CHAP, iv.] THE VASCULAR MECHANISM. 289 



and we have at present no satisfactory indications of the way in 

 which they do or may intervene. 



Two questions naturally suggest themselves here. The first is, 

 Why does the cardiac cycle begin with the sinus beat ? We have 

 previously, 15-t, given the evidence that the sinus has a greater 

 potentiality of beating than the other parts ; in and by itself it 

 beats more readily and with a quicker rhythm than the other 

 parts. When we ask the further question, why has it this greater 

 potentiality ? the only answer we can at present give is that it is 

 inborn in the substance of the sinus. The problem is somewhat 

 of the same kind as why the heart of one animal beats so much 

 quicker than that of another. All we can say at present is that 

 the rate is the outcome of the molecular constitution of tissue, 

 without being able to define that molecular constitution. 



The second question is, Why does not the contraction wave 

 starting at the sinus spread as a continuous wave over the whole 

 heart ? why is it broken up into sinus beat, auricle beat, ventricle 

 beat ? We may here call to mind the fact mentioned in 153, of 

 the existence, more or less marked in all hearts, and well seen in 

 the heart of the tortoise, of a muscular ring or collar between the 

 sinus and the auricle, and of a similar ring between the auricle 

 and ventricle. The muscular tissue in these rings seems to be of 

 a somewhat different nature from the muscular tissue forming the 

 body of the sinus, or of the auricle or of the ventricle. If we 

 suppose that this tissue has a low conducting power, it may offer 

 sufficient resistance to the progress of the contraction to permit 

 the sinus for example to carry out or to be far on in the development 

 of its beat, before the auricle begins its beat (and thus bisect so to 

 speak the beat which otherwise would be common to the two), and 

 yet not offer so much resistance as to prevent the contraction wave 

 passing ultimately on from the sinus to the auricle. We may in 

 the tortoise by careful clamping or section of the auricle in its 

 middle, by which an obstacle to the contraction wave is introduced, 

 bisect the single auricular beat into two beats, one of the part 

 between the sinus and the obstacle, and another between the 

 obstacle and the ventricular. We may thus consider the breaking 

 up the primitive unbroken peristaltic wave of contraction from 

 sinus to bulbus to be due to the introduction of tissue of lower 

 conducting power at the junctions of the several parts. 



We do not say that this is the complete solution of the problem, 

 but it at least offers an approximate solution; and here as elsewhere 

 we have no satisfactory evidence of nervous elements being main 

 factors in the matter. 



In the above we have dealt chiefly with the heart of the cold- 

 blooded animal, but as far as we know the same conclusions hold 

 good for the mammalian heart also. 



The question now arises, If the ganglia are not the prime cause 

 of the heart's rhythmic beat, or of the maintenance of the normal 



F. 19 



