PHYSICAL SUPPORT AND MOVEMENT 



97 



traces a line on smoked paper on a revolving drum, the single twitch is 

 recorded by a curve of characteristic form (Fig. 86). The twitch as a 

 whole lasts about 0.1 second in the frog. It takes a very short time 

 (0.01 second) for the muscle to start to contract. This brief period of 

 inaction is known as the latent period; by the time it is ended the change 



Fig. 87. — Curves of jcontraction of muscle in response to repeated stimuli. Rate of 

 stimulation is slow at the bottom, but gradually increases toward the top. {From Howell, 

 "Textbook of Physiology," W. B. Saunders Company.) 



of electric potential which is the sign of stimulation has usually reached 

 a peak and subsided. Then the muscle contracts for about 0.04 second, 

 and the succeeding relaxation lasts about 0.05 second. Following the 

 tw'itch there is a period of recovery lasting a number of seconds in which 

 the muscle returns to its previous condition. If stimuli are applied 

 repeatedly before the recovery is complete, the muscle shows fatigue 



