CIRCULATION. 443 



Other muscles in which no nerve-cells have been discovered can contract 

 rhythmically. Thus the bulbus aortse of the frog beats regularly after its 

 removal from the body, even the smallest pieces showing under the microscope 

 rhythmical contractions. Engelmann, who observed this fact, declares that 

 the entire bulbus is lacking in nerve-cells. This is contradicted by Dogiel ; 

 yet it seems hardly reasonable that these " smallest pieces " which Engelmaun 

 mentions were each provided with ganglion-cells. It is more probable that the 

 contractions were the result of a constant artificial stimulus. 1 Curarized stri- 

 ated muscles placed in certain saline solutions may contract from time to time. 2 

 The hearts of many invertebrates in which ganglion-cells are apparently absent 

 beat rhythmically. 3 



Much has been made of the fact that the ganglion-cells grow into the heart 

 long after the cardiac rhythm is established, 4 showing that the embryonic heart 

 muscle has rhythmic contractile powers. The adult heart muscle, it is alleged, 

 retains certain embryonic peculiarities of structure, and as structure and func- 

 tion are correlated, should also retain the embryonic power of contraction 

 without nerve-cells. 5 



It cannot be denied that these facts prove that the embryo heart muscle 

 possesses rhythmic contractility, that the apical half of the heart of the adult 

 frog and tortoise may be made to contract rhythmically, and that even fully 

 striated muscle will under some conditions show more or less periodic contrac- 

 tions. They can, however, hardly be said to prove that the beat of the mam- 

 malian or even the batrachian adult heart is not dependent on discharges from 

 the cardiac nerve-cells. Even the freedom of the apex from ganglion-cells, 

 which is the very foundation of the doctrine of muscular origin, has recently 

 been questioned. 6 This problem is still unsolved. 



The Excitation-wave. The change in form which constitutes what com- 

 monly is called the cardiac contraction is preceded by a change in electrical 

 potential, supposed to be a manifestation of the unknown process by which the 

 heart-muscle is excited to contract. Both the contraction and the electrical 

 change sweep over the heart in the form of waves, and it has become the cus- 

 tom to speak of the electrical change as the excitation-wave. It should not be 

 forgotten, however, that this usage rests merely on an assumption, for the real 

 nature of the excitation is still a mystery. The contraction-wave begins nor- 

 mally at the great veins, travels rapidly through the auricle, and, after a dis- 

 tinct interval, spreads through the ventricle. The excitation-wave, which pre- 

 cedes and is the cause of the contraction, probably takes the same course, 7 and 

 in fact it is possible to show that the change in electrical potential actually 

 begins under normal conditions at the great veins and passes thence over the 

 entire heart. But this sequence is not invariable. The ventricle under abnor- 



1 Engelmann, 1882, p. 446 ; Dogiel, 1894, p. 225. 2 Biedermann, 1880, p. 259. 



3 Concerning the cardiac apex in fishes, see Ludwig and Luchsinger, 1881, p. 247; Kazera- 

 Beck and Dogiel, 1882, p. 259 ; McWilliam, 1885, p. 197 ; Mills, 1886, p. 91. 



4 Wagner, 1854, p. 227 ; Schenck, 1867, p. Ill ; His, Jr., 1893, p. 25 ; Pickering, 1893, p. 391. 



5 Gaskell, 1883, p. 77. 6 Berkeley, 1894. p. 90. 7 Compare Kaiser, 1895, p. 447. 



