72 EXPERIMENTAL PHYSIOLOGY 



A tracing obtained by this arrangement is shown in curve n, fig. 

 67, and for purposes of comparison a simple twitch taken immediately 

 after, under precisely the same conditions, by attaching a thread from 

 the tendon to a crank lever is reproduced in curve i. The two curves 

 show the following points of difference. 



(i.) The latent period in the case of n is less than in i. The 

 measurement given by the curve of thickening is just under -005 sec. 

 This method is the one commonly employed for determining the latent 

 period, the points to be especially observed being (a) to make all 



Fig. 67 Curve I the Shortening, and Curve II the Thickening of a Semi- 

 membranosus and Gracilis Preparation. Time Tracing, 200 per Sec. 



moving parts of the lightest possible character, and (b) to stimulate 

 the muscle exactly at the spot on which the recording lever rests. 



(ii.) The amount of movement is very much less in the second 

 tracing. This follows as a necessity when we remember that in the 

 first instance the curve is a summation of the movements along the 

 length of the whole muscle. The second is a summation of the 

 different thickenings over one transverse section ; and as the length 

 is much greater than the thickness, that of itself explains the 

 main difference. Further thickening is not proportional to the 

 amount of shortening, which therefore further accentuates the dif- 

 ference. 



(iii.) The total duration of the second is a little less than that of 

 the first curve. This is because, in a contraction, the whole length of 

 the muscle fibre does not contract simultaneously, but successively, 

 whereas in the particular conditions of the experiment thickening 



