CHAP, iv.] THE VASCULAR MECHANISM. 307 



The stimulus need not necessarily be the interrupted current ; 

 mechanical, chemical or thermal stimulation of the vagus will 

 also produce inhibition ; but in order to get a marked effect it is 

 desirable to make use of not a single nervous impulse but a series 

 of nervous impulses ; thus it is difficult to obtain any recognisable 

 result by employing a single induction shock of moderate intensity 

 only. As we shall see later on ' natural' nervous impulses descend- 

 ing the vagus from the central nervous system, and started there, 

 by afferent impulses or otherwise, as parts of a reflex act, may 

 produce inhibition. 



The stimulus may be applied to any part of the course of the 

 vagus from high up in the neck right down to the sinus ; indeed 

 very marked results are obtained by applying the electrodes 

 directly to the sinus where as we have seen the two nerves plunge 

 into the substance of the heart. The stimulus may also be applied 

 to either vagus, though in the frog, and some other animals, one 

 vagus is sometimes more powerful than the other. Thus it not 

 unfrequently happens that even strong stimulation of the vagus on 

 one side produces no change of the rhythm, while even moderate 

 stimulation of the nerve on the other side of the neck brings the 

 heart to a standstill at once. 



If during the inhibition the ventricle or other part of the heart 

 be stimulated directly, for instance mechanically by the prick of a 

 needle, a beat may follow ; that is to say, the impulses descending 

 the vagus, while inhibiting the spontaneous beats, have not wholly 

 abolished the actual irritability of the cardiac tissues. 



With a current of even moderate intensity, such a current for 

 instance as would produce a marked tetanus of a muscle-nerve 

 preparation, the standstill is complete, that is to say, a certain 

 number of beats are entirely dropped ; but with a weak current 

 the inhibition is partial only, the heart does not stand absolutely 

 still but the beats are slowed, the intervals between them being 

 prolonged, or weakened only without much slowing, or both 

 slowed and weakened. Sometimes the slowing and sometimes the 

 weakening is the more conspicuous result. 



158. It sometimes happens that, when in the frog the vagus 

 is stimulated in the neck, the effect is very different from that 

 just described ; for the beats are increased in frequency, though 

 they may be at first diminished in force. And, occasionally, the 

 beats are increased both in force and in frequency : the result 

 is augmentation, not inhibition. But this is due to the fact 

 that in the frog the vagus along the greater part of its course is a 

 mixed nerve and contains fibres other than those of the vagus proper. 



If we examine the vagus nerve closely, tracing it up to the 

 brain, we find that just as the nerve has pierced the cranium, 

 just where it passes through the ganglion (GV, Fig. 70), certain 

 fibres pass into it from the sympathetic nerve of the neck, Sy, of 

 the further connections of which we shall speak presently. 



202 



