534 



NERVOUS SYSTEM. 



it is restored more promptly by stimulation with the ascending current than 

 by absolute repose, and vice versd. This phenomenon, observed by Volta, is 

 known as " voltaic alternation." 



Many of the phenomena illustrating the law of contraction may be ob- 

 served without the use of complicated apparatus. A form of battery, very 



convenient for some of these experiments, 

 is the one described by Bernard. It con- 

 sists 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 

 (20 centimetres) long, which, when moist- 

 ened with water slightly acidulated with 

 acetic acid, will give a constant current of 

 about the required strength. 



The law of contraction is applicable to 

 inhibitory nerves, as the inhibitory nerve of 

 the heart, the difference being that the 

 stimulation produces inhibition instead of 



FIG. 187. Electric forceps (LiSgeois). 

 c, copper ; z, zinc. 



FIG. 188. Arrangement of frog^s legs prepared so as to 

 show induced contraction (Liegeois). 



contraction. It also holds good for sensory nerves, the effects being observed 

 by noting the reflex contractions produced '(Pfliiger). 



A peculiar phenomenon, discovered by Matteucci, has been called " in- 

 duced muscular contraction." If the nerve of a galvanoscopic frog's leg be 

 placed in contact with the muscles of another leg prepared in the same way, 

 stimulation of the nerve, giving rise to contraction of the muscles with which 

 the nerve of the first leg is in contact, will induce contraction in the muscles 

 of both. This experiment may be extended, arid contractions may thus be 

 induced in a series of legs, the nerve of one being in contact with the mus- 

 cles of another. It is shown that " induced contraction " is not due to an 

 actual propagation of the electric current but to a stimulus attending the 



