CHAP, iv.] THE VASCULAR MECHANISM. 



291 



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

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

 duce inhibition. 



on 



off 



\ u 



\ v V 



FIG. 54. INHIBITION OF FROG'S HEART BY STIMULATION OF VAGUS NERVE. 



on marks the time at which the interrupted current was thrown into the vagus, 

 off when it was shut off. The time marker below marks seconds. The beats were 

 registered by suspending the ventricle from a clamp attached to the aorta and 

 attaching a light lever to the tip of the ventricle. 



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 pre- 

 paration, the standstill is complete, that is to say a certain num- 

 ber 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 pro- 

 longed, 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 



192 



