CHAP, iv.] THE VASCULAR MECHANISM. 369 



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 dis- 

 tinguish 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 

 nervous influences from without, but of intrinsic events. For 

 instance, 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 product of a disordered nutrition of 

 the cardiac substance. The normal nutrition sets the pace of the 

 normal rhythm. We cannot explain how this comes about; nor 

 can we explain why in one individual the normal pace is set as 

 low as 50, or even 30 beats a minute, and in another as high as 90 

 a minute, or even more, while in most persons it is about 70 a 

 minute. The slower or the quicker pace, though not normal to 

 the species, must be considered as normal to the individual, for it 

 may be kept up through long years in an organism capable of 

 carrying on a normal man's duties and work. So long as we 

 cannot explain these differences we cannot hope to explain how it 

 is that a disordered nutrition brings about an irregular heart beat, 

 either the more regular irregularity of a " dropping" pulse, that is 

 a failure of sequence rather than an irregularity, or a more dis- 

 tinctly irregular rhythm. We may, however, distinguish two 

 kinds of irregularity ; one, in which, in spite of all favourable 

 nutritive conditions, the cardiac substance cannot secure, even 

 perhaps for a minute, a steady rhythm ; and another in which the 

 rhythm, though normal under ordinary circumstances, is, so to 

 speak, in a condition of unstable equilibrium, so that a very slight 

 change in conditions, too much or too little blood, or some small 

 alteration in the composition of the blood, or the advent of some, 

 it may be slight, nervous impulse, augmentor or inhibitory, 

 developes a temporary irregularity. 



191. No one thing, perhaps, concerning the heart is more 

 striking than the fact that a heart which has gone on beating for 

 many years, with only temporary irregularities, and those few and 

 far between, a heart which must therefore have executed with long- 

 continued regularity many millions of beats, should suddenly, 

 apparently without warning, after a brief flickering struggle, cease 

 to beat any more. But we must remember that each beat is 

 an effort, an effort moreover, which, as we have seen ( 155), is 

 F. 24 



