444 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



mal conditions has been seen to contract before the auricle, the normal sequence 

 of great veins, auricle, and ventricle being reversed. 1 The energy of the ven- 

 tricular muscle-cell may, therefore, be discharged by an excitation arising 

 within the ventricle itself. Evidence of this is afforded also by the experi- 

 ment of Wooldridge, 2 who isolated the ventricles by drawing a silk ligature 

 tightly about the auricles at their junction with the ventricles, completely 

 crushing the muscle and nerves of the auricle in the track of the ligature with- 

 out tearing through the more resistant pericardium. This experiment was 

 repeated the following year by Tigerstedt, 3 who devised a special clamp for 

 crushing the auricular tissues. Both observers found that the auricles and 

 ventricles continued to beat. The rhythm, however, was no longer the 

 same. The ventricular beat was slower than before 4 and was independent of 

 the beat of the auricle. Thus the ventricle, no longer connected physiologically 

 with the auricle, develops a rhythm of its own, an idio-ventricular rhythm. It 

 seems improbable that the very small part of the auricular tissue which cannot 

 be included in Wooldridge's ligature for fear of closing the coronary arteries 

 should be able to maintain the ventricular contractions. 



Independent contraction is said to be secured by properly regulated excita- 

 tion of the cardiac end of the cut vagus nerve. Stimuli of one second duration 

 applied to the vagus at intervals of six .to seven seconds arrest the auricles 

 completely, but do not stop the ventricles, except during the second of stimu- 

 lation. The ventricles, now dissociated from the auricles, beat with a rhythm 

 different from that which characterized the normal heart. 5 The force of this 

 demonstration is somewhat weakened by the possibility that the auricles, 

 although not beating themselves, might still excite the ventricles to contraction. 



Conduction of the Excitation. If the points of non-polarizable electrodes 

 are placed on the surface of the ventricle and connected with a delicate galvan- 

 ometer, a variation of the galvanometer needle will be seen with each ventric- 

 ular beat. If one electrode is placed near the base of the heart and the other 

 near the apex it is seen that the former electrode becomes negative before the 

 latter, indicating that the part of the heart muscle on which the basal electrode 

 rests is stimulated before the apical portion, and that the difference in electrical 

 potential, or excitation-wave, according to the prevailing hypothesis, travels as 

 a wave over the ventricle from the base to the apex (see Fig. 112). Burdon- 

 Sanderson and Page 6 have found that the duration of the difference of poten- 

 tial is about two seconds in the frog's heart at ordinary temperatures. Cooling 

 lengthens the period of negativity, warming diminishes it. Some observers 



1 Recently studied by Engelmann, 1895, p. 275; see also Knoll, 1894, p. 306, who observed 

 fibrillary contraction of the auricle coincident with strong co-ordinated contractions of the ven- 

 tricles. 



2 Wooldridge, 1883, p. 527. 



3 Tigerstedt, 1884, p. 500; see also Krehl and Komberg, 1892, p. 54. 



4 The isolated ventricle may, however, beat as rapidly as the auricle, although independ- 

 ently of it (Bayliss and Starling, 1892, p. 408). 



5 Roy and Adami, 1892, p. 236 ; see also Knoll, 1884, p. 312. 



6 Burdon-Sanderson and Page, 1884, p. 338. 



