THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 147 



of the excitation over the heart and the co-ordination of its parts 

 is in the same position as the question of the automatism and 

 rhythmicity. In the horseshoe crab, as already remarked, the 

 mechanism appears to be a nervous one. In higher hearts, on the 

 other hand, facts have been discovered which favour each of the 

 rival hypotheses. In the frog's heart the probability that the con- 

 traction wave is propagated from fibre to fibre of the muscle without 

 the intervention of nerves has been much insisted upon, since the 

 muscular tissue, although presenting certain variations in its 

 character in the different divisions of the heart and at their junctions, 

 forms a practically continuous sheet over the whole organ from 

 base to apex. In support of this view has been brought forward 

 the observation that the delay of the wave at the auriculo- ventricu- 

 lar groove is much greater than it ought to be if the excitation were 

 transmitted by nerves, since the velocity of the nerve-impulse is 

 exceedingly great (p. 793) ; and the further observation that, when 

 the ventricle is caused to contract by artificial stimulation of the 

 auricle, this delay is appreciably greater when the stimulus is applied 

 as far from the ventricle as possible than when it is applied as near 

 to it as possible. The delay has been attributed to the ' embryonic ' 

 character of the muscular tissue at the junction of the sinus with the 

 auricles and of the auricles with the ventricles. But it has never 

 been demonstrated that muscular fibres with the histological char- 

 acters described do, as a matter of fact, conduct the contraction 

 wave so much more slowly than the other cardiac muscular fibres. 

 It is just as probable, and indeed more so, that, whether the con- 

 traction travels in any particular division of the heart directly from 

 muscle-fibre to muscle-fibre or not, the impulse to contraction is 

 transferred from each division of the heart to the next by a nervous 

 mechanism whose action is timed with the very object of securing 

 a certain interval between the systoles of successive divisions. In 

 any case, since we know that the velocity of the nerve-impulse is 

 very different in different varieties of nerves, the question cannot be 

 decided by general arguments of this kind. In Limulus, as a matter 

 of fact, the velocity in the intrinsic heart nerves is only one-tenth as 

 great as in the ordinary motor (limb) nerves of the animal (Carlson). 

 In the mammalian heart the alleged absence of muscular con- 

 nection between the auricles and ventricles was long the foundation 

 of the general belief that the link was a nervous one. Certainly 

 there is no dearth of nerves which might serve as such a bridge. 

 But it has been shown (Kent, His, etc.) that in the mammalian 

 heart, too, a slender band of muscular fibres, arising at a definite 

 point (the auriculo- ventricular nodeLnear the coronary sinus on the 

 right side of the interaurimlar septum heiow ihe lossaovalisTpasses^ 

 forwards and downwards through the fibrous ring between the 

 auricles and ventricles under the septa! cusp of the tricuspid valve. 

 It then divides into two branches, one for each ventricle, which run 



