CIRCULATION. 



455 



tbe experiment. The tip of the auricle is injured by immersion in hot water, 

 and the demarcation current (the injured tissue being negative toward the unin- 

 jured) is led off to a galvanometer. Orr exciting the vagus in the neck, the 

 demarcation current is markedly increased. No visible change of form is seen 

 in the auricular strip. 



FIG. 117. The tortoise heart prepared for the demonstration of the electrical change in the cardiac 

 muscle accompanying the excitation of the vagus nerve : V, vagus nerve ; C, coronary nerve ; S, sinus 

 and part of auricle in connection with it ; G, galvanometer, in the circuit formed by two non-polarizable 

 electrodes and the part of the auricle between them ; E, induction coil (Gaskell, 1887). 



Changes in the Auricle. There is little probability that the action of 

 the vagus on the auricle l differs essentially from the action on the ventricle. 

 The force of the auricular contraction is diminished. The diastole is length- 

 ened. The change in force appears earlier than in the change in periodicity, 

 and sometimes without it. On the whole, the auricle is more easily affected 

 by vagus excitation than the ventricle. 



Action on Bulbus Arteriosus. If the bulbus arteriosus of the frog's 

 heart is extirpated in such a way as to leave untouched the nerve-fibres that 

 connect it with the auricular septum, the contractions of the isolated bulbus 

 will be arrested when the peripheral end of the vagus is excited. 2 



Diminished Irritability of Heart. During vagus excitation with cur- 

 rents of moderate strength, the arrested heart will respond to direct stimula- 

 tion by a single contraction. With strong vagus excitation, however, the 

 directly stimulated heart contracts not at all or less readily than before. 3 



Effects of Varying the Stimulus. A single excitation of the vagus does 

 not stop the heart. 4 Morat has investigated the effect of excitations of varied 



1 Eckhard, 1860, p. 140; Nuel, 1874, p. 86; Gaskell, 1882, p. 1010; 1883, p. 89; Mills, 

 1885, p. 250; 1886, p. 550; McWilliam, 1885, p. 225; 1887, p. 309; 1888, p. 348; Johansson 

 and Tigerstedt, 1889 ; Franck, 1891, p. 581; Bayliss and Starling, 1892, p. 410; Eoy and 

 Adami, 1892, p. 219. * Dogiel, 1894, p. 227. 



3 Schiff, 1850, p. 64; 1877, p. 494; Einbrodt, 1859, p. 353; Eckhard, 1883, p. 25; McWil- 

 liam, 1885, p. 222; 1888, p. 351 ; Mills, 1888, p. 3. 



4 Bonders, 1868, p. 344; 1872, p. 5 ; Tarchanon", 1876, p. 303; Heidenhain, 1882, p. 386. 



