CHAP, iv.] THE VASCULAR MECHANISM. 189 



probably in both cases, reaching the medulla from the brain. In 

 succeeding pages we shall have occasion more than once in discuss- 

 ing the effects of stimulating a given nerve, to consider how far 

 those effects are due to a reflex inhibition of the heart; and probably 

 there are few events taking place in the body which have not a 

 tendency thus to affect the central vascular pump, though in many 

 cases the tendency is counteracted by interfering agencies. But 

 we must be careful to avoid falling into the error of supposing 

 that every arrest, or slowing or weakening of the heart, is due to 

 impulses descending the vagus fibres. In many instances cardiac 

 troubles are due to events originating in the heart itself, so far 

 independent of the inhibitory processes which we are studying 

 now, that they are in no way whatever counteracted by atropin. 



Direct stimulation of the cardio-inhibitory centre itself, such as 

 occurs during the destruction of or results from injury to the 

 medulla, also produces inhibition. 



And the question naturally arises, Has this cardio-inhibitory 

 centre any constant automatic action ? 



In the dog, and also, though to a far less extent, in the rabbit, 

 section of both vagi is followed by a quickening of the heart's beat. 

 This result may be interpreted as shewing that the centre in the 

 medulla exercises a permanent restraining influence on the heart ; 

 that organ in fact being habitually curbed. The argument that 

 the effects of an artificial stimulation of the vagus soon wear off, 

 and that therefore a permanent stimulation of the vagi, leading to 

 permanent inhibitory action, would be impossible, may be met by 

 the reflection that a natural stimulation is, possibly, not wholly 

 identical with artificial stimulation, and its effects need not 

 necessarily wear off. 



We need not now stay to discuss the question whether this 

 central action is really automatic, i.e. kept up by molecular pro- 

 cesses originating in certain nerve cells, or reflex, that is, maintained 

 by nervous impulses reaching it along certain or various afferent 

 nerves. Granting, however, the existence of a centre in the 

 medulla, which either automatically or otherwise is in permanent 

 action, it is obviously open to us to speak of reflex inhibition as 

 being brought about by influences which augment the action of 

 that centre. But we have seen that active nervous centres are 

 subject, not only to augmentative, but also to inhibitory influences. 

 Hence the cardio-inhibitory centre might itself be inhibited by 

 impulses reaching it from various quarters. In other words, the 

 beat of the heart might be quickened by a lessening of the normal 

 action of the inhibitory centre in the medulla. It is in fact 

 probable, that many cases of quickening of the heart's beat are 

 produced in this way ; though the matter requires further investi- 

 gation. 



Accelerator nerves. The heart's beat may in the mammal be 

 quickened, even after division of both vagi, by direct stimulation 



