CHAP, iv.] THE VASCULAR MECHANISM. 309 



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. 70, 

 cutting the nerve below so as to block all impulses from passing 

 downwards, and only allow impulses to pass up to the vagus and 

 thence down the mixed vagus trunk to the heart, we get very re- 

 markable 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 vigour 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 ensure 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 inhibitory 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 vagus proper and the other by the cervical sym- 

 pathetic nerve, and these two sets have opposite and antagonistic 

 effects upon the heart. 



The one set, those belonging to the vagus proper, are inhibitory ; 

 they weaken the systole and prolong the diastole, the effect with a 

 strong stimulation being complete, so that the heart is for a time 

 brought to a standstill. Sometimes the slowing, sometimes the 

 weakening is the more prominent. When the nerve and the heart 

 are in good condition, it needs only a slight stimulus, a weak 

 current, to produce a marked effect, and it may be mentioned that 

 the more vigorous the heart, the more rapidly it is beating, the 

 easier is it to bring about inhibition. Although as we have said 

 the effect is at its maximum soon after the beginning of stimula- 

 tion, a very prolonged inhibition may be produced by prolonged 

 stimulation ; indeed by rhythmical stimulation of the vagus the 

 heart may be kept perfectly quiescent for a very long time and 

 yet beat vigorously upon the cessation of the stimulus. In other 

 words, the instruments of inhibition, that is, the fibres of the vagus 

 and the part or substance of the heart upon which these act to 

 produce inhibition, whatever that part or substance may be, are 

 not readily exhausted. Further, the inhibition when it ceases is, 

 frequently at all events, followed by a period of reaction, during 

 which the heart for a while beats more vigorously and rapidly 

 than before. Indeed the total effect of stimulating the vagus 



