398 ELECTRO- PHYSIOLOGY CHAP. 



recurs, or a weak after-effect remains in the direction of nega- 

 tivity of the point nearest to the seat of excitation, which, under 

 all conditions, is at first negative in its reaction. The failure of 

 the second phase in the last cases may he explained on the pre- 

 sumption that the two variations follow so closely in time as not to 

 be clearly distinguished. For with a short tract led off at normal 

 rate of propagation, the wave of negativity can obviously arrive 

 at the second electrode before it reaches its maximum at the 

 first contact. We learn in detail from Engelmann's experi- 

 ments on the time -relations of the variation that it seems 

 to begin at the seat of excitation immediately after the impact 

 of the stimulus, i.e. with no perceptible latent period. The stage 

 of increasing negativity lasts on an average for 0'09 sec., so that as, 

 according to Engelmann's measurements on the frog's heart, the 

 contraction does not begin till O'l sec. later, the maximum of 



negativity occurs before the 

 twitch begins. 



The continuous and fairly 

 level increase of negativity is very 

 remarkable, showing as it does 

 that the systole is a simple 

 twitch, and not a tetanus. 

 Contrary observations have 

 been made by Fredericq on 



F,o. 120 -Diphasic variation in the ventricle of t l ie( J O 's heart, The Stage of 

 the frog s heart (rheotome experiment). N, 



negative ; P, positive phase. The time (in -^ diminishing negativity USUally 

 sec.) is counted from the moment of excita- n ., -, , -, 



tion. (Engelmann.) exhibits a much longer period, 



and more complicated curve 



of variation. When (as in most cases) the current is reversed, 

 the E.M.F. passes rapidly, almost in a straight line, from the 

 maximum of negativity to the maximum of positivity, and then 

 falls again gradually to its zero. The total duration of the variation 

 is conditioned by many factors. In the diphasic variation Engel- 

 mann estimates it at an average of 0'436, in the monophasic at 

 0'211 sec. The local duration of negative electromotive activity 

 may therefore be computed as at least 0'2 sec. on an average. 

 As regards the absolute magnitude of the E.M.F. of the varia- 



O O 



tion, it can only be said with certainty that it is of the same 

 order as that of the artificial cross -section. On leading off from 

 the natural longitudinal and fresh transverse section (which 



