NERVE FORCE. 233 



modification of it. A third view supposes it to be a special form of 

 the common force of nature, correlated to electricity, and, through it, 

 to all the other forms of that force. 



In considering these views, it is necessary to bear in mind that there 

 are many reasons why the physiological energy of nerve substance, or 

 nerve force, is to be regarded as something different from electricity. 

 First, if it were electricity, it would be conductible along a piece of 

 copper or other metallic conductor; whereas, if a nerve be divided, and 

 its cut ends be connected, by laying a piece of metal wire between 

 them, the one portion of the nerve does not act when the other portion 

 is excited ; or, in other words, the nerve force cannot pass along the 

 metal wire. Nerve fibres, or nerve cells, are the only structures along 

 which the nerve force can be propagated. Secondly, cold diminishes 

 the conducting power of nerves, for the nerve force ; whereas it in- 

 creases the conducting power of solids or fluids, for electricity. 

 Thirdly, the crushing of a nerve, or tying it tight, and afterwards 

 loosening it, interferes with the future passage of the nerve current ; 

 whereas the mere bruising of a wire, does not stop electricity from 

 passing through it. It may be, however, that the case of an injured 

 nerve, should rather be compared with that of a compound telegraph 

 wire, in which the internal copper conducting wires are broken, whilst 

 the outer supporting coils and coverings of iron wire, rope, and gutta- 

 percha, are uninjured. Fourthly, as has already been mentioned, the 

 nerve current travels at an extraordinarily slow rate, as compared with 

 that of electricity. Lastly, from careful experiments performed by 

 Pfliiger, it appears that the nerve force increases in power, in propor- 

 tion to the length of nerve excited ; that is to say, the effect in .causing 

 muscular contraction is greater, the further from the muscle, or the 

 nearer to the nervous centre, the nerve is excited ; and so, in the reflex 

 action of nerves, the nearer to the peripheral ends of the nerve the 

 stimulus is applied, the greater the effect. It would seem, therefore, 

 that either the nerve force gathers strength as it passes along a nerve, 

 or excites the development of additional nerve force as it travels along. 

 This peculiarity distinguishes it from electricity, which has no such 

 power of exciting new force within a conductor, but rather tends 

 gradually to become itself exhausted. From these various facts, it 

 appears safe to conclude that nerve force is, at least, something differ- 

 ent from electricity ; and a force so far peculiar to living animals, and 

 to specially organized living tissues in animals, viz., to nerve cells and 

 nerve fibres, that it cannot be manifested, conducted, or propagated, 

 excepting in and through those tissues. There remain, however, the 

 further important questions, whether, and in what manner, it is related, 

 or correlated, to electricity, and through it to the common force of 

 nature. It has been shown by the physicist that mechanical force, 

 producing motion, is correlated with and convertible into heat, heat 

 into chemical force, chemical force into electrical force, and electrical 

 into magnetic force ; moreover, that each of these is correlated, and 

 convertible into the other, or, indeed, any one of them into any other, 

 all being thus interchangeable. Now, it is not supposed that the force 

 acting in a nerve, is identical with electrical force, nor yet, a peculiar. 



