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A TEXTBOOK OF PHYSIOLOGY 



stimulation. (2) Muscle will contract in response to certain chemical 

 stimuli e.g., ammonia which do not excite nerve. (3) The South 

 American arrow-poison curare abolishes the action of nerve by 

 paralyzing the nerve-endings in the muscles; yet, under these condi- 

 tions, the muscle is directly excitable. The experiment is generally 

 made as follows: Both the sciatic nerves are dissected out in the 

 thighs of a frog in which the cerebral hemispheres have been destroyed. 



FIG. 271. DIAGRAM TO SHOW THE ACTION OF THE HELMHOLTZ SIDE-WIRE. 



Round one thigh, but not including the nerve, a ligature is tied. 

 Curare is injected into the dorsal lymph sac, and circulates everywhere 

 but in the ligated thigh. The upper ends of both sciatic nerves are 

 certainly exposed to the action of the drug. It is found that stimula- 

 tion of the nerve-supply to the ligated side produces a contraction of 

 that leg, whereas stimulation of the nerve to the other leg does not. 

 Direct excitation of the muscles, however, causes a response in both 

 legs. The block is therefore in the nerve-endings. 



