106 NERVOUS SYSTEM. 



mals just killed ; and, of all classes, the cold-blooded animals 

 present the most favorable conditions, an account of the 

 persistence of nervous and muscular irritability for a consid- 

 erable time after death. Experimenters most commonly use 

 frogs, on account of the long persistence of the irritability 

 of their tissues and the facility with which certain portions 

 of the nervous system can be exposed. For ordinary experi- 

 ments 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 rela- 

 tive to the action of electricity upon the nervous system 

 have been developed. 



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 periph- 

 ery, and, in the other, be opposite to the course of the nerve. 

 These currents have been called respectively the direct, or 

 descending, and the inverse, or ascending. 1 "When the posi- 

 tive 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 direct current. When the poles are reversed, and the 

 direction of the galvanic current is from the periphery 

 toward the centre, it is called the inverse current. It will 

 be convenient to speak of these two currents respectively as 

 direct and inverse, in detailing experiments upon the action 

 of electricity upon the nerves. 



The points to be noted with regard to the effects of the 



1 The direct current is sometimes called centrifugal, and the inverse, centrip- 

 etal. 



