SOME FEATURES OF THE CIRCULATION. 239 



traction, may distend the ventricle to a greater or to a less extent, and so 

 produce a stronger or weaker ventricular systole. 



175. Still more efficient, perhaps, as a direct governor of the heart's 

 beat is the quality and quantity of blood passing in the mammal through 

 the coronary arteries and regulating the nutrition of the cardiac substance. 

 In the absence of all interference by inhibitory and augmentor fibres the 

 heart will continue beating with a certain rhythm and force, determined by 

 the metabolism going on certainly in its muscular, and possibly to a certain 

 extent also in its nervous elements. We have seen that the energy set free 

 in an ordinary skeletal muscle, in response to a stimulus, may vary from 

 nothing to a maximum according to the metabolism going on according to 

 the nutritive vigor of the muscular fibres. The spontaneous rhythmic beat 

 of the cardiac substance may be regarded as the outcome of a metabolism 

 more highly pitched, more elaborate, of a higher order than that which 

 simply furnishes the ordinary skeletal fibre with mere irritability toward 

 stimuli. All the more, therefore, may the beat be expected to be influenced 

 by any change in the metabolism of the cardiac substance, and so by any 

 change in the blood which furnishes the basis for that metabolism. Hence 

 the beat of the heart, quite apart from 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 organization (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. 



176. 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 irregularity is a striking proof of the com- 

 plete 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 im- 

 pulses which ascending from the mucous membrane of the stomach along cer- 

 tain 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 rapid- 

 ity and force of the beat. Such a passing failure of the heart-beat, in its sud- 

 den onset, in its brief duration, and in the reaction which follows, very 

 closely resembles the temporary inhibition brought about by artificial stimu- 

 lation of the vagus. But these characters are not essential to cardiac inhibi- 

 tion. For it must be remembered that the central nervous system possesses, 

 in the form of natural nervous impulses of various origin, a means of stimu- 

 lation 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 faint- 

 ing from emotion, pain, digestive troubles, etc., 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 145.) 



