116 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. X. 



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. 145). 



The upper line (m) is traced by the end of the lever in connection 



FIG. 145. Simple muscle curve. (M. Foster.) 



wilh a muscle after stimulation of the muscle by a single indue tion- 

 shock : the middle-line (I) is that described by a lever, which indicates 

 by a sudden drop the exact instant at which the induction-shock is 

 given. The lower wavy line (t) is traced by a tuning-fork vibrating 

 200 times a second, and serves to measure precisely the time occupied 

 in each part of the contraction. 



It will be observed that after the stimulus has been applied as 

 indicated by the vertical line s, there is an interval before the con- 

 traction commences, as indicated by the line c. This interval,, termed 

 the latent period, when measured by the number of vibrations of the 

 tuning-fork between the lines s and c, is found to be about y^-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, indicated 



