Physiology of the Heart. 99 



between it and the first associated metamere below, leads to 

 acceleration and augmentation. The same laws apply to this 

 as to vagus acceleration. 



The Results of the Stimulation of the Vagus. — The vagus 

 when stimulated may arrest the auricle and ventricle in the 

 manner described for the alligator. In no Chelonian thus 

 far examined is the heart arrested by the vagus through the 

 reduction of the force of the beat to zero, as in the frog. As 

 in the terrapin, unilateral vagus effects are comparatively 

 common ; i.e., a strength of current sufficing to arrest one 

 auricle, for example, when the corresponding vagus is stim- 

 ulated, has much less effect on its fellow. 



Comparative Effect of each Vagus. — A large number of 

 experiments have given results very closely resembling 

 those obtained for the terrapin, i.e., in the great majority 

 of instances the right vagus has greater power, especially 

 in maintaining the heart in stand-still or continued stimu_ 

 lation, than the left; but in both terrapin and sea turtle this 

 difference appears to be less than in the land tortoise. This 

 difference in the vagi seems to extend to other families of 

 cold-blooded animals, and is to be explained not by a differ- 

 ence in the number of inhibitory fibres in each vagus, nor 

 by inequality in the distribution of them, but by the fact that 

 the right pulsatile venous area (right sinus and veins), to 

 which the right vagus is mostly distributed, is the chief or 

 dominating part in the driving machinery of the heart, for 

 arrest of the heart is practically dependent on arrest of the 

 sinus. In one case, in the sea turtle, prolonged alternate 

 stimulation of the vagi gave perfect cardiac inhibition for 

 more than six hours. This is the longest case of heart 

 stand-still of this kind yet recorded for any animal. 



Structure of the Chelonian Heart. — Between the sinus and 

 the conspicuous auricle there is a part of the heart some- 

 what different in appearance, structure, and physiological 

 qualities from either the sinus or auricle proper. A similar 

 structure is found in some fishes. Especially have my 

 experiments shown that the capacity for independent 

 rhythm in this part is greater than in the so-called " bulged " 

 part (G-askell) of the auricles, I have, therefore, thought it 



