VARIATIONS IN THE LATENT PERIOD. 463 



exhibits any change of form, then in .04 sec. attaining the maxi- 

 mum height of contraction, and without waiting in the contracted 

 condition, spends .05 sec. in relaxing. 



The latent period which appears in a single contraction curve 

 drawn by a muscle stimulated in the usual way, through the 

 medium of a nerve, is not entirely occupied by preparatory 

 changes going on in the substance of the muscle, but a certain 

 part of the time recorded as latent period corresponds to the time 

 required for the transmission of the impulse along the nerve. 

 This may be shown by stimulating first the far end of the nerve 

 and then the muscle itself. In this case two curves will be drawn 

 having different latent periods, that obtained by direct stimula- 

 tion of the muscle being shorter, and representing the real latent 



FIG. 186. 



Curves drawn by the same muscle in different stages of fatigue. A, when fresli ; B, 

 C, D, E, each immediately after the muscle had contracted 200 times. Showing that 

 fatigue causes a low, long contraction. 



period of the muscle, while the longer one includes the time taken 

 by the impulse to travel along the piece of nerve between the 

 electrodes and the muscle (see p. 504). 



The latent period varies much in different kinds of muscle, in 

 the same kind of muscle in different animals, and in the same 

 individual muscle under different conditions. As a rule, the slow- 

 contracting muscles have a longer latent period. Thus the non- 

 striated, slow-contracting muscles found in the hollow viscera 

 have a latent period of some seconds. The striated muscles of 

 cold-blooded animals have a longer latency than the same kind 

 of muscle in birds and mammalia. The same gastrocnemius of 

 a frog has a shorter latent period when strongly stimulated, or 

 when its temperature is raised, and vice versa. 



