PHYSIOLOGY OF MOVEMENT. 



713 



Such a curve (as produced by the pendulum myograph, Fig. 277) 

 is represented in Fig. 278. To study the characteristics of such a curve 

 more fully certain additional apparatus is necessary. In the first place, 

 it is necessary to know the rate of motion of the recording surface. 

 This may be accomplished by means of a recording tuning-fork writing 

 on the traveling surface. It is further necessary to indicate the instant 

 at which the nerve or muscle receives the stimulus. This may be done 

 by including an electro-magnet writing on the traveling surface in the 

 current through which the stimulus to the muscle passes. If the curve 

 be examined, it will be noticed that the muscle does not commence to 

 shorten instantaneously with the entrance of the stimulus into the nerve, 

 but an appreciable interval elapses after the application of the stimulus 

 before contraction commences. This interval is termed the latent period, 

 and is usually about one-seventieth part of a second. The duration of 

 thelatent period will depend upon the distance through which the stimulus 

 a. 3 



Fig. 278.— Muscle Curve Obtained by means of the Pendulum Myo- 

 graph. (Foster.) 

 (To be read from left to right.) 

 a indicates the moment at which the induction shock is sent into the nerve ; &, the commencement; c, 

 the maximum ; and d, the close of the contraction. The two smaller curves are due to oscillations of the 

 lever. Below the muscle curve is the curve drawn by a tuning-fork, making one hundred and eighty 

 double vibrations a second, each complete curve, therefore, representing 1-180 of a second. 



has to pass through the nerve before entering the muscle. If the elec- 

 trodes be moved along the sciatic nerve farther from the muscle, the 

 latent period will be increased. If moved down closer to the muscle, 

 or applied directly upon the muscle, although not absent, the duration 

 of the latent period will be greatly reduced (Fig. 279). It is, therefore, 

 evident that while part of the latent period is consumed in the conduc- 

 tion of the stimulus through the nerve, yet a considerable fraction of 

 it is taken up in changes in the muscle itself which precede active con- 

 traction, and this process occupies the greater portion of the latent 

 period. 



When the stimulus or induction shock is applied to the nerve 

 the latent period is partly due, in the first place, to the production 

 of a nerve impulse in the nerve ; and, second, the progression of that 

 impulse through the nerve to the muscle ; and, third, the changes already 

 alluded to which occur in the muscle itself. 



