144 THE CIRCULATION OF THE BLOOD AND LYMPH 



especially the conclusions as to rhythm, conduction, and co-ordina- 

 tion. Nevertheless the Limulus heart affords one absolutely un- 

 ambiguous example of a heart whose rhythmical beat is sustained 

 by nervous impulses. And in the case of the higher animals also 

 facts may be adduced in favour of the neurogenic origin of the beat. 

 The isolated auricular appendices of the mammalian heart, in which 

 no ganglion-cells have been found, refuse to beat spontaneously. 

 If in the frog we divide the sinus, which is conspicuously rich in 

 ganglion-cells, from the lower portion of the heart, it continues to 

 pulsate. A fragment from the base of the ventricle will go on 

 contracting if it includes Bidder's ganglion, but not otherwise. We 

 cut off the lower two-thirds of the frog's ventricle, the so-called 

 apex preparation, which either contains no ganglion-cells or is 

 relatively poor in them, and it remains obstinately at rest. Further, 

 if, without actually cutting off the apex, we dissever it physiologically 

 from the heart by crushing a narrow zone of tissue midway between 

 it and the auriculo- ventricular groove, we abolish for ever its power 

 of spontaneous rhythmical contraction. The frog may live for 

 many weeks, but in general the apex remains in permanent diastole. 

 It can be caused to contract by an artificial stimulus, but it neither 

 takes part in the spontaneous contraction of the rest of the heart, 

 nor does it start an independent beat of its own. 



What can be simpler than to assume that the sinus beats because 

 it has numerous ganglion-cells in its walls, and that the apex refuses 

 to beat because it has comparatively few or none ? Could we pick 

 out the nerve-cells from the sinus, without injuring the muscular 

 tissue, as easily as we can extirpate the median nerve-cord in 

 Limulus, we may well suppose that it would lose its power of auto- 

 matic contraction. And although, if we pursue our investigations 

 a little farther, facts may emerge which seem to contradict the 

 neurogenic hypothesis, the contradiction is usually only apparent. 

 Let us inquire, for instance, what happens to the auricles and ven- 

 tricle of the frog's heart when the sinus is cut off. The answer is 

 that as a rule, while the sinus goes on beating, the rest of the heart 

 comes to a standstill, in spite of the numerous ganglion-cells in the 

 auricular septum and the auriculo- ventricular groove. Not only 

 so, but if the ventricle be now severed from the auricles by a section 

 carried through the groove, it is the former, poor in nerve-cells 

 though it be, which will usually first begin to beat. We shall again 

 have to discuss this experiment (p. 165). It, at any rate, cannot be 

 interpreted as proving that the automaticity of the heart does not 

 depend upon the presence of ganglion-cells. For although a portion 

 of the heart rich in ganglion-cells may, under the circumstances 

 mentioned, refuse for a time to beat, there is good evidence that 

 this is due either to a peculiar condition called inhibition into which 

 the muscular tissue or the nerve-cells of the lower portions of the 



