Chemical Agents on the Nervous System. 255 



nected by copper wires, were used in the cases in which I 

 wanted the more considerable power ; and two glasses of the 

 " couronne de tasses" of Volta, with arcs of zinc and silver, 

 connected with fine platinum wires, in those cases in which a 

 mild Voltaic current was required ; lastly, a single arc of 

 zinc and silver applied to the nervous tissue itself, or to the 

 nervous and muscular tissues, when this form of experiment 

 was preferred. 



The frog which was made the subject of the experiment 

 about to be detailed, was prepared by my friend, Mr Smith, 

 in the diflPerent manners represented in figures 1, 2, 3, 4, 5, 

 6, 7, 8, or merely with the spinal marrow divided. 



As I shall have to state more particularly hereafter, it is 

 necessary sometimes to remove the skin, which is a very im- 

 perfect conductor., sometimes to insulate the nerves from all 

 muscular or other structures, which are good conductors ; 

 and it is especially necessary to prevent the neurilemma and 

 the nervous substance itself from becoming dry, yet without 

 an actual layer of humidity extending along the external sur- 

 face of the nerve. 



II. Precautions— Effects of Dryness— of External Moisture— of 

 Extent of Contact. 



Many circumstances combine to render experiments on the 

 electrogenic condition of nerves extremely difficult, and liable 

 to error. Of these are the three which I have just enume- 

 rated, and of which I propose to give a brief account before 

 I proceed to the actual investigation of my subject. 



It frequently happened that a phenomenon appeared in one 

 experiment which was absent in another ; and that we could 

 not always produce the phenomena of the electrogenic state, 

 which we had succeeded in doing in a previous experiment, 

 apparently identical ; and on one occasion, in which we ex- 

 changed our copper wires of communication for others of fine 

 platinum wire, our experiment was altogether unsuccessful. 

 It was of the utmost importance to ascertain and remove the 

 causes of these discrepancies. ■ 



We prepared a frog in the manner represented in fig. 3, 



