34 



EXPERIMENTAL PII YSIOLOG Y 



inertia, it oscillates for a time about a mean position which it 

 ultimately reaches. The difficulty therefore is purely instrumental, 

 and should be reduced to a minimum by working with as light a lever 

 as possible. It is particularly marked when the relaxation process is 

 carried out very rapidly, and is completely absent when, from any 

 cause, the time is prolonged. An examination of this part of the 

 curve shows practically the same changes as the preceding portion, 

 though in the reverse order. It is at first concave, and then, after an 

 intermediate portion in which the change of curvature is but slight, 

 it becomes convex to the abscissa line. These changes mean that at 

 first the rate of relaxation increases slowly, then more rapidly, until 

 a maximum rate is attained, and from this gradually diminishes until 

 relaxation ceases. The length of time occupied by this process, in the 

 curve of fig. 38, is seen, by measuring c'df, to be oV^ths of a second, or 



i 



Fig. 38. — Isotonic Twitch of a Hyoglossus Muscle. Time Tracing, 200 per sec. 

 Magnification, 3 (i.e. the Vertical Ordinate represents 3 times the 

 amount of shortening at that instant). 



•075 sec. ; but this time measurement is not to be regarded as quite so ac- 

 curate as the two preceding— it is probably estimated a little too high. 



By adding up these three time measurements it is seen that for 

 this twitch the total time occupied was -16 sec. 



So far we have been mainly occupied in a study of the curve with 

 regard to its^time relations, but there are other points which the curve 

 can teach us. In the first place the height of the curve will tell us 

 the amount of the shortening that took place. The height of c from 

 the abscissa line is found to be exactly 20 mm. ; and as the magnifica- 

 tion of the movement was 3, the amount the muscle contracted was 



20 



mm. The length of the muscle when loaded by the lever was 



o 



20 1 



28 mm. Consequently the muscle contracted x-. , i.e. nearly a 



quarter of its whole length. 



We may in the! next place estimate the amount of work per 

 formed by the muscle during its twitch. This is given by the product 



