AND ITS MODE OF ACTION. 395 



usually adopted in the galvanic machines used in medical practice, 

 for the treatment of certain paralytic affections. In these machines, 

 the electric circuit is alternately formed and broken with great 

 rapidity, thus producing the greatest effect upon the nerves with 

 the smallest expenditure of electricity. Such alternating currents, 

 however, if very powerful, exhaust the nervous irritability more 

 rapidly and completely than any other kind of irritation ; and in 

 an animal killed by the action of a battery used in this manner, the 

 nerves may be found to be entirely destitute of irritability from the 

 moment of death. 



The irritability of the nerves is distinct from that of the muscles; and 

 the two 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 attached, the muscles contract, as we 

 have seen, 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. The two properties may be separated from each 

 other, however, by the action of woorara, which has the power, as 

 first pointed out by Bernard, 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 ; show- 

 ing that the nervous irritability has ceased to exist. But if the 

 galvanic discharge be passed directly through the muscles, contrac- 

 tion at once takes place. The muscular irritability has survived 

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

 distinct from it. 



It will be recollected, on the other hand, that in cases of death 

 from the action of sulphocyanide of potassium, the muscular irri- 

 tability is itself destroyed ; so that no contractions occur, even when 

 the galvanic discharge is made to traverse the muscular tissue. 



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

 paralysis, in which the muscular fibres themselves are directly 

 affected ; and second, a nervous paralysis, in which the affection is 

 confined to the nervous filaments, the muscles retaining their natural 

 properties, and being still capable of contracting under the influence 

 of a direct stimulus. 



Nature of the Nervous Force. The special endowment by which 

 a nerve acts and manifests its vitality is a peculiar one, inherent in 

 the anatomical structure and constitution of the nervous tissue. It is 



