290 INHIBITION OF THE BEAT. [BOOK i. 



sequence, what purposes do they serve ? But before we even 

 attempt to answer this question we must deal with the nervous 

 mechanisms by which the beat of the heart, thus arising spon- 

 taneously within the tissues of the heart itself, is modified and 

 regulated to meet the requirements of the rest of the body. 



The Government of the Heart-Beat by the Nervous System. 



157. It will be convenient to begin with the heart of the 

 frog, which as we have seen is connected with the central nervous 

 system through and therefore governed by the two vagi 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 one of the vagi, 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 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 vigour and frequency. If the duration of the current 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. 54, it will 'frequently be found that one beat at 

 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. 



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 recognisable 

 result by employing a single induction shock of moderate intensity 

 only. As we shall see later on ' natural ' nervous impulses descend- 



