CHAP, iv.] THE VASCULAR MECHANISM. 183 



If a transverse incision be carried through the ventricle at 

 about its upper third, leaving the base of the ventricle still 

 attached to the auricles, the portion of the heart left in the body 

 will go on pulsating regularly, with the ordinary sequence of 

 sinus, auricles, ventricle, but the isolated lower two-thirds of the 

 ventricle will not beat spontaneously at all however long it be 

 watched. Moreover in response to a single stimulus such as an 

 induction-shock or a gentle prick it gives, not as in the case of the 

 entire ventricle or of the ventricle to which the auricles are attached, 

 a series of beats, but a single beat. 



Lastly, to complete the story we may add, that when the 

 heart is bisected longitudinally, each half continues to beat 

 spontaneously, with an independent rhythm, so that the beats of 

 the two halves are not necessarily synchronous, and this continuance 

 of spontaneous pulsations after longitudinal bisection may be seen 

 in the conjoined auricles and ventricle, or in the isolated auricles, 

 or in the isolated but entire ventricle. Moreover the auricles 

 may be divided in many ways and yet many of the segments 

 will continue beating; small pieces even may be seen under 

 the microscope pulsating, feebly it is true but distinctly and 

 rhythmically. 



The various parts of the frog's heart thus form, as regards 

 the power of spontaneous pulsation, a descending series: sinus 

 venosus, auricles, entire ventricle, lower portions of ventricle, 

 the last exhibiting under ordinary circumstances no spontaneous 

 pulsations at all. 



Now ganglia, containing nerve cells, are found in great abun- 

 dance in the sinus venosus, are seen in various parts of the auricles, 

 and occur as the so-called Bidder's ganglia at the junction of the 

 auricles and ventricle, from whence they also spread into the 

 upper part of the ventricle; in the lower two-thirds of the 

 ventricle they are entirely wanting. It is natural to infer from 

 this that the ganglia are in some way the agents of the spontaneous 

 pulsation. 



The uncertainty, and in most cases temporary character of the 

 pulsations, occurring with seeming spontaneity, in the auricles or 

 ventricle separated from the sinus venosus, have led many to the 

 opinion that these are not really spontaneous, but of the nature of 

 reflex acts, induced by some obscurely acting stimuli, and that 

 really spontaneous pulsations proceed only from the sinus venosus. 

 And a view has been generally adopted which teaches that the 

 spontaneous beats of the frog's heart are due to rhythmic nervous 

 impulses started in the ganglia of the sinus venosus and spreading 

 thence to other parts, the ganglia of the auricles and of the 

 auriculo-ventricular groove acting in subordination to those of 

 the sinus, or behaving under certain circumstances independently 

 as reflex centres, or performing other functions which we shall 

 have to speak of immediately as of a restraining or inhibitory 



