460 MANUAL OF PHYSIOLOGY. 



1. A short period between the moment of stimulation and that 

 at which the lever begins to rise, during which the muscle does 

 not move. This is known as the latent period. In the skeletal 

 muscles of the frog this period lasts about .01 sec. 



2. A period during which the lever rises, at first slowly, then 

 more quickly, then again slowly, until it ceases to rise. This 

 stage has been called the period of rising energy. It lasts about 

 .04 sec. 



3. When the highest point is attained the lever commences to 

 fall, at first slowly, then more quickly, and at last slowly. There 

 is then no pause at the height of contraction. The stage of 

 relaxing has been called the period of falling energy. It is said 

 to occupy a somewhat longer time than the second period, lasting 

 about .05 sec. 



Thus we see that a stimulus occupying an immeasurably short 

 time sets up a change in the molecular condition, which taking 

 nearly .1 sec. to run its course, and requiring .01 sec. before it 

 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 ot the muscle being shorter, and representing the real latent 

 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. 502). 



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- 



