188 REFLEX INHIBITION. [BOOK i. 



clearer views as to that main event we cannot expect to under- 

 stand exactly how inhibition is brought about. The conception 

 of an inhibitory mechanism, in which certain of the fibres of the 

 vagus end, must be regarded as a temporary hypothesis, useful 

 only until we gain further light; and we have ventured to 

 dwell on so obscure a topic at so great a length only because 

 inhibition of the heart through the vagus is not only a factor 

 of immense importance in the general operations of the economy, 

 and plays so prominent a part in the action of many drugs, but 

 because it is a type of other inhibitory processes in the nervous 

 system and elsewhere, which, perhaps even more than itself, con- 

 tribute to render the working of the complicated machine of the 

 animal body, at once both uniform when regularity is required 

 and delicately responsive when variety is needed. 



Reflex inhibition. For it must not be thought that cardiac 

 inhibition by means of the vagus nerve is a mere experiment of 

 the laboratory; we have reason to think that it is an incident 

 continually recurring in daily life. For we have evidence 

 that the inhibitory action of the vagus may be brought about by 

 reflex action. If the abdomen of a frog be laid bare, and the 

 intestine be struck sharply, as 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 sympathetic chain be stimulated with the inter- 

 rupted current, cardiac inhibition is similarly produced. If in 

 these two experiments both vagi are divided, or the medulla 

 oblongata destroyed, inhibition 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 in- 

 hibition through one vagus may be brought about by stimulation 

 of the central end of the other. 



If the peritoneal surface of the intestine be inflamed, very 

 gentle stimulation of the inflamed surface will produce marked 

 inhibition; and in general the alimentary tract seems in closer 

 connection with the cardio-inhibitory centre than other parts of 

 the body : the injurious, sometimes fatal effects of a violent blow 

 on the stomach are known to all. But apparently stimuli if suffici- 

 ently powerful will through reflex action produce inhibition from 

 whatever be the part of the body to which they are applied. Thus 

 crushing a frog's foot will stop the heart. In ourselves the fainting 

 from emotion or from severe pain is the result of a reflex inhibition 

 of the heart, the afferent impulses in the one case at least, and 



