CIRCULATION. 



153 



different from that which characterized the normal heart. 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. 27). Burdon- 

 Sanderson and Page 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 

 believe that the excitation-wave under certain conditions returns toward the 

 base after having reached the apex. The speed of the excitation-wave has been 

 measured by the interval between the appearance of negative variation in the 

 ventricle when the auricle is stimulated first near and then as far as possible 



Fig. 27.— The electrical variation in the spontaneously contracting heart of the frog, recorded by a 

 capillary electrometer, the apex being connected with the sulphuric acid ami the base with the mercury 

 of the electrometer. The changes in electrical potential are shown by the line e, > , which is obtained by 

 throwing the shadow of the mercury in the capillary <>n a travelling sheet of sensitized paper. The con 

 traction of the heart is recorded by the line h, h : time, in „',, second, by I. t. The curves read from hit 

 P. right. The electrical variation is diphasic; in the first phase the base is negative to the apex ; in the 

 second, the apex is negative to the base; the negative variation passes as a wave from base to apex 

 (Waller, 1887, p. 231). 



from the non-polarizable electrodes. The interval is the time which the excita- 

 tion-wave requires to pass the distance between the two points stimulated. The 

 average rate is at least 50 millimeters per second. 1 The negative variation 

 begins apparently instantly ai'ter the application of the stimulus. Its phases 

 and their characteristics have been described by Engelmann. 

 The latent period of a frog's heart muscle is about 0.08 second. 



1 Burdon-Sanderson and Page (Journal of Physiology, 1880, ii. j>. 4Jti i give 125 millimeters 



per second. 



