422 NERVOUS IRRITABILITY 



No. 1, and the negative pole, 6, in that containing log No. 2, it will be 

 seen that the galvanic current will traverse the two legs in opposite 

 directions. In No. 1, it will pass in a direction contrary to the course 

 of its nervous fibres, that is, it will be for this leg an inverse current ; 

 while in No. 2 it will pass in the same direction with that of the nervous 

 fibres, that is, it will be for this leg a direct current. It will now be 

 found that at the moment when the circuit is completed, a contraction 

 takes place in No. 2 by the direct current, while No. 1 remains at rest ; 

 but at the time the circuit is broken, a contraction is produced in No. 1 

 by the inverse current, while no movement takes place in No. 2. A suc- 

 cession of alternate contractions may thus be produced in the two legs 

 by repeatedly closing and opening the circuit. If the position of the 

 poles, a, 6, be reversed, the effects of the current will be changed in a 

 corresponding manner. 



After a nerve has become exhausted by the direct current, it is still 

 sensitive to the inverse ; and after exhaustion by the inverse, it is still 

 sensitive to the direct. It was even found by Matteucci that after a 

 nerve has been exhausted for the time by the direct current, the return 

 of its irritability is hastened by the subsequent passage of the inverse 

 current ; so that it will become again sensitive to the direct current 

 sooner than if allowed to remain at rest. Nothing, accordingly, is so 

 exciting to a nerve as the passage of direct and inverse currents, alter- 

 nating with each other in rapid succession. Such a mode of applying 

 the electric stimulus is that afforded by the Faradic apparatus, in which 

 momentary currents of induced electricity are made to traverse the cir- 

 cuit in two opposite directions in rapid alternation. 



The irritability of motor nerves is distinct from that of the muscles. 

 This is shown by the fact that the two properties may be destroyed 

 or suspended independently of each other. When the frog's leg has 

 been prepared and separated from the body, with the sciatic nerve at- 

 tached, the muscles contract, as shown above, whenever the nerve is 

 irritated. The irritability of the nerve, therefore, is manifested in this 

 instance only through that of the muscle, and that of the muscle is 

 called into action only through that of the nerve. But the action of 

 woorara has the power, as first pointed out by Bernard, 1 of destroying 

 the irritability of the nerve without affecting that of the muscles. If a 

 frog be poisoned by this substance, and the leg prepared as above, the 

 poles of a galvanic battery applied to the nerve will produce no effect. 

 But if the galvanic discharge be passed directly through the muscles, 

 contraction takes place. The muscular irritability has survived that 

 of the nerves, and must therefore be regarded as essentially distinct 

 from it. 



There are, therefore, two kinds of paralysis : first, a muscular par- 

 alysis, in which the muscular fibres themselves are directly affected ; 



1 Le<jons sur la Physiologic du Systfeme nerveux. Paris, 1858, tome i. p. 199. 



