CONDITIONS AFFECTING A NERVOUS IMPULSE 291 



be shown in mammals by poisoning the animal with curare, and then 

 stimulating a motor nerve continuously while the animal is kept alive 

 by means of artificial respiration. As the effect of the curare on the 

 end- plates begins to wear off in consequence of its excretion, the muscles 

 supplied by the stimulated nerve enter into tetanus. The action of the 

 curare may be cut short at any time by the injection of salicylate 

 of physostigmin, when the muscles will at once begin to react to the 

 excitation. 



The same fact may be shown on the excised nerve-muscle prepara- 

 tion of the frog. The gastrocnemii of the two sides with the sciatic 



A B 



FIG. 108. Arrangement of experiment for demonstrating the absence of 



fatigue in medullated nerve-fibres. 

 EC, exciting circuit ; CP, polarising circuit. 



nerves are dissected out, and an exciting circuit is so arranged that the 

 interrupted secondary currents pass through the upper ends of both 

 nerves in series (Fig. 108). At the same time a constant cell is connected 

 with two non-polarisable electrodes (np, np) placed on the nerve of B, 

 so that a current runs in the nerve in an ascending direction. The effect 

 of passing a constant current through a nerve is to block the passage 

 of impulses through the part traversed by the current. When the 

 constant polarising current is made, the muscle B may give a single 

 twitch, and then remains quiescent. The exciting current is then sent 

 through both nerves by the electrodes e and e 2 . The muscle A enters 

 into tetanus, which gradually subsides owing to " fatigue." When A 

 no longer responds to the stimulation, the constant current through 

 the nerve of B is broken. B at once enters into tetanus, which lasts as 

 long as the contraction did in the case of A, and gradually subsides as 



