CHAP, iv.] THE VASCULAR MECHANISM. 305 



into sinus beat, auricle beat, ventricle beat instead of sweeping 

 over the whole heart as a continuous wave ? these and allied 

 questions touch problems concerning which our knowledge is at 

 present too imperfect to render any discussion profitable here. 

 We may however venture to say that the phenomenon in question 

 cannot be explained by an appeal to the grosser features of the 

 arrangement of ganglia and nerves which we described in 153. 



156. In the above we have dealt chiefly with the heart of s 

 the cold blooded animal, but so far as we know the same general 

 conclusions hold good for the mammalian heart also. There is, it 

 is true, in the mammal, no prepotent sinus venosus, but as in the 

 frog the auricles are dominant, and their beat determines the beat 

 of the ventricles. Even more clearly than in the frog however, the 

 ventricles, though they normally follow the auricles in their beat, 

 being initiated as it were by them, possess an independent 

 rhythmic power of their own. By a mechanical contrivance all 

 conduction of nervous or muscular impulses between the auricles 

 and ventricles may be abolished, though the blood may continue 

 to flow from the cavities of the former to those of the latter. 

 When this is done the ventricles go on beating rhythmically, 

 but at a rate which is quite independent of that of the auricular 

 beats. In one respect however the mammalian heart seems at 

 first sight quite different from the heart of the frog. In the 

 latter muscular continuity is provided between the sinus venosus 

 and the auricles, between the auricles and the ventricle ; this 

 muscular continuity it may be argued is, without the aid of any 

 distinct nervous paths, sufficient for the propagation of the beat 

 along the several parts. In the mammalian heart the connective 

 tissue rings w T hich separate the auricles from the ventricles seem 

 to form complete breaks in the muscular continuity between 

 the upper and lower chambers, and to necessitate nervous ties for 

 carrying on the beat from the former to the latter. But it 

 would appear that even in the highest mammals, the ring in 

 question is broken by bundles of muscular fibres passing between 

 the auricles and ventricles ; and it may be argued that these 

 afford sufficient muscular continuity to justify the view that 

 the beat of the mammalian heart is carried out in a manner 

 not essentially different from that which obtains in the frog or 

 the tortoise. 



We may now turn to 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 require- 

 ments of the rest of the body. 



The Government of the Heart Beat by the Nervous System. 



157. It will be convenient to begin with the heart of the 

 frog, which as we have seen is connected with the central nervous 



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