CHAP, iv.] THE VASCULAR MECHANISM. 295 



nerve, ///, by the anterior root of which they reach the spinal 

 cord. 



159. Both sets of fibres then may be traced to the central 

 nervous system ; and we find accordingly that the heart may be 

 inhibited or augmented by nervous impulses which are started in 

 the nervous system either by afferent impulses as part of a reflex 

 act or otherwise, and which pass to the heart by the inhibitory or 

 by the augmenting tract. 



Thus if the medulla oblongata or a particular part of the 

 medulla oblongata which is specially connected with the vagus 

 nerve be stimulated, the heart is inhibited ; if for instance a 

 needle be thrust into this part the heart stands still. This region 

 in question may be stirred to action, in a 'reflex' manner, by 

 afferent impulses reaching it from various parts of the body. 

 Thus if the abdomen of a frog be laid bare, and the intestine be 

 struck sharply with the handle of a scalpel, the heart will stand 

 still in diastole with all the phenomena of vagus inhibition. If 

 the nervi mesenterici or the connections of these nerves with the 

 spinal cord be stimulated with the interrupted current, cardiac 

 inhibition is similarly produced. If in these two experiments 

 both vagi are divided, or the medulla oblongata is destroyed, inhi- 

 bition is not produced, however much either the intestine or the 

 mesenteric nerves be stimulated. This shews that the phenomena 

 are caused by impulses ascending along the mesenteric nerves to 

 the medulla, and so affecting a portion of that organ as to give 

 rise by reflex action to impulses which descend the vagi as 

 inhibitory impulses. The portion of the medulla thus mediating 

 between the afferent and efferent impulses may be spoken of as 

 the cardio-inhibitory centre. 



Reflex inhibition through one vagus may be brought about by 

 stimulation of the central end of the other. In general the 

 alimentary tract seems in closer connection with the cardio-inhi- 

 bitory centre than other parts of the body ; and if the peritoneal 

 surface of the intestine be inflamed, very gentle stimulation of 

 the inflamed surface will produce marked inhibition. But 

 apparently stimuli if sufficiently powerful will through reflex 

 action produce inhibition whatever be the part of the body to 

 which they are applied. Thus crushing a frog's foot will stop 

 the heart, and adequate stimulation of most afferent nerves will 

 produce some amount of inhibition. 



The details of the reflex chain and the portion of the centre 

 concerned in the development of augmenting impulses have not 

 been worked out so fully as in the case of inhibitory impulses, 

 but there can be little doubt that the former like the latter are 

 governed by the central nervous system. 



160. So far we have been dealing with the heart of the 

 frog, but the main facts which we have stated regarding inhi- 

 bition and augmentation of the heart-beat apply also to other 



