CHAP, iv.] THE VASCULAR MECHANISM. 301 



be introduced into the circulation (when the experiment is con- 

 ducted on a living animal, or be applied in a weak solution to 

 the heart itself when the experiment is conducted, as in 1 1n- 

 case of a frog, on an excised heart or after the circulation has 

 ceased), it will after a short time be found, not only that the stimu- 

 lation, the application of a current for instance, which previously 

 when applied to the vagus produced marked inhibition, now 

 produces no inhibition, but even that the strongest stimulus, the 

 strongest current applied to the vagus will wholly fail to affect 

 the heart, provided that there be no escape of current on to the 

 cardiac tissues themselves; under the influence of even a small 

 dose of atropin, the strongest stimulation of the vagus will not 

 produce standstill or appreciable slowing or weakening of the beat. 



Now it might be supposed that the atropin produces this re- 

 markable effect by acting on some ganglionic or other mechanism 

 intervening between the vagus fibres and the cardiac muscular 

 tissue ; but we have evidence that the atropin acts either on the 

 muscular tissue itself or on the very endings of the nerves in the 

 muscular fibres. We have said, 155, that a properly prepared 

 strip of the ventricle of the tortoise will execute for a long time 

 spontaneous rhythmic contractions, it will go on " beating "for a 

 long time. A strip of the auricle will exhibit the same phenomena 

 even still more readily. If now while such a strip from the 

 auricle is satisfactorily beating a gentle interrupted current be 

 passed through it, it will stop beating : the current inhibits the 

 spontaneous beats ; a very gentle interrupted current must be 

 used, otherwise the effect is obscured by the more direct stimu- 

 lating action of the current. If now the strip be gently bathed 

 with a weak solution of atropin no such inhibitory effect is 

 produced by the interrupted current ; the beats go on regardless 

 of the action of the current. The interpretation of this experiment 

 is that in the first case the interrupted current stimulated the fine 

 termination of the inhibitory fibres in the muscular strip, and that 

 in the second case the atropin produced some effect either on 

 these fine fibres, or on their connections with the muscular 

 substance or on the actual muscular substance itself by virtue 

 of which they ceased to act. But if this be so, if the same 

 inhibitory effects are produced alike by stimulating the vagus 

 trunk, and stimulating the very endings of the nerves in the 

 muscles of the heart, if not the actual muscular tissue itself, then 

 there is no need to suppose the existence of any special inhibitory 

 mechanism placed between the fibres in the vagus branches and 

 the cardiac muscular tissue. 



The action of atropin on the heart is so to speak com- 

 plemented by the action of muscarin, the active principle of 

 many poisonous mushrooms. If a small quantity of muscarin be 

 introduced into the circulation, or applied directly to the heart, 

 the beats become slow and feeble, and if the dose be adequate the 



