MUSCLE. 25 



When a current passes through the thread, it is pulled towards one 

 or the other magnet according to the direction of the current. The 

 thread is illuminated by a strong light, and a magnified image of its 

 movements can be thrown on a screen and observed directly ; and if 

 the screen is replaced by a moving photographic plate, these movements 

 can be recorded. 



When a perfectly uninjured muscle, e.g. the sartorius of a frog, is 

 connected by means of non-polarisable electrodes at B and C (fig. 4), 

 with the string galvanometer, no electrical current can be detected, 

 and the muscle is said to be isoelectric. The application of a single 

 induction shock at the point A causes a contraction travelling as a wave 



FIG. 4. Diagram to show the method of investigating electrical changes in muscle. 

 D, E is the thread ; F and G are the electro-magnets of the string galvanometer. 



from A to C, and at the same time an electrical change is recorded by 

 the galvanometer. 



The stimulus produces a chemical change, which travels along 

 the muscle at the same rate as, and which precedes, the wave of 

 contraction. As this wave of excitation passes along the muscle, it 

 gives rise to an electrical change of such a kind that the excited part 

 of the muscle is negative to the resting part. The electrical changes 

 also precede the mechanical shortening of th^ muscle ; they take place 

 chiefly during the latent period, and a,re completed long before the 

 contraction is over. 



Since excited muscle is negative to resting muscle, the electrical 

 change is diphasic, as is shown diagram matically in fig. 5. The first 

 movement of the thread (represented by the ascending part of the 

 curve in the figure) occurs when the muscle at B is excited, and becomes 

 negative to the resting muscle at C ; the current flows through the 



