368 IRREGULAR HEART BEAT. [BOOK i. 



or from excess of one or more of the normal constituents, may 

 unfavourably or, it may be, favourably affect the heart beat, by 

 directly influencing the cardiac tissues through the coronary 

 arteries. These changes in the blood may of course also work 

 upon the heart through the central nervous system, and this 

 indirect effect may possibly be different from the direct effect. 

 Thus, when the breathing is interfered with, the too highly 

 venous blood, while it acts directly on the cardiac tissues and that 

 unfavourably, also stimulates the cardio-inhibitory centre, whereby 

 the heart is slowed and its expenditure of energy lessened. 



190. As is well known, the beat of the heart may become 

 temporarily or permanently irregular. That many hearts go on 

 beating day after day, year after year, without any such irregu- 

 larity is a striking proof of the complete balance which usually 

 obtains between the several factors of which we are speaking. 

 Sometimes such cardiac irregularities, those of a transient nature 

 and brief duration, are the results of extrinsic nervous influences. 

 Some events taking place in the stomach, for instance, give rise to 

 afferent impulses which ascending from the mucous membrane of 

 the stomach along certain afferent fibres of the vagus to the 

 spinal bulb, so augment the action of the cardio-inhibitory centre 

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

 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 follows, 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 nervous 

 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 cardio-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 



