the beat. It has been seen 

 that in the Chelonians and Alligator, arrest of the sinus leads 

 almost invariably to arrest of the rest of the heart, whether 

 that arrest be brought about by the vagus or by a ligature 

 placed between the sinus and auricle ; and in those cases 

 in which one vagus is unable to maintain the rest of the 

 heart in standstill, it is always because the sinus is not 

 controlled. 



Gaskell 2 has shown that the part of the vagus known as 

 the " coronary " nerve is that which influences the force 

 of the beat of the auricle and ventricle while the rate 

 depends on the sinus. 



The peculiar unilateral vagus effects, pointed out in my 

 paper on the Terrapin, and in this one, seem to me to throw 

 new light on this question. 



1 Hemmungsnerven System des Herzens, Berlin, 1869. 



2 Journal of Physiology, vol. iii. Nos. 5 and 6. 



