98 WARFARE IN THE HUMAN BODY 



of the whole heart. The vagus fibres are thus conceived 

 as actually depressant fibres, while the accelerator (or aug- 

 mentor) acts, it is said, in exactly the opposite way. Since 

 such views are founded on observations first made by the 

 Webers, which cannot be placed in the category of ordinary 

 reflex inhibition with substituted action, some further 

 examination of the deductions drawn may surely be 

 made with a view of determining whether the cardiac 

 vagus really plays the part assigned to it, and whether 

 the view stated above can throw light not only upon 

 ordinary " inhibition," but on the real nature of the cardiac 

 diastole. 



Although there is great reluctance on the part of physio- 

 logists to acknowledge that experiments, however great 

 their value, are often misleading, it is just as true to say so 

 as to say that what happens in vitro is not always repeated 

 in vivo. If it were, practical medicine would be less 

 uncertain than it is. But just as the living organism with 

 its unmeasured complexities thwarts and disappoints both 

 physician and pharmacologist, so the animal experimented 

 on, in conditions which are pathological and unnatural from 

 the very beginning, cannot always show, and cannot be 

 expected to show, the reactions due to natural stimuli when 

 subjected, probably under conditions of trauma, to stimuli 

 with which evolution has not made it acquainted. To say 

 so much is not to urge any vital objection to experiment, 

 but merely to caution those who, when they obtain interest- 

 ing results, believe they are physiological. If in general a 

 negative effect obtained by nerve stimulation requires 

 explanation, how much more is needed to make it credible 

 that evolution has contrived, by the mechanism of the 

 cardiac vagus, a means not only of weakening the organism, 

 but of actually bringing about its death ? 



