462 MANUAL OF PHYSIOLOGY. 



The curve drawn on the recording surface of a pendulum myo- 

 graph, by such a single contraction, is represented in Fig. 185. 

 The short vertical stroke on the abscissa or base line is drawn by 

 touching the lever when the muscle is in the uncontracted state, 

 and indicates the time of stimulation. The upper curved line is 

 drawn by the lever and during the contraction of the muscle. 



In such a curve the following stages are to be distinguished : 



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 



FJG. 185. 



Curve drawn by a frog's gastrocnemius on the Pendulum Myograph. Below is seen 

 the tuning-fork record of the time occupied by the contraction. Parallel to the latter is 

 the abscissa. The little vertical mark at the left shows the moment of stimulation, and 

 the distance from this to the beginning of the rise of the curve gives the latent period, 

 which is followed by the ascent and descent of the lever. 



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 



