126 STUDIES IN ADVANCED PHYSIOLOGY. 



peratures the muscles cease to react. Third, chemical 

 stimuli: A number of chemical agents will stimulate the 

 muscles. Such agents are fumes of nitric oxide, sulphur 

 dioxide, hydrochloric acid, but especially ammonia gas. 

 Fourth, electrical stimuli: A current of electricity may 

 in different forms induce the muscles to very vigorous 

 activity, and one of the best worked-out chapters on muscle 

 physiology is its reaction to electrical stimuli. 



The question whether these stimuli affect the muscles 

 themselves, and not the nerves in the muscles, can be defi- 

 nitely answered by saying that the stimuli affect the muscles 

 themselves directly. For instance, ammonia stimulates the 

 muscles, but does not affect the nerve. Still again some 

 muscles, for instance the sartorius of the thigh, have no 

 nerves in their ends, yet the muscle there may be stimu- 

 lated to contraction. The best proof is, however, that 

 furnished by the South American poison called curare. 

 This curare lames the motor nerves, but a muscle with such 

 a lamed nerve will still react to all of the preceding stimuli. 

 This curare is used by many of the South American Indians 

 in their chase and warfare. The end of the arrow is im- 

 pregnated with the curare, and when the arrow is intro- 

 duced into the flesh of an animal it drops down motionless 

 to the ground. Its circulation and all of its sensory nerves 

 are left intact, but there is a complete paralysis of all vol- 

 untary action. The animal of course dies by suffocation, 

 being unable to move the muscles of respiration. 



A SINGLE CONTRACTION. 



Having now called attention to the stimuli which produce 

 muscular contraction, a closer study of such a contraction 

 itself naturally follows. If in order to study it more advan- 

 tageously, a frog's muscle be entirely cut out from the body 

 and then suspended from a hook, it may still be made to 

 contract, and on account of its isolation, the phenomena of 

 such a contraction much better studied. Usually a piece of 

 nerve running to the muscle is left connected with it and 



