THE VASCULAR MECHANISM. 201 



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 stand- 

 still at once. 



If during the inhibition the ventricle or other part of the heart be stimu- 

 lated directly for instance, mechanically by the prick of a needle a beat 

 may follow ; that is to say, the impulses descending the vagus, while inhibit- 

 in n 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 stand- 

 still 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 weaken- 

 ing is the more conspicuous result. 



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. 



144. 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 ( G. V., Fig. 74), certain fibres pass into it from 

 the sympathetic nerve of the neck, Sy., of the further connections of which 

 we shall speak presently. 



This being the case, we may expect that we should get different results 

 according as we stimulated (1) the vagus in the cranium before it was joined 

 by the sympathetic, (2) the sympathetic fibres before they join the vagus, 

 and (3) the vagus trunk containing the real vagus and the sympathetic fibres 

 added. What we have previously described re the ordinary results of 

 stimulating the mixed trunk, and these, as we have said, are not wholly 

 constant, though usually and in the main most distinct inhibitory results 

 follow. 



If we stimulate the sympathetic in the neck, as at Sy., Fig. 74, cutting 

 the nerve below, so as to block all impulses from passing downward, and only 

 allow impulses to pass up to the vagus and thence down the mixed vagus 

 trunk to the heart, we get very remarkable results. The beat of the heart, 

 instead of being inhibited, is augmented ; the beats are increased either in 

 frequency or in force, or most generally both in frequency and in force. 

 The effect is, perhaps, best seen when the heart before stimulation is beating 

 slowly and feebly ; upon stimulation of the cervical sympathetic the beats 

 at once improve in vigor and frequency ; indeed, a heart which, for one 

 reason or another, has almost ceased to beat may, by proper stimulation of 

 the sympathetic, be called back into vigorous activity. 



If, on the other hand, we stimulate the vagus before it has been joined 

 by the sympathetic fibres (and to insure the result not being marred by any 

 escape of the stimulating current on to the sympathetic fibres it is necessary 

 to stimulate the vagus within the cranium), we get pure and constant inhib- 

 itory results the beats are for a time wholly abolished, or are slowed, or are 

 weakened, or are both slowed and weakened. 



Obviously, then, the heart of the frog is supplied through the vagus by 

 two sets of fibres coming from the central nervous system, the one by the 



