302 IRREGULAR HEART BEAT. [BOOK i. 



as to stop the heart for a beat or two, the stoppage being fre- 

 quently followed by a temporary increase in the rapidity and force 

 of the beat. Such a passing failure of the heart beat, in its 

 sudden onset, in its brief duration, and in the reaction which fol- 

 lows, very closely resembles the complete but temporary inhibition 

 brought about by artificial stimulation of the vagus. And as we 

 have seen the inhibitory action of the vagus is especially prone to 

 be set going by afferent impulses passing up to the central ner- 

 vous system from the viscera. 



The effects however which we produce by our rough means of 

 direct stimulation of the trunk of the vagus do not afford a true 

 picture of the action of the cardie-inhibitory mechanism in the 

 living body ; we come nearer to this when we obtain inhibition in 

 a reflex manner. From the knowledge gained in this way we 

 may infer that the fainting which comes from pain, emotions and 

 the like, is due to the action of the inhibitory mechanism. 

 Several forms of irregular heart beat are probably brought about 

 by the same mechanism ; we may in this relation call to mind 

 that one effect of the action of the inhibitory fibres is to produce 

 not merely slowing or weakening but distinct irregularity of the 

 heart beat. 



Many observations shew that the cardio-inhibitory mechanism 

 may be affected by afferent impulses or otherwise in two different 

 ways. On the one hand the cardio-inhibitory centre may be 

 thrown into action, or when already in action may have its action 

 increased ; on the other hand if already in action, that action may 

 be lessened ; the inhibition may itself be inhibited. The division 

 of both vagus nerves in the dog affords an instance of the effect 

 on the heart of arresting previously existing inhibitory impulses. 

 Hence it becomes difficult in the complex living body to distin- 

 guish between an augmentation due to activity of the augmentor 

 mechanism and one due to suspension of the previously active 

 inhibitory mechanism. The two may probably be distinguished 

 by studying the details of the behaviour of the heart in the two 

 cases. Failing this it is difficult to say whether a case of that 

 irregularity of the heart which we call ' palpitation ' has been 

 brought about positively by the one mechanism or negatively by 

 the other. 



We must remember, moreover, that irregularity in the heart 

 beat in at least a large number of cases is the result not of ner- 

 vous influences from without, but of intrinsic events. For in- 

 stance, in many cases the irregularity of the heart beat is wholly 

 unaffected by atropin, and therefore cannot be due to vagus 

 action. It is very often the result of what we may call a dis- 

 ordered nutrition of the cardiac substance, though we cannot state 

 the exact nature of the disorder. 



169. We may repeat that the effect of inhibitory action is 

 to lessen the expenditure of energy and so to assist the heart for 



