600 



NERVOUS SYSTEM. 



long persistence of the irritability of their tissues and the facility with which certain por- 

 tions of the nervous system can be exposed. For ordinary experiments upon the nervous 

 conduction, the parts are prepared by detaching the posterior extremities, removing the 

 skin, and cutting away the bone and muscles of the thigh, so as to leave the leg with the 

 sciatic nerve attached. A frog's leg thus isolated presents a nervous trunk one or two 

 inches in length, attached to the muscles, which will respond to the slightest stimulus. 

 It is by experiments made upon frogs prepared in this way that most of the important 

 facts relative to the action of electricity upon the nervous system have been developed. 

 A form of galvanic apparatus which we have long used and found very convenient for 

 these experiments is essentially the one described by Bernard. It consists simply of 

 alternate copper and zinc wires wound around a piece of wood bent in the form of a 

 horseshoe and terminating in two platinum points representing the positive and negative 

 poles. This forms a sort of electric forceps, about eight inches long, which, when moist- 

 ened with water slightly acidulated with acetic acid, will give a current of about the 

 strength required for most experiments. 



It is evident that the galvanic current may be applied to a nerve so that the direction 



may, in the one case, follow the course of the 

 nerve, that is, from the centre to the periphery, 

 and, in the other, be opposite to the course of the 

 nerve. These currents have been called respec- 

 tively the descending, or direct, and the ascending, 

 or inverse. When the positive pole (the copper) is 

 placed nearer the origin of the nerve, and the 

 negative pole (the zinc), below this point in the 

 course of the nerve, the galvanic current follows 

 the normal direction of the motor conduction, and 

 this is called the descending current. When the 

 poles are reversed, and the direction of the galvan- 

 ic current is from the periphery toward the centre, 

 it is called the ascending current. It will be con- 

 venient to speak of these two currents respectively 

 as descending and ascending, in detailing experi- 

 ments upon the action of electricity upon the nerves. 

 The points to be noted with regard to the effects 

 of the application of electricity to an exposed nerve 

 are the action of constant currents of different de- 

 grees of intensity, the phenomena observed on 

 closing and opening the circuit, and the effects 

 of an interrupted current. 



During the passage of a feeble constant current 

 through an exposed nerve, whatever be its direc- 

 tion, there are no convulsive movements and no 

 evidences of pain. This fact has long been recog- 

 nized by physiologists, who at first limited the 

 effects of electricity upon the nerves to two pe- 

 riods, one at the closing of the circuit and the 

 other at its opening. We shall see, however, that 

 the passage of electricity through a portion of a 

 nervous trunk produces a peculiar condition in 

 parts of the nerve in the neighborhood of tho 

 poles of the battery, described under the name of 

 electrotonus ; but the fact that neither motion nor sensation is excited in a mixed nerve 

 during the actual passage of a feeble constant current is not invalidated. 



FIG. 189. Electric forceps. (Liegeois.) 



C, copper ; 2", zinc ; P, P, positive poles ; N, N, 



negative poles. 



