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, i.e. 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 of the muscle 

 begins when the discharging stimulus has ceased, and is marked 

 by the rising of the lever to the summit of the curve. From 



