24 EXPERIMENTAL PHYSIOLOGY. 



averages 0.04 sec. while that used in relaxation is slightly more, 

 0.05 sec.. Roughly the total time involved in the various changes 

 is 0.1 sec. for the frog's gastrocnemius (Fig. 6). 



The duration of the simple twitch varies in different animals. 

 It is less in the rabbit, 0.07 sec., and considerably less in insects, 

 0.003 sec. 



The real LATENT PERIOD is primarily due to the time consumed 

 in inducing the changes in muscle leading up to contraction. Such 

 changes vary with the onset of fatigue, modification of temperature, 

 and intensity of the stimulus. Besides this, other factors are 

 added. If a load is attached to the muscle, the preliminary effect 

 of attempted contraction is a slight elongation on account of the 

 inertia of the load. With the ordinary apparatus friction also 

 slows the response. 



FIG. 6. Record of an isotonic contraction of a frog's gastrocnemius muscle. 

 Stimulated at X. Tuning fork tracing, 1/100 sec. 



Contraction Period. A muscle requires time for the develop- 

 ment of the maximum contraction because each individual muscle 

 fibre is not stimulated in all parts simultaneously, but receives its 

 impulse at the motor end-plate located near the middle of the fibre. 

 Hence the contraction spreads outward at the rate of 3 to 4 meters 

 per sec. In human muscle the velocity of such a wave is 10 to 

 13 meters per sec. Therefore, even though each muscle fibre of a 

 mass may be stimulated simultaneously through a nerve trunk, 

 time is required for the spread of the contraction throughout the 

 fibres. 



This can be shown experimentally in a muscle with parallel 

 fibres such as the sartorius. Levers are arranged resting on each 

 end of the muscle so that when the corresponding part of the muscle 



