1 86 THE CONTRACTION OF CARDIAC MUSCLE. 



takes place in muscular rather than in nervous tissue, so that when he con- 

 siders that his experiments prove that the ventricle may contract after the sinus 

 without an intermediate contraction of the auricles, he still considers at the 

 same time that the stimulus to contraction of the ventricular muscle reaches 

 the ventricle by way of the muscular tissue of the auricle, although the mole- 

 cular change in the auricular muscle is not sufficient to cause a change of form 

 in it. 



Such a conclusion is greatly strengthened by Fletcher's l recent observation 

 that water rigor in striated muscle does not cause a discharge of C0 2 similar 

 to heat rigor, and by Biedermann's observation that the electromotive pheno- 

 mena of water-clotted muscle are not at all comparable 'with those of true 

 rigor, since the difference of potential between a normal and water-clotted part 

 of the muscle is insignificant. Both these observations point to the conclusion 

 that the action of distilled water upon the muscle is physical and not chemical ; 

 that therefore the wave of contraction would travel along the water-clotted 

 auricle in Engelmann's experiment without showing any alteration of form, 

 just as it would along an auricle which was mechanically prevented from 

 altering its form. 



Considering, then, the difficulty of deciding whether in any given case a 

 contraction does or does not take place, we may sum up this section by saying 

 that in all cases the sequence of the contractions of the different parts of the 

 heart is most probably due to the passage of a wave of contraction along 

 muscular tissue, even though under certain conditions this contraction may be 

 so small as not to be visible by ordinary methods of investigation. 



The causation of the beat of the heart and of the sequence of the 

 contractions of the separate cavities is the same in warm-blooded 

 as in cold-blooded animals. A considerable amount of very difficult 

 and laborious experimental investigation has been carried out by various 

 observers upon the mammalian heart on the assumption that it is not 

 safe to draw the conclusion that what is known to be true of the cold- 

 blooded heart will necessarily be true of the warm-blooded ; and it has 

 even been argued again and again, that although it may be proved up to 

 the hilt that the beat of the frog's heart is due to the rhythmical pro- 

 perties of its muscular tissue, yet the beat of the dog's heart may be 

 totally different, and may be due to the presence of special nerve centres. 

 This view of a fundamental difference between the innervation of the 

 hearts of the cold- and warm-blooded animals has received especial 

 support from the supposed impossibility in the case of warm-blooded 

 animals, of attributing the sequence of ventricular upon auricular beat 

 to the passage of a peristaltic wave over muscular tissue at the auriculo- 

 ventricular junction. The explanation of the sequence of beats in 

 the mammal must, it is said, be due to special nervous arrangements, 

 because the muscular tissue of the auricles is absolutely disconnected 

 from that of the ventricles. As might have been anticipated, all recent 

 work on the mammalian heart tends more and more strongly to prove 

 that there is no fundamental difference between it and the cold-blooded 

 heart, any difference there may be being a difference in degree, not 

 in kind. 



Thus the observations of Kent, 2 and of Krehl and Eomberg, 3 have 

 shown that there is muscular continuity between the auricles and 

 ventricles quite sufficient to account for the passage of a contraction 



1 Journ. PhysioL, Cambridge and London, 1897, vol. xxiii. p. 85. 



- Ibid., 1892, vol. xiv. p. 223. 



3 Arch.f. cxper. Path. u. Pharmakol., Leipzig, 1892, Bd. xxx. S. 71. 



