1 80 THE CONTRA CTION OF CA RDIA C MUSCLE. 



We may then sum up this section by saying that the beat of the 

 heart of cold-blooded vertebrates depends upon the rhythmical power of 

 the muscular tissue of the large veins and sinus being greater than 

 the rhythmical power of the other parts of the heart, and that in 

 all cases the greater or less rhythmicity of any part of the heart 

 depends upon the nature of the muscular fibre of which that part 

 is composed, and not upon the presence or absence of ganglion cells. 



On the Sequence of the Contractions of the Different 



Parts of the Heart. 



In the same way precisely as we have been driven to the conclusion 

 that the beat of the heart depends upon the inherent rhythmical power 

 of its muscular tissue, and that some parts beat more easily than others, 

 because their rhythmical power is greater, so also are we driven to the 

 conclusion that the sequence of the contractions of the different parts 

 of the heart is due to a peristaltic wave of contraction which starts 

 from the most automatically rhythmical tissue, and travels over the 

 rest of the heart with varying speed, quickest over the tissue which has 

 become modified so as to approach more nearly in its properties to 

 ordinary striated muscle, namely, the reticulated bulged portion of the 

 auricle and ventricle, more slowly over those parts which retain a more 

 embryonic character, namely, the auriculo-ventricular muscular ring, 

 and the bulbus or conus arteriosus. Starting with a tube of equal 

 character throughout, w T e see how, by the modification of portions of 

 that tissue in the two regions of the auricle and ventricle into a more 

 rapidly contracting, more rapidly conducting, less rhythmical tissue, we 

 obtain an efficient heart, which shall not merely pass the blood on, but, 

 by the forcible and nearly simultaneous contraction of all parts of its 

 auricular and ventricular w T alls respectively, keep up in a most efficient 

 manner a high blood pressure in the arterial system, while at the same 

 time, owing to the absence of modification of the tissue between the 

 auricles and ventricles, i.e. the canalis auricularis, whereby it still retains 

 its original more rhythmical but slower conducting power, the useful 

 purposeful pause between the auricular and ventricular contractions is 

 Drought about. 



The experimental evidence which I brought forward in 1883, 1 shows 

 clearly that such an explanation is the true one, and that therefore the 

 sequence of the contractions of the different parts of the heart is not 

 dependent upon the presence of ganglion cells any more than the heart- 

 beat itself. 



The fundamental experiment, and it is one of very great importance 

 for all questions connected with the heart, is an experiment of the same 

 character as that originally performed by Romanes 2 on the muscular tissue 

 of the Medusae. ; an experiment, namely, of cutting the muscular tissue 

 of the heart, so as to leave only a small bridge of tissue over which the 

 contraction must pass, and then studying the block to the passage of 

 that contraction caused by reducing this bridge more and more. 



If the heart of the tortoise be suspended, as in Fig. 104, and one auricle 

 be cut away, then a section through the basal part of the auricles into 

 the auricular tissue proper will divide all the nerves with their ganglion 



1 Journ. Physiol., Cambridge and London, 1883, vol. iv. pp. 61-81. 



2 Phil, Trans., London, 1875, vol. clxvi. p. 269, 





