CHAP. iv.J THE VASCULAR MECHANISM. 



245 



and therefore governed by, the two vagus nerves, each of which 

 though apparently a single nerve contains, as we shall see, fibres 

 of different origin and nature. 



If while the beats of the heart of a frog are being carefully 

 registered an interrupted current of moderate strength be sent 

 through the vagus nerve, the heart is seen to stop beating. It 

 remains for a time in diastole, perfectly 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 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 vigour and frequency. If the duration of the stimulation 

 be very long, the heart may recommence beating while the stimula- 

 tion 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 com- 

 mencement of the application of the stimulus, gradually declining 

 afterwards ; 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 shew 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. 68, it will frequently be found that one beat at 



FIG. 68. INHIBITION OF FROG'S HEART BY STIMULATION OF VAGUS NERVE. 



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

 ojfwhen 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. 



least occurs after the 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. 



