CHAP, iv.] THE VASCULAR MECHANISM. 187 



ferred that atropin, unlike urari, paralyses this intrinsic inhibitory 

 mechanism itself. 



After the application of muscarin 1 or pilocarpin, the heart stops 

 beating, and remains in diastole in perfect standstill. Its ap- 

 pearance is then exactly that of a heart inhibited by profound and 

 lasting vagus stimulation. This effect is not hindered by urari. 

 The application however of a small dose of atropin at once restores 

 the beat. These facts are interpreted as meaning that muscarin 

 (or pilocarpin) stimulates or excites the inhibitory apparatus 

 spoken of above, which atropin paralyses or places hors de combat. 



There are many other effects of various poisons which have been 

 appealed to as throwing light on the action of the heart ; but we 

 must not enter into the discussion of these here. We may however 

 in this connection call attention to a remarkable experiment 

 known as that of Stannius. If a ligature be drawn tightly- 

 round the junction of the sinus venosus with the auricles, or if 

 the auricles be separated from the sinus by an incision carried 

 along the boundary-line between them, a standstill is produced 

 closely resembling a very prolonged vagus inhibition. Quiescence 

 thus induced may last a very considerable time. During the 

 standstill, a pulsation may be induced by a stimulus applied 

 directly to the heart, a whole series of beats being evoked when a 

 mechanical stimulus, such as the prick of a needle, is applied over 

 the seat of Bidder's ganglia at the junction of the auricles with the 

 ventricles, or to the ganglia in the auricles or to those in the 

 bulbus ; and when the ventricle is separated by an incision from 

 the auricles, the former will recommence beating, while the latter 

 remain as quiescent as before. The condition of the heart in 

 this experiment so closely resembles the standstill produced by 

 vagus stimulation, that the effect might be supposed to be caused 

 by the ligature (or section) stimulating the vagus fibres or the 

 inhibitory mechanism at the sinus; but this view is clearly dis- 

 proved by the fact that the experiment succeeds perfectly well 

 after atropin has been given. Another explanation attributes the 

 standstill to the section depriving the heart of the prepotent 

 ganglia in the sinus, and the recommencement of pulsation in the 

 ventricle after separation by incision or ligature from the auricles 

 to the incision or ligature acting as a stimulus to the ventricle but 

 not to the auricle. The experiment in fact is brought forward in 

 support of the views enunciated on p. 183. But these, as we have 

 said, are not satisfactory, and an adequate interpretation of the 

 experiment has yet to be supplied. Indeed, did it seem profitable, 

 we might relate many other puzzling results which have been 

 obtained in experimenting on the heart. We have already warned 

 the reader that the problem of the causes of the normal spon- 

 taneous beat is as yet far from being solved, and until we get 



1 The poisonous effects of many mushrooms are probably in large measure due 

 to a similar action on the heart. 



