4 88 NERVOUS SYSTEM 



is more vigorous the greater the extent of the nerve included between 

 the poles of the battery. This fact has long been observed, and its ac- 

 curacy may easily be verified. It would naturally be expected that the 

 greater the amount of stimulation, the more marked would be the mus- 

 cular action ; and the stimulation seems to be increased in proportion to 

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



The excitability of a nerve, it is well known, may be exhausted by 

 repeated applications of electricity, whatever be the direction of the 

 current ; and it is more or less completely restored by repose. When it 

 has been exhausted for the descending current, it will respond to the 

 ascending current, and vice versd ; and after it has been exhausted by 

 the descending current, it is restored more promptly by stimulation with 

 the ascending current than by repose, and vice versd. This phenom- 

 enon, observed by Volta, is known as "voltaic alternation." 



Many of the phenomena illustrating the law of contraction may be 

 observed without the use of complicated apparatus. A simple* form of 

 battery, very convenient for some of these experiments, consists of alter- 

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

 f<?rm 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 centimeters) long, which, when moistened with 

 water slightly acidulated, will give a current of about the proper strength 

 for many simple experiments. 



The law of contraction is applicable to inhibitory nerves, as the in- 

 hibitory nerve of the heart, the difference being that stimulation pro- 

 duces inhibition instead of contraction. It also holds good for sensory 

 nerves, the effects being observed by noting the reflex contractions 

 produced (Pfliiger). 



A peculiar phenomenon, described by Matteucci, has been called 

 " induced muscular contraction." If the nerve of a galvanoscopic frog's 

 leg is 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, 

 and contractions may thus be induced in a series of legs, the nerve of 

 one being in contact with the muscles of another. It is shown that 

 " induced contraction " is not due to an actual propagation of the elec- 

 tric current, but to a stimulus attending the muscular contraction itself, 

 by the fact that the same phenomena occur when the first contraction is 

 produced by mechanical or chemical excitation of the nerve. It prob- 

 ably is due to the negative variation of the muscle-current during con- 

 traction (see page 427). 



