THE ISOMETRIC TWITCH 



41 



as for a simple twitch, p. 30, and then dissect out a muscle preparation. 

 This should be the semimembranosus and gracilis (see p. 19). If we choose 

 a small muscle which will only exert a small tension, a long piece of the 

 torsion wire must be taken ; if a powerful muscle a short piece. The muscle 

 may be stimulated directly or indirectly. Record the twitch and tuning-fork 

 tracing as before, and then determine the point of stimulation and draw an 

 abscissa line. Next remove the muscle and to the lever attach a thread, 

 which passes over a pulley held in the muscle forceps and to its free end hang 

 a weight. This pulls up the lever and the writing point. By rotating the 

 drum draw a line parallel to the zero abscissa. Now hang on a larger weight, 

 and so obtain a second line, and by a series of these evaluate the movements 

 of the writing point. 



The form of curve obtained by this method is seen in fig. 45. 

 These curves were all taken from the same double semimem- 



Fig. 45. — Three Isometric Twitches with Different Initial Tensions. The 

 Numbers on the Left indicate the Number of Grams required to Bring 

 the Lever to the Level of the Corresponding Horizontal Line. 



branosus and gracilis preparation with maximal stimuli, the only 

 alteration in the conditions of the three experiments being with regard 

 to the initial tension of the muscle. The horizontal lines indicate 

 the amount of displacement of the lever by given weights, which are 

 expressed in grams. 



The general form of the curve is seen to closely correspond to 

 that of an isotonic twitch, but measurement shows a few important 

 differences. There is a latent period, a period of rising tension, and 

 one of falling tension. The apex of the curve is seen to be rounded. 



