96 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. IX. 



time-marker is a tuning-fork vibrating 100 times a second. This is 

 struck, and by means of a writing-point fixed on to one of the prongs 

 of the fork, these vibrations may be written beneath the myogram. 

 More elaborate forms of electrical time-markers or chronographs are 

 frequently employed. 



The Simple Muscle Curve. 



We can now pass on to results, and study first the result of a 

 single instantaneous stimulus upon a muscle. This causes a single 

 or simple muscular contraction, or, as it is often called, a twitch. The 

 graphic record of such a contraction is called the simple muscle curve. 

 One of these is shown in the accompanying figure (fig. 118). 



FIG. 118. Simple muscle curve. 



The muscle was stimulated by a single induction-shock, at the 

 instant marked P upon the base-line. The lower wavy line is traced 

 by a tuning-fork vibrating 100 times a second, and serves to measure 

 the time occupied in each part of the contraction. 



It will be observed that after the stimulus has been applied 

 there is an interval before the contraction commences. This 

 interval, termed the latent period, when measured by the tuning- 

 fork tracing is seen to be about T Q- sec. During the latent period 

 there is no apparent change in the muscle. 



The second part is the stage of contraction proper. The lever 

 is raised by the shortening of the muscle. The contraction is at first 

 very rapid, but then progresses more slowly to its maximum. 



The next stage is the stage of elongation. After reaching its 

 highest point, the lever descends in consequence of the elongation 

 of the muscle. The small waves which follow the main curve are 

 simply due to the elasticity of the muscle and recording apparatus, 

 and are most marked when the contraction is rapid and vigorous. 



