98 



On. Alternate Stimulation of the Vagi. [Feb. 14, 



the pressure fell. At 2h. 27m. 15s. the pressure quickly rose, not- 

 withstanding the continued stimulation ; until at 2h. 27m. 45s. the 

 current was broken and the experiment brought to a close. 



It seems to the authors that the generally accepted theory of the 

 connection of the vagus nerves with a common intermediate apparatus 

 is fully competent to cover the facts established by A. B. Meyer, 

 Tarchanoif and Puelma, and themselves. Each vagus abuts against a 

 common nervous apparatus, through which every inhibitory stimulus 

 must pass to reach the cardiac muscle. It is only necessary to 

 suppose — and the supposition, as it will be shown, is not gratuitous — 

 that the exhaustibility of the vagi is far greater than that of the com- 

 mon mechanism they lead to, in order to gain a consistent view of 

 the facts. When one vagus is stimulated with a strong current, the 

 heart, after a longer or shorter period of diastolic rest, gradually 

 escapes from the inhibitory influence of the stimulus. This escape 

 must not be supposed to be due to the gradual exhaustion of the whole 

 inhibiting apparatus, but merely to that of the vagus which is stimu- 

 lated. The heart escapes, not because its guard is enfeebled, but be- 

 cause the warrant for detention miscarries. On shunting the current 

 into the other vagus, the intermediate apparatus is at once roused and 

 the heart stopped. 



So much greater are the enduring powers of the intermediate ap- 

 paratus that, even in the mammal, where, presumedly, they are more 

 easily exhausted than in the frog, the intermediate apparatus seems 

 capable of tiring out relay after relay of reinvigorated vagi. Indeed, 

 it would appear to be next to impossible, by the most careful and 

 judicious alternate stimulation of the vagi, to cause exhaustion of the 

 intermediate apparatus ; for, after stimulating the vagi alternately a 

 few times, the interval during which each vagus can hold the heart in 

 check becomes too brief for the recovery of the resting vagus. Never- 

 theless, when this stage has been reached, it is always possible (in the 

 frog) at once to stop the heart by applying an induced, interrupted 

 current to the sinus venosus, even if the current so applied be weaker 

 than that passing previously through the vagus nerves. 



But, although the authors believe that the intermediate apparatus 

 can, in health, easily wear out the organised attacks of the vagi, 

 particularly in the case of frogs, yet they think it not improbable 

 that, in conditions unfavourable to the restorative processes, the in- 

 termediate apparatus may occasionally succumb to successive vagus- 

 stimulations, especially in the case of mammals. They are, however, 

 convinced that the latter alternative very rarely occurs. 



The assumption of such enormous powers of endurance for the in- 

 termediate mechanism is not gratuitous. As long ago as 1869, A. B. 

 Meyer* pointed out that the heart of the frog could be inhibited 

 * A. B. Meyer, loc. cit. 



