CHANGES IN ELECTRICAL STATE. 455 



takes place in the current, but, owing to its extremely short dura- 

 tion, the galvanometric needle is prevented by its inertia from 

 following the change. Only the most sensitive and well-regulated 

 instruments show the electric change of a single contraction, but 

 when the muscle is kept contracted by a series of rapidly re- 



FIG. 184. 



Diagram illustrating the arrangement in the Rheoscopic Frog. A = 

 stimulating limb. B = stimulated limb. The current from the electrodes 

 passes into nerve (N) of stimulating limb (A), causing its gastrocnemius to 

 contract. Whereupon the negative variation of the natural current between 

 -{- and stimulates the nerve (N / ), and excites the muscles of B to action. 



peated stimulations then the inertia of the needle is readily over- 

 come. The negative variation of a single contraction can, how- 

 ever, be easily shown on the sensitive animal tissues. For this 

 purpose "the nerve of one nerve-muscle preparation* is placed 

 upon the surface of another muscle so as to pass over the middle 

 of the transverse and longitudinal sections. Then the second 

 (stimulating) muscle is made to contract, the negative variation 

 acts as a stimulus to the nerves lying on it, and so the first (stim- 

 ulated) muscle contracts. Not only does this show the negative 

 variation of a single contraction, but it also demonstrates that 



* By a nerve-muscle preparation is meant a muscle of a frog (commonly 

 the gastrocnemius and the half of the femur to which it is attached) and 

 its nerve which has been carefully separated from other parts and removed 

 from the body. 



