200 



THE VASCULAR MECHANISM. 



motionless and flaccid ; all the muscular fibres of the several chambers are 

 for the time being in a state of relaxation. The heart has been inhibited 

 by the impulses descending down the vagus from the part of the nerve 

 stimulated. 



If the duration of the stimulation be short and the strength of the current 

 great, the standstill may continue after the current has been shut off; the 

 beats when they reappear are generally at first feeble and infrequent, but 

 soon reach or even go beyond their previous vigor and frequency. If the 

 duration of the current be very long, the heart may recommence beating 

 while the stimulation is still going on, but the beats are feeble and infrequent 

 though gradually increasing in strength and frequency. The effect of the 

 stimulation is at its maximum at or soon after the commencement of the 

 application of the stimulus, gradually declining afterward ; but even at the 

 end of a very prolonged stimulation the beats may still be less in force or in 

 frequency, or in both, than they were before the nerve was stimulated, and 

 on the removal of the current may show signs of recovery by an increase in 

 force and frequency. The effect is not produced instantaneously ; if on the 

 curve the point be exactly marked when the current is thrown in, as at on, 

 Fig. 73, it will frequently be found that one beat at least occurs after the 



FIG 73. 



A/\M) 



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. 



current has passed into the nerve; the development of that beat has taken 

 place before the impulses descending the vagus have had time to affect the 

 heart. 



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 recognizable result by employing a single induction-shock of moderate 

 intensity only. As we shall see later on, "natural" nervous impulses 

 descending 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 maybe 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 infrequently happens that even strong stimulation of the vagus on one 



