NERVE-FORCE. 97 



pain, the posterior roots will be found to be distinctly sen- 

 sible. TTe have lately been in the habit, in class-demonstra- 

 tions, of dividing the fifth pair in the cranium without using 

 an anaesthetic, as the operation is instantaneous and the 

 effects are much more striking in this way ; but when we 

 have used an anaesthetic, we could never push the effects 

 sufficiently to abolish the sensibility of the root of the nerve. 

 In an animal brought so fully under the influence of ether 

 that the conjunctiva, supplied with branches of the fifth, 

 had become absolutely insensible, the instant the instrument 

 touched the root of the nerve in the cranium, there were 

 evidences of acute pain. Nothing could more strikingly 

 illustrate the mode of disappearance of the sensibility of the 

 nerves from the periphery to the centres. 



The nervous irritability may be momentarily destroyed 

 by severe shock in killing an animal. This is sometimes 

 illustrated in preparing frogs for experiments on the nerves ; 

 the shock of killing the frog by decapitation, tearing off 

 the skin, etc., abolishing the irritability of the nerves for 

 the moment. The observations of Longet and Masson have 

 shown, also, that a galvanic shock sufficiently powerful to 

 destroy life abolishes instantly the excitability of the motor 



Nerve-Force. The so-called nervous irritability, artifi- 

 cially manifested by the application of a stimulus directly to 

 the nerve-tissue, enables the nerves to conduct from the cen- 

 tres to the periphery a force which is generated in the gray 

 substance. This we may call the nerve-force. Its produc- 

 tion is one of the most remarkable of the phenomena of 

 life ; and its essence, or the exact mechanism of its genera- 

 tion, is one of the problems that has thus far eluded the 

 investigations of physiologists. We know, however, that in 

 the operations of the nervous system, the nerves serve sim- 

 ply as conductors and the nerve-cells generate the nerve- 



1 LONGET, Traite de physiologic, Paris, 1869, tome ii., p. 602. 



