THE PHENOMENON OF CONTRACTION. 27 



interval of rhr 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 



Fig. 8. Curve of simple muscular contraction. 



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 certain 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 antecedent physical 

 and chemical changes caused by the stimulus, and the effect of 

 these changes in putting the mechanism of contraction 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 may be a trifle longer, 

 0.05 sec. In muscles whose duration of contraction differs from 



