NERVOUS IRRITABILITY. 363 



has been restored by repose, that voluntary motion and sensation are 

 reestablished. 



Different Action of the Direct and Inverse Currents. A galvanic 

 current which traverses the nerve in the direction of its motor fibres, 

 namely, from the centre toward the periphery, as from a to b (Fig. 95), 

 is called a direct current. If made to pass in the contrary direction, 

 from b to a, it is called an inverse current. When the nerve is exceed- 

 ingly irritable, or with a galvanic current of considerable intensity, 

 muscular contraction takes place at both the commencement and 

 termination of the current, whether direct or inverse. But when the 

 activity of the nerve has become somewhat diminished, or when the 

 current employed is of feeble intensity, contraction takes place only at 

 the commencement of the direct and at the termination of the inverse 

 current. If both hind legs of a frog be prepared in such a way that 

 they remain connected with each other by the sciatic nerves and a 

 portion of the spinal column, when the positive pole of a battery is 

 applied to the right foot and the negative pole to the left, the current 

 passing through the sciatic nerves will be an inverse current for the 

 right nerve, and a direct current for the left nerve. At the moment 

 of completing the circuit, a contraction will take place in the left 

 leg, but not in the right ; and when the current is broken, the right 

 leg contracts, while the left remains at rest. If the position of the poles 

 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 when a 

 nerve has been temporarily exhausted 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, follow- 

 ing each other in quick succession. Such a form of galvanism is that 

 afforded by the Faradic apparatus, in which rapidly alternating currents 

 of induced electricity traverse the circuit in opposite directions. 



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

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

 independently of each other. In the experiment above described, the 

 irritability of the nerve is manifested only through that of the muscle, 

 and that of the muscle is called into action only through that of the 

 nerve. But under the influence of woorara, the action of the motor 

 nerve, as shown by Bernard,* may be suspended without affecting 

 the irritability of the muscles. In a frog, poisoned by this substance, 

 the poles of a galvanic battery applied to the sciatic nerve will produce 

 no effect. But if the galvanic current be passed directly through the 



* Le9ons sur la Physiologic du Systeme Nerveux. Paris, 1858, tome i., p. 199. 



