984 PHYSIOLOGY 



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 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 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 tlio 

 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 sug- 

 gested 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 Engelmami . 

 The arguments against the ' neurogenic ' hypothesis may be summarised 

 as follows : 



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

 excited by various means to rhythmic contraction. When, in the living fro^. 

 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 tin* intraventricular pressuiv 

 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 .1 

 strip free from ganglion cells can be cut from the apex of the tortoi 

 ventricle (Fig. 432) which, on keeping in a moist chamber and moistening 

 occasionally with normal salt solution, enters into rhythmic contractions. 



(6) In the frog it is possible to excise the intcrauricular 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 

 rhythm. This experiment is still easier to carry out in the tortoise's heart 



