40 



EXPERIMENTAL PHYSIOLOGY 



merits of the writing point are previously determined experimentally, 

 any position of the writing point will be known to be caused by the 



Fig. 43. — The Principle of the Isometric Method. 



exertion of a certain force by the muscle. In this way, then, we may 

 record variations in the force exerted by the muscle at different 

 instants of its contraction. 



Experiment 2. — Record an isometric twitch by means of the apparatus 

 shown in fig. 44. The upper end of the muscle is to be fixed in the muscle 

 forceps. The lower end by means of an S-shaped hook is attached to the 

 lever x at a point near its axis. This axis consists of a stiff steel wire, w, 

 which can be clamped at any position by the screw s 2 . About 2 mm. from 

 the end of the axis the lever x is rigidly fixed perpendicular to it, and the project- 

 ing piece fits loosely into a small socket in the brass support a. The brass arm b 

 carrying the axis can be adjusted horizontally and clamped in any position 



Muscle Attached to an Isometric Lever. 



by the screw s 1 , so that the free end of the axis just rests in its socket. Any 

 rotation of the lever now produces a torsion in the steel axis, and this torsion 

 is proportional to the amount of rotation of the lever. By hanging weights 

 on the lever and observing the torsions produced we shall then know that 

 when that same displacement is produced by a muscle the tension exerted, 

 which is exactly opposed by the torsion of the wire, is to be measured by the 

 previously applied weights. Arrange the recording and stimulating apparatus 



