82 



ELECTRO-PHYSIOLOGY 



CHAP. 



FIG. 41. Isometric Method. (Gad.) 



and allowed by means of a writing-point to record its movements 

 on a travelling surface, a curve will be obtained (with almost 

 total exclusion of change of form in the muscle) which represents 



essentially the increase 

 and decrease of tension 

 during contraction, with 

 approximately constant 

 length of muscle (Figs. 

 41 and 42). Such a 

 curve is termed by Fick 

 an " isometric " curve of 

 contraction, because it is 

 recorded with uniform 

 length of muscle, while 

 with the usual " iso- 

 tonic " method, on the 

 contrary, the tension re- 

 mains approximately 

 constant, and the muscle 

 contracts freely. It is 

 naturally impossible to obtain a tracing of an absolutely iso- 

 metrical muscular contraction ; for if a lever is to be moved, 

 and serve as index of increasing and decreasing tension, the 

 muscle cannot be stretched quite immovably between two points, 

 as would be required in 

 absolutely mathematical 

 and exact isornetry. The 

 force opposing the tension 

 is rather exercised by a 

 movable body, which draws 

 a writing-point nearer or 

 farther, according to the 



. . -, n , i FIG. 42. a. Isotonic curve of twitch : b, isometric curve 



magnitude ot the tension. of twitch. 



Yet this occurs in so 

 slight a degree with Fick's tension -indicator that the highest 



o . o 



appreciable tension- value in the shortening of the muscle is only a 

 fraction of a millimeter. The tension-values to which a muscle 

 attains its contraction are under some conditions very consider- 

 able. If isotonic be compared with isometric curves, described 

 by the same muscle under uniform conditions, we find invariably 



a 

 b- 



