184 INHIBITION OF THE BEAT. [BOOK i. 



character. And the same view with possibly some slight modi- 

 fications has been supposed to hold good for the hearts of all 

 vertebrate animals. 



Facts however are met with which appear to oppose this con- 

 ception. If the "perfusion" cannula previously described be 

 introduced into a frog's ventricle and secured by a ligature 

 carried round the ventricle some little distance below the base, the 

 lower part of the ventricle remains motionless and free from 

 pulsations in the same way as when it has been removed by 

 an incision. If however the cavity be regularly supplied with 

 serum or diluted blood (that of the rabbit being practically the 

 most useful), after a longer or shorter time, this portion of the 

 ventricle begins to pulsate with a more or less regular rhythm 

 and will continue these apparent spontaneous beats for an almost 

 indefinite time. It is usual to explain these pulsations, which 

 may be witnessed even when only the extreme tip of the ventricle 

 is tied on to the cannula, as not really spontaneous but as excited 

 by the serum or dilute blood, supplied under pressure, acting as a 

 stimulus; such an explanation is however hardly satisfactory. 

 Then again, though it is quite true that the beats of an isolated 

 frog's ventricle are uncertain and temporary, so much so as 

 perhaps to justify the view that they are not really spontaneous, 

 the isolated ventricle of the tortoise beats with such regularity and 

 for so long a time, that it seems almost impossible to avoid the 

 conclusion that in this animal, at all events, the ventricle by itself 

 possesses a real power of spontaneous pulsation. Moreover even 

 in the frog, section at various points, of the nerves with which the 

 ganglia are connected, may be effected and indeed Bidder's ganglia 

 carefully extirpated, without the natural sequence of beat of the 

 several parts being changed. And careful investigation has dis- 

 closed many other facts, which we cannot discuss here but which 

 go far to shew that the generation of the beat of the heart is 

 a very complex matter indeed. While we must admit that the 

 ganglia of the sinus venosus (in the frog, or what corresponds to 

 these in other animals) are prepotent in the work of producing 

 the beat, our knowledge will not at present allow us to make 

 a definite and consistent statement as to what it is they exactly 

 do, or as to the share in generating and carrying out the beat, 

 which is taken by the other ganglia, and their respective nerves, or 

 by the muscular fibres themselves. 



Inhibition of the Beat. The beat of the heart may be 

 stopped or checked, i.e. may be inhibited by efferent impulses 

 descending the vagus nerve. 



If while the beats of the heart of a frog are being care- 

 fully registered (Fig. 40) 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 



