CHAP, iv.] THE VASCULAR MECHANISM. 257 



in the same way as when the augmentor fibres are directly stimu- 

 lated ; there is a marked increase in the force of the auricular and 

 of the ventricular systole, and at times an obvious acceleration of 

 the rhythm. We may infer that in such a case the augmentor 

 fibres are thrown into activity through the afferent impulses as 

 part of a reflex act. At the same time it must be remembered, 

 that afferent impulses may increase the beat of the heart not by 

 exciting the augmentor mechanism, but by depressing, that is 

 by inhibiting a previously existing activity of the cardio-inhibitory 

 centre ; to this point we shall again have to refer. 



We may however conclude that both the inhibitory and the 

 augmentor mechanisms of the heart can be brought into action by 

 means of the central nervous system. Speaking broadly the effect 

 of the former is to diminish the work of the heart, and so to lower 

 the blood pressure, and that of the latter to increase the work of 

 the heart, and so to heighten the blood pressure. 



139. If, either in a frog or a mammal, or other animal, after 

 the vagus fibres have been proved, by trial, to produce, upon stimu- 

 lation, the usual inhibitory effects, a small quantity of atropin 

 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, in the 

 frog for instance, 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. 



Further, this special action of atropin on the heart is so 

 to speak complemented 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 heart is brought to a complete standstill. The effect 

 is in some respects like that of powerful stimulation of the vagus. 

 Now if, in a frog, the heart be brought to a standstill by a dose of 

 muscarin, the application of an adequate quantity of atropin will 

 bring back the beats to quite their normal strength and rhythm. 

 The one drug is so far as the heart is concerned (and indeed in 

 many other respects) the antidote of the other. These and other 

 results have been taken to indicate that there exists in the heart 

 a special inhibitory mechanism, and that it is through this special 

 mechanism that the inhibitory fibres of the vagus produce inhibi- 

 tion, while atropin produces the effect just mentioned by paralys- 



