26 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



will indicate not only the true extent of the shortening, but also 

 the amount of shortening or subsequent relaxation at any moment 

 during the entire period. To obtain such records from the rapidly 

 contracting frog's muscle it is evident that the recording surface 

 must move with considerable rapidity and with a uniform velocity. 

 A curve of this kind is represented in Fig. 8, on page 25. C rep- 

 resents the axis of abscissas and gives the factor of time. A 

 vertical ordinate erected at any point on C gives the extent of 

 shortening at that moment. Below the curve of the muscle is 

 the record of the vibrations of a tuning fork giving 50 double vibra- 

 tions per second ; that is, the distance from crest to crest represents 

 an interval of -g^ of a second. Three principal facts are brought 

 out by an analysis of the curve: I. The latent period. By this is 

 meant that the muscle does not begin to shorten until a certain 

 time after the stimulus is applied. On the curve the stimulus 

 enters the muscle at S, and the distance between this point and the 

 beginning of the rise of the curve interpreted in time is the latent 

 period. II. The phase of shortening, which has a definite course 

 and at its end immediately passes into III the phase of relaxation. 



The Latent Period. In the contraction of the isolated frog's 

 muscles as usually recorded the latent period amounts to 0.01 sec, 

 but it is generally assumed that this period is exaggerated by the 

 method of recording used, since the elasticity of the muscle itself 

 prevents the immediate registration of the movement. By improve- 

 ments in methods of technique the latent period for a fresh muscle 

 may be reduced to as little as 0.005 or even 0.004 sec. Under the 

 conditions in the body, however, the muscle contracts against a 

 load, as when lifting a lever; hence, we may assume that normally 

 there is a lost time of at least 0.01 sec. after the stimulus enters the 

 muscle. In addition to the latent period due to the elasticity 

 of the muscle it is probable that a brief amount of time actually 

 elapses after the stimulus enters the muscle before the act of 

 shortening begins ; some time is taken up in the chemical changes 

 and the effect of these changes in putting the mechanism of con- 

 traction into play (see below on the Theory of Muscle Contractions). 

 The latent period varies greatly in muscles of different kinds, and in 

 the same muscle varies with its conditions as regards temperature, 

 fatigue, load to be raised, etc. 



The Phases of Shortening and of Relaxation. In the normal 

 frog's muscle the phase of shortening for a simple contraction occu- 

 pies about 0.04 second, while the relaxation is a trifle longer, 0.05 sec. 

 Naturally in muscles whose duration of contraction differs from 

 that of the frog the time values for the shortening and the relaxation 

 exhibit corresponding differences. Many conditions, some of which 

 will be described below, alter the duration of the simple contraction, 



