284 THE HEART-BEAT AND CARDIAC GANGLIA. [BOOK i. 



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

 entire ventricle when stimulated at the base 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. 



In these experiments then the various parts of the frog's heart 

 also form, as regards the power of spontaneous pulsation, a descend- 

 ing series : sinus venosus, auricles, entire ventricle, lower portions 

 of ventricle, the last exhibiting under ordinary circumstances no 

 spontaneous pulsations at all. 



155. Now we have seen ( 153) that these parts form 

 to a certain extent a similar descending series as regards the 

 presence of ganglia; at least so far that the ganglia are very 

 numerous in the sinus venosus, that they occur in the auricles, 

 and that while Bidder's ganglia are present at the junction of 

 the ventricle with the auricles, ganglia are wholly absent from 

 the rest of the ventricle. Hence on the assumption (which we 

 have already, 100, seen reason to doubt) that the nerve cells 

 of ganglia are similar in general functions to the nerve cells of 

 the central nervous system, the view very naturally presents 

 itself that the rhythmic spontaneous beat of the heart of the frog 

 is due to the spontaneous generation in the ganglionic nerve cells 

 of rhythmic motor impulses which passing down to the muscular 

 fibres of the several parts causes rhythmic contractions of these 

 fibres, the sequence and coordination of the beating of the several 

 divisions of the heart being the result of a coordination between 

 the several ganglia in regard to the generation of impulses. 

 Under this view the cardiac muscular fibre simply responds to the 

 motor impulses reaching it along its motor nerve fibre in the same 

 way as the skeletal muscular fibre responds to the motor impulses 

 reaching it along its motor nerve fibre; in both cases the muscular 

 fibre is as it were a passive instrument in the hands of the motor 

 nerve, or rather of the nervous centre (ganglion or spinal cord) 

 from which the motor nerve proceeds. And the view, thus based 

 on the fact of the frog's heart, has been extended to the hearts of 

 (vertebrate) animals generally. 



There are reasons however which shew that this view is not 

 tenable. 



For instance the lower two-thirds, or lower third or even the 



