314 Canadian Record of Science. 



the vagi, the rest of the heart, including sinus extension, 

 may beat as usual ; and this holds equally well, as I have 

 observed, for the Alligator and the Fish. 



3. Arrest of the heart by diminution of the force of the 

 contractions to zero, as often occurs in the Frog, does not, 

 so far as my observations go, occur in any Chelonian. 



4. The ventricle with the auricles may cease, the sinus 

 and sinus extension continuing to beat. But such stop is 

 likely to be very brief, the wave of contraction soon passing 

 on. 



5. Preliminary acceleration, which is very rare in the S. 

 Teri'apin, occurs more frequently in the sea-turtle, but never 

 except with the stimulation of a weak current. 



I have noticed brief preliminary acceleration soon followed 

 by slowing, the strength of the current remaining the same, 

 when the heart's powers have been much enfeebled. 



Arrest of the sinus, auricles and ventricles continuing to 

 beat, is unknown. 



Diastolic relaxation during stimulation is, perhaps, in the 

 marine turtle rather less marked than in the Terrapin ; but 

 it does occur, and equally well in the bloodless heart. 



The After-Effects of Vagus Stimulation. — These are very 

 similar in all the Chelonians ; in all, stimulation of the vagus 

 may be followed by a rhythm without increase ; a rhythm 

 with slight increase or with marked increase ; and the same 

 law holds equally well in the sea-turtle as in the Terrapin 

 that the rate of increase in the force and frequency of the beats 

 of the heart is in inverse proportion, to those prevailing at the time 

 of stimulation ; from this it follows that a weak heart, one 

 needing help most, is the one the vagi nerves can actually 

 improve most effectually. This has been illustrated to me 

 over and over again when working on the marine turtles ; 

 thus, when, as the heart is getting weak, the left auricle, as 

 is the rule, falls into a condition of great enfeeblement, 

 while the right is comparatively strong, the stimulation 

 of the vagus will restore the left, for a time at least, to 

 harmony of rhythm with its fellow, and produce marked 

 improvement in the strength of the beats. 



The after acceleration in the sea-turtles, especially in C. 



