602 NERVOUS SYSTEM. 



of the spinal column. The legs are then placed each one in a vessel of water, and a 

 feeble galvanic current is passed from one glass to the other. It is evident that, with 

 this arrangement, the current will pass through both nerves, being descending for the 

 .one and ascending for the other. In this case, if the irritability of the nerves be not too 

 great, there will be a contraction in the leg for which the current is descending at the 

 time of closing the circuit, and the other leg will contract when the circuit is opened. 

 This experiment has been modified by Chauveau and applied to the two facial nerves in a 

 living horse. A Leyden jar is very feebly charged, and the two facials are exposed. 

 The current is then passed instantaneously through both the nerves, which gives but a 

 single stimulus, and that corresponds to the time of closing the circuit. In this experi- 

 ment, the current is descending for one nerve and ascending for the other, and con- 

 traction takes place only in those muscles supplied with the nerve for which the current 

 is descending. 



The muscular contraction produced by galvanic stimulation of a nerve is more vig- 

 orous the greater the extent of the nerve included between the poles of the battery. 

 This fact has long been observed, and its accuracy is easily verified. It would naturally 

 be expected that, the greater the amount of stimulation, the more marked would be the 

 muscular action ; and the stimulation seems to be increased in proportion to the extent 

 of nerve through which the galvanic current is made to pass. 



The irritability of a nerve, it is well known, may be exhausted by the repeated appli- 

 cation of electricity, whatever be the direction of the current, and it is more or less com- 

 pletely restored by repose. It is a curious fact, in this connection, that, when the irrita- 

 bility of a nerve has been exhausted for the descending current, it will respond to the 

 ascending current, and vice versa; and it is even more remarkable that, after the irrita- 

 bility has been exhausted by the descending current, it is restored more promptly by 

 stimulation with the ascending current than by absolute repose, and vice versa. This 

 phenomenon, observed by Volta, is sometimes known as " voltaic alternation." It is very 

 strikingly illustrated in frogs prepared as above described, with the two posterior ex- 

 tremities, the nerves attached through a portion of the spinal cord, placed in vessels of 

 water so that a current may be simultaneously passed through both nerves, being descend- 

 ing for the one and ascending for the other. As we have already seen, after a time, con- 

 traction occurs only in one leg, for which the current is descending, on closing the cir- 

 cuit, and in the other, only on opening the circuit. By repeatedly passing the current 

 in this way, after a time there will be no contraction in either leg, the irritability of the 



nerves having become exhausted. If the 

 poles of the battery be now reversed, so as 

 to make the ascending current take the place 

 of the descending, contractions on closing 

 and opening the circuit will again occur. 



Induced Muscular Contraction. A cu- 

 rious phenomenon was discovered by Mat- 

 teucci, in experimenting upon nervous and 

 muscular irritability, which has been call- 

 ed " induced muscular contraction." It 

 was found that, if the nerve of a gal- 

 vanoscopic frog's leg (the leg prepared 

 with the nerve attached in the way al- 

 FIG. m.-Arraiwement offrotfs legs prepared so as read describec l) be placed in contact with 

 to sfiow induced contraction. (Liegeois.) , n , . 



the muscles of another leg prepared in 



the same way, galvanization of the nerve, giving rise to contraction of the mus- 

 cles with which the nerve of the first leg is in contact, will induce contraction in 

 the muscles of both. This experiment may be, extended, and contractions may thus be 



