GENERAL PROPERTIES OF THE NERVES. 597 



animal. This is sometimes illustrated in preparing frogs for experiments upon the 

 nerves; as the shock of killing the frog by decapitation, tearing off the skin, etc., 

 abolishes the irritability of the nerves for the moment. It has been observed, also, 

 that a galvanic shock sufficiently powerful to destroy life instantly destroys the excita- 

 bility of the motor nerves. 



Nerve-Force. The so-called nervous irritability, artificially manifested by the applica- 

 tion of a stimulus directly to the nerve-tissue, enables the nerves to conduct from the 

 centres to the periphery a force which is generated in the gray substance. This we may 

 call the nerve-force. Its production is one of the most remarkable of the phenomena of 

 life ; and its essence, or the exact mechanism of its generation, 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, tho nerves serve simply as conductors and the 

 nerve-cells generate the nerve-force. It is evident, also, that nearly all of the so-called 

 vital phenomena are more or less influenced and controlled through this wonderful agent ; 

 and, throughout our study of the nervous system, we shall be constantly investigating the 

 phenomena attending the operation of nerve-force, while we are compelled to admit our 

 ignorance of its essential nature. 



Non-identity of Nerve-Force with Electricity. When we come to study fully the 

 action of electricity upon the nerves, we shall see that this is by far the most convenient 

 stimulus for exciting the nervous action and one by which we closely imitate the true 

 nerve-force. So great is the similarity, indeed, between certain of the phenomena pro- 

 duced by the application of electricity and those attending the physiological action of 

 nerves, that some physiologists have regarded the nerve-cells as generators of an electric 

 current. This hypothesis explains the nature of nerve-force, in so far as it assimilates it 

 to a force, with the action of which, as artificially generated, we are more or less familiar. 

 No one at the present day, however, pretends that the nerve-force has been demonstrated 

 to be identical with any form of electricity ; and the question does not now demand 

 extended discussion. 



A series of experiments made by Pre" vost and Dumas, in 1823, are worthy of note as 

 showing the absence of a true electric current in nerves in action ; but these have been 

 confirmed in later years with apparatus sufficiently delicate to settle the question beyond 

 a doubt. The most conclusive experiments upon this subject are those of Matteucci and 

 Longet, made upon horses, at the veterinary school at Alfort. These physiologists 

 exposed the sciatic nerves in the living animal, and, when there was evidently a conduc- 

 tion in both directions, as evinced by pain and muscular action, they failed to detect the 

 slightest evidence of an electric current with the most delicate galvanometer that could 

 be constructed. The fact of the absence of a galvanic current in nerves during their 

 physiological action was even more strikingly illustrated by Matteucci, who demonstrated, 

 in the electric eel, that, although the electric discharges from the peculiar organs of this 

 animal were under the control of the nervous system and could be excited by galvanic 

 stimulation of the proper nerves immediately after death, no galvanic current existed in 

 these nerves during their physiological action. 



When we abandon the hypothesis of the identity of nerve-force with electricity, we 

 are compelled to admit that the agent generated by the nerve-centres is sui generis 

 and not to be compared with any force known outside of living organisms or artificially 

 produced by direct stimulation of the nerves ; but we admit, nevertheless, the fact that 

 electricity may be generated by animals, as the electric fishes, and that electric currents 

 exist in different anatomical structures in the living body, including the nerves, under 

 certain conditions. Our study of the nerve-force, then, leaving its essential nature unex- 

 plained, is mainly confined to a description of its characteristic phenomena. 



Rapidity of Nervous Conduction. The first rigorous estimates of the velocity of the 

 nerve-current were made in 1850, by Helmholtz, and were applied to the motor nerves. 



