LECT. XIV. XV. ELECTRIC AND NERVOUS FORCES. 257 



sider it necessary to give here a summary of the principal 

 differences which have been ascertained experimentally, 

 between the effects produced by electric irritation of the 

 nerves, and those determined by other stimulating agents, 

 such as heat, chemical and mechanical actions, &c. The 

 following are these different distinctions : 



1. Electricity is, the only irritant which can excite, at one 

 time sensation, and at another contraction, according to the 

 direction in which it traverses a nerve. 



2. The electric current alone, in passing transversely 

 across a nerve, produces no phenomena due to the excita- 

 bility of the nerve. 



3. The electric current has no effect on the nerves, that 

 is, it neither causes contraction nor sensation, when its ac- 

 tion on the nerve is prolonged. 



4. The electric current alone can modify the excitability 

 of a nerve, and even rapidly destroy it, when the current 

 circulates in a certain direction, and can preserve or aug- 

 ment the excitability, when passing in the opposite direc- 

 tion. 



5. Lastly, of all irritating agents, the electric current is 

 the only one which possesses, for a long space of time, the 

 power of reviving the excitability of the nerve, when it has 

 become very much enfeebled in respect to other stimu- 

 lants. 



Analogy between the Electric and Nervous Forces. These 

 differences between the action which the electric current 

 exercises on the nerves, and that of other irritants, evi- 

 dently show that the first is more simple than the others. 

 Hence arises the analogy between the nervous force and 

 the electric current, which the earliest observers of galvan- 

 ism faintly perceived. 



But ought we, from this analogy, to conclude that the 

 nervous force is merely an electric current ? Let us be 

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