CHAP, iv.] THE VASCULAR MECHANISM. 301 



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 

 mere tip of the frog's ventricle, that is to say parts which are 

 admitted not to contain nerve cells, may, by special means, be 

 induced to carry on for a considerable time a rhythmic beat, which 

 in its main features is identical with the spontaneous beat of the 

 ventricle of the intact heart. If such a part of the frog's ventricle 

 be tied on to the end of a perfusion cannula (Fig. 67), the portion 

 of the ventricular cavity belonging to the part may be adequately 

 distended and the part may at the same time be ' fed ' by making 

 a suitable fluid, such as blood, to flow through the cannula. It will 

 then be found that the portion of ventricle so treated will, after a 

 preliminary period of quiescence, commence to beat, apparently 

 spontaneously, and will continue so beating for a long period of 

 time. It may be said that in this case the distension of the 

 cavity and the supply of blood or other fluid act as a stimulus ; 

 but if so the stimulus is a continuous one, or at least not a 

 rhythmic one, and yet the beat is most regularly rhythmic. 



Then again the reluctance of the ventricle to execute spon- 

 taneous rhythmic beats is to a certain extent peculiar to the frog. 

 The ventricle of the tortoise for instance, the greater part of the 

 substance of which is as free from nerve cells as is that of the 



