THE CAUSATION OF THE HEART-BEAT 941 



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 functionally 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 pos- 

 sible 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 parts of the ventricle, where it is apparently 

 absent. From this fact it has be^n 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 ventricle, 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, how- 

 ever, 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 Oaskell 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 circulation of the blood is 

 maintained by the contraction of all the parts of the heart except the apex, 

 which never resumes its activity. If, however, the intra ventricular pressure 

 be raised by clamping the aorta the apex begins to beat at its own rhythm, 

 which is independent of the rhythm of the rest of the heart. Moreover a 

 strip can be cut from the apex of the tortoise's ventricle (Fig. 423), free from 

 ganglion-cells, which on keeping in a moist chamber and moistening occasion- 

 ally with normal salt solution enters into rhythmic contractions. 



(b) In the frog it is possible to excise the inter- auricular septum with 

 its ganglia, and a considerable portion of the ganglia in the sinus venosus and 

 at the base of the ventricles, without interfering in any way with the cardiac 



