

THE VASCULAR MECHANISM. 199 



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 140 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 would otherwise be common to the two), and yet 

 not offer so much resistance as to prevent the contraction wave passing ulti- 

 mately 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 sequence, what 

 purposes do they serve ? But before we even attempt to answer this 

 question we must deal with the nervous mechanisms by which the beat 

 of the heart, thus arising spontaneously within the tissues of the heart 

 itself, is modified and regulated to meet the requirements of the rest of 

 the body. 



The Government of the Heart-beat by the Nervous System. 



143. It will be convenient to begin with the heart of the frog, which, 

 as we have seen, is connected with the central nervous system through 

 and therefore governed by the two vagi nerves, each of which though 

 apparently a single nerve contains, as we shall see, fibres of different origin 

 and nature. 



If while the beats of the heart of a frog are being carefully registered an 

 interrupted current of moderate strength be sent through one of the vagi, 

 the heart is seen to stop beating. It remains for a time in diastole, perfectly 



