THE CAUSATION OF THE HEART-BEAT 1059 



slowly and then more rapidly, but never attaining the rate of the sinus: 

 The auricles beat first, and then the ventricle. If now the ventricle 

 be cut away by an incision in the auriculo-ventricular groove from 

 the auricles, the latter go on beating ; while the former, after a few 

 beats due to the excitation of the incision, stops still, and only after a 

 considerable time may begin again to contract very slowly. 



On the other hand, a ventricle -apex preparation (that is to say, the 

 lower two-thirds of the ventricle separated functionally from the rest 

 of the heart) never beats again under normal circumstances. To single 

 stimuli it responds with a single beat, not with a series of beats as the 

 whole heart does. If the lower third of the ventricle be separated func- 

 tionally in the living frog by crushing the ring of tissue between it and 

 the upper third, it never gives a spontaneous beat again, although it is 

 under the most normal conditions possible in the circumstances. There 

 is thus a descending scale of automatic power in the different parts of 

 the frog's heart from the sinus, where it is highest, to the lower part 

 of the ventricle, where it is apparently absent. From this fact it has 

 been thought that the automaticity of the frog's heart is dependent on 

 the ganglia present in it. The contraction was supposed to be started 

 by impulses proceeding from the sinus ganglion. If this were cut off, 

 Bidder's ganglia, or the scattered cells in the upper third of the ven- 

 tricle, could, it was thought, take up its task of originating impulses. 

 The muscle-cells under this hypothesis act as the servants of the 

 ganglion-cells, just as the voluntary muscles wait on the commands of 

 the cells in the spinal cord and brain. 



The view that the ganglion-cell sends out rhythmic impulses had, 

 however, to be discarded when it was discovered that the muscle 

 forming the lower third of the ventricle either of the frog or the tortoise, 

 though free from ganglion-cells, could be excited by various means to 

 rhythmic contractions. Thus it could be set into rhythmic action 

 when supplied with salt solution under pressure, through a perfusion 

 cannula, or when excited by the passage of a constant current or of 

 weak induction shocks. The fact that the heart muscle responded to 

 continuous stimulation by a rhythmic discharge suggested that the 

 function of the ganglion-cells was to furnish a constant stimulation to 

 the muscle-cells and so maintain these in rhythmic activity. 



The theory of the ganglionic origin of the cardiac rhythm was 

 seriously affected by a series of researches carried out by Gaskell and 

 by Engelmann. The arguments against the ' neurogenic ' hypothesis 

 may be summarised as follows : 



(a) The cardiac muscle, free from any ganglion-cells whatsoever, 

 can be excited by various means to rhythmic contraction. When, in 

 the living frog, the apex of the ventricle is crushed off from the base 

 so as to leave only material continuity between the two parts, the 



