1 84 THE CONTRACTION OF CARDIAC MUSCIE. 



other ; we must, in fact, be able to obtain a reversal of beats, as in 

 the heart of the Ascidian. The ease with which a reversal of the 

 sequence of the beats of the heart can be observed is well exemplified 

 in the hearts of the skate x and tortoise, and in both cases it is clearly 

 seen that the only factor requisite is to start a rhythm with a rate 

 quicker than that of the natural sinus rhythm ; that, as might be 

 expected, the heart-beat starts from the place where the rate of the 

 spontaneous contractions is quickest. If the heart of a large skate is 

 taken out of the body, and the auricle slit up so as to make a long strip 

 of tissue, then it is very easy to see the peristaltic wave travel along 

 this strip to the ventricle, and reappear after the quick, apparently 

 simultaneous contraction of the ventricle as a peristaltic wave slowly 

 traversing the long muscular conus arteriosus. If, then, the conus 

 arteriosus be stimulated by any pointed instrument, it is excited to a 

 rhythm of excitation, the rate of which at first may be quicker than that 

 of the sinus rate. Immediately the peristaltic wave is reversed, and all 

 the beats can be seen travelling in the reverse direction along the 

 elongated auricular strip, until, with the subsidence of the stimulation, 

 the rate of the conus beat becomes just less than that of the sinus beat, 

 when forthwith all the contractions pass along in the normal direction. 

 The same thing can be seen in the tortoise 2 by starting a spontaneous 

 beat in the ventricle and auriculo-ventricular junction with a rhythm 

 quicker than that of the sinus. This can be obtained as follows : The 

 auricle is slit up, not sufficiently to cause any blocking, but so as to allow 

 every contraction to travel easily, and a cannula is placed in the aorta 

 in connection with a pressure bottle filled with a blood solution. The 

 pressure of the fluid closes the aortic valves, and forces blood into the 

 coronary system, which is developed to a certain extent in the tortoise. 

 This blood drops out of the cut coronary veins, and does not reach the 

 sinus. In this way the rhythmical tissue of the auriculo-ventricular ring is 

 brought nearer the condition of spontaneous rhythmical activity, but 

 still the sinus holds its own. Now cool down the sinus, which, owing to 

 the conditions of the experiment, can easily be done without affecting 

 the ventricle, the sinus beat gets slower and slower, and at last every 

 contraction suddenly passes in the reverse direction, first V, then Av, 

 then As, and finally sinus. If now the sinus be cut away, there is no 

 Stannius standstill, but the ventricle and auricle continue to beat with 

 the same rate of rhythm as before removal of the sinus. 



We come, then, to the conclusion that the beat of the heart starts 

 from that part which is most rhythmical, i.e. which beats spontaneously 

 at the quickest rate, and travels as a wave of contraction over the rest 

 of the heart at rates of speed which vary in different parts according 

 to the nature of the muscular tissue. 



So far I have not attempted to decide the question whether the sequence 

 in question is due to the actual passage of a wave of contraction over muscular 

 fibres, or "whether, owing to the presence of a nerve plexus coincident with the 

 muscular structures, or to the nervous nature of the muscular substance itself, 

 it is possible to conceive of a stimulation wave travelling along the neuro- 

 muscular tissue without an accompanying contraction. Granted, in fact, that 

 the nerve trunks and nerve cells are not concerned in the sequence, is it still 

 possible that the ventricle may contract, in response to a contraction of the 

 sinus, without an intermediate contraction of the auricle 1 



1 Gaskell, op. clt., p. 78. - Op. cit., p. 88. 



