. Electro-physiology. Electrophysiology. 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 fi
. Electro-physiology. Electrophysiology. 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, 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 tlie(JO£»'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 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
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