I 3 4 A MANUAL OF PHYSIOLOGY . 



proofs of this is that, if the sinus is heated the ventricle beats 

 much more rapidly in unison with the rapidly beating sinus and 

 auricles, while if the ventricle itself is heated no change takes place 

 in its rhythm. Now, if the ventricle responds to a constant 

 stimulus by rhythmical beats, the condition of the ventricular 

 tissue ought to affect the rate of its beat. In the mammalian 

 heart, too, an alteration in the temperature of a definite area of 

 the wall of the right auricle lying between the mouths of the 

 venae cavae, produces a change in the rate of the whole heart, 

 while no effect is caused by altering the temperature of any other 

 portion of the heart. It has already been stated that the 

 impulses from the nerve-cord which maintain the rhythm in the 

 Limulus heart are also discontinuous. 



Conduction and Co-ordination. The question of the con- 

 duction 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 contraction 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- 

 ventricular 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. 689) ; 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 characters 

 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 contrac- 

 tion is transferred from each division of the heart to the next by 

 a nervous mechanism whose action is timed with the very object 



