Electro-physiology (1896-98) Electro-physiology electrophysiolog01bied Year: 1896-98 56 ELECTRO-PHYSIOLOGY CHAP. graphic tracing of the process to be measured, upon a rapidly- moving surface. If such a surface (of smoked glass or paper) moves with sufficient velocity past the point of a recording lever attached to, and following the contraction of, the twitching muscle to which it is attached, a curve is obtained, the abscissa of which corresponds with the time, the ordinates on the other hand with the magnitude of the contraction (expansion) of the muscle. This is the principle of the Helmho
Electro-physiology (1896-98) Electro-physiology electrophysiolog01bied Year: 1896-98 56 ELECTRO-PHYSIOLOGY CHAP. graphic tracing of the process to be measured, upon a rapidly- moving surface. If such a surface (of smoked glass or paper) moves with sufficient velocity past the point of a recording lever attached to, and following the contraction of, the twitching muscle to which it is attached, a curve is obtained, the abscissa of which corresponds with the time, the ordinates on the other hand with the magnitude of the contraction (expansion) of the muscle. This is the principle of the Helmholtz Myograph, which has been followed by a number of similar instruments, as described in every text-book. The time value of the abscissa in every such ' contraction curve ' is easily determined if the speed of the travelling-surface is known, or if a tuning-fork tracing is taken simultaneously with the ' myogram.' The application of the graphic method enables us at once, and simultaneously, to recognise the peculiarities characteristic of the FIG. 33.—Curve of muscular contraction. (Helmholtz.) n, Moment of excitation. process to be examined, in magnitude, form, and duration. If the moment of stimulation is marked upon the recording surface, the rise of the lever from the abscissa does not usually coincide with the moment of stimulation, but occurs distinctly later, the muscle does not begin to shorten at the moment at which the induction shock takes effect, but a given time elapses before the charges produced by excitation bring about contraction, as ex- pressed in the movement of the lever (Fig. 33). The length of this time, measured between a and the begin- ning of the curve along the abscissa, was estimated by Helmholtz at about O'Ol sec. for a loaded frog's muscle directly excited by an induction shock. It is known as the period of latent stimulation (latency period), because during this time 110 visible mechanical effect is produced by the stimulus. Contraction
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