CHAP, iv.] THE VASCULAR MECHANISM. 345 



extrinsic nervous influences, may vary largely in consequence of 

 changes in its own metabolism, which in turn may result from 

 alterations in the blood-supply, or may have a deeper origin, and 

 be due to the fact that the cardiac substance, owing to failure in 

 its molecular organisation (a failure which may be temporary or 

 permanent), is unable to avail itself properly of the nutritive 

 opportunities afforded by a normal quantity of normal blood. 



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

 porarily or permanently irregular ; that many hearts go on beating 

 day after day, year after year, without any such irregularity 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 

 medulla oblongata, 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 temporary inhibition brought about by 

 artificial stimulation of the vagus. But these characters are not 

 essential to cardiac inhibition. For it must be remembered that 

 the central nervous system possesses, in the form of natural 

 nervous impulses of various origin, a means of stimulation far 

 finer, more delicate and more varied than anything we can effect 

 by our rough means of induction coils and electrodes. Thus in 

 many cases of fainting, the heart-beats, instead of stopping 

 abruptly, gradually die away or fade away it may be to an absolute 

 brief arrest, but more frequently merely down to a feebleness 

 which is insufficient to supply the brain with a quantity of blood 

 adequate to maintain consciousness, and then, in many cases, at 

 all events, are resumed, or recover strength gradually and quietly 

 without any boisterous reaction. In all probability all cases of 

 simple fainting, from emotion, pain, digestive troubles, &c., as 

 distinguished from the syncope of actual heart disease, are instances 

 of vagus inhibition, and though we cannot accurately reproduce 

 their varied phases by direct stimulation of the vagus trunk, 

 we may approach them more nearly by producing reflex inhibition, 

 as by mechanical irritation of the abdomen, see 159. 



Whether definite temporary irregularity is ever brought about 

 by means of the augmentor fibres, we have at present no clear 

 evidence ; but cases do occur of palpitation without previous 

 stoppage, cases in which a few hurried strong beats come on, pass 

 off, and are followed by feebler beats ; and these may possibly be 

 due to some transient influence of augmentor fibres thrown into 



