602 NERVOUS SYSTEM. 



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

 vanic current is passed from one glass to the other. It is evident that, with this arrange- 

 ment, the current will pass through both nerves, being direct for the one and inverse for 

 the other. In this case, if the irritability of the nerves be not too great, there will be 

 a contraction in the leg in which the current is direct at the time of making the circuit, 

 and the other leg will contract when the circuit is broken. This experiment has been 

 modified by Ohauveau and applied to the two facial nerves in a living horse. A Leyden 

 jar is very feebly charged with electricity, 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 making the circuit with the constant current. In this 

 experiment, the current is direct for one nerve and inverse for the other, and contraction 

 takes place only in those muscles supplied with the nerve for which the current is direct. 

 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 direct current, it will respond to the inverse 

 current, and vice versa; and it is even more remarkable that, after the irritability has 

 been exhausted by the direct current, it is restored more promptly by stimulation with 

 the inverse 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 extremities, 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 direct for the one and inverse 

 for the other. As we have already seen, after a time, contraction occurs only in one leg, 

 for which the current is direct, on making the circuit, and in the other, only on breaking 

 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 inverse current take the place 

 of the direct, contractions with making and 

 breaking the circuit will again occur. The 

 irritability may again be exhausted and re- 

 stored by changing the poles, and this may 

 be repeated several times with the same 

 preparation. 





Induced Muscular Contraction. A cu- 

 rious phenomenon was discovered by Mat- 

 teucci, in experimenting upon nervous and 

 muscular irritability, which has been called 

 "induced muscular contraction." It was 

 found that, if the nerve of a galvanoscopic 

 frog's leg (the leg prepared with the nerve 

 in the way already described) be placed in contact with the muscles of another 

 leg prepared in the same way, galvanization of the nerve giving rise to contraction of 

 scles 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 



