NERVE 



69 



it. It is influenced by many chemical substances, some of 

 which increase its excitability in small doses, and diminish 

 it in larger doses ; some again even in the smallest dose 

 depress its activity, e.g. potassium salts, and such drugs as 

 chloroform and ether. 



Continued activity has no effect on the excitability of axons, 

 and the phenomena of fatigue are not manifested in them. 



This may be proved by taking two nerve-muscle prepar- 

 ations, A and B, and stimulating both nerves repeatedly with 



Stimulating Current. 



5^ 



Blocking Current. 



A B 



Fig. 29. — Experiment to show that a nerve cannot be fatigued. Two muscle 

 nerve preparations, A and B, are stimulated by the faradic current. 

 B is blocked by a galvanic current or by cooling, till the muscle of A 

 no longer contracts. The block is then removed, and B contracts. 



the same electric current, but preventing the stimulus from 

 reaching one of the muscles, B, by blocking its passage by 

 passing a galvanic current through the nerve (see p. 71) or 

 by applying ice to it (fig. 29). 



If, after muscle A no longer contracts, the block 

 is removed, muscle B will contract, showing that the nerve 

 is not fatigued. 



O. 



Conduction in Neurons. 



When a neuron is stimulated at any point, some time 

 elapses before the result of the stimulation is made manifest. 



