NATURE OF THE CHANGES IN THE NERVOUS SYSTEM. 99 



there is a great analogy between the propagation of nervous and that of elec- 

 trical influence, cannot be denied. But the reasons in favour of their identity 

 are not greater than those which might be adduced to prove that nervous in- 

 fluence is identical with other physical forces ; since mechanical and chemical 

 stimulation will, equally with electricity, imitate, to a certain extent, the natural 

 changes in this system. On the other hand, there are many valid reasons 

 against such a supposition ; of which one of the most cogent is, that by putting 

 a ligature round a trunk, its functions as a conductor of nervous influence are 

 paralyzed, whilst it is still capable of conveying electricity. The various fibrils, 

 too, are not as completely insulated from each other in regard to the passage of 

 electricity, as we know them to be in respect to nervous agency. To the 

 influence (whatever its nature may be) which the nerves convey, the term vis 

 nervosa has been provisionally applied. ; and it is convenient to employ a term 

 of this nature, when the laws according to which it operates are being speci- 

 fied. It must be remembered, however, that nothing is really gained by the 

 use of such a term, which resembles one of the unknown quadpies in algebra. 

 It is quite possible that the changes in the afferent nerves may differ from those 

 that take place in the efferent ; and that the changes which convey some kinds 

 of impressions through the former, may differ from those concerned in others. 

 No real progress is made, therefore, by attributing any phenomena of the ner- 

 vous system to the vis nervosa ; any more than by referring the various mate- 

 rial changes in the organism to the operation of the vital principle. The 

 laws according to which these changes take place are, however, legitimate 

 subjects for physiological investigation. Those regulating the propagation of 

 nervous agency may be briefly stated as follows. They evidently result from 

 the facts already mentioned, respecting the isolated character of each fibril, 

 and the identity of its endowments through its whole course. They are here 

 stated, with some modification, in the language of Mil Her. 



I. When the whole trunk of a sensory nerve is irritated, a sensation is pro- 

 duced, which is referred by the mind to the parts to which its branches are 

 ultimately distributed ; and if only part of the trunk be irritated, the sensation 

 will be referred to those parts only which are supplied by the fibrils it con- 

 tains. This is evidently caused by the production of a change in the senso- 

 rium, corresponding with that which would have been transmitted from the 

 peripheral origins of the nerves, had the impression been made upon them. 

 Such a change only requires the integrity of the afferent trunk between the 

 point irritated and the sensorium ; and is not at all dependent upon the state 

 of the extremity to which the sensations are referred. This may have been 

 paralyzed by the division of the nerve ; or altogether separated, as in ampu- 

 tation ; or the relative position of its parts may have been changed. It results 

 from the foregoing, that, when different parts of the thickness of the same 

 trunk are separately subjected to irritation, the sensations are successively 

 referred to the several parts supplied by these divisions. This may be easily 

 shown by compressing the ulnar nerve, in different directions, where it passes 

 at the inner side of the elbow-joint. 



II. The sensation produced by irritation of a branch of the nerve, is con- 

 fined to the parts to which that branch is distributed, and does not affect the 

 branches which come off from the nerve higher up. The rationale of this 

 law is at once understood : but it should be mentioned that there are certain 

 conditions, in which the irritation of a single nerve will give rise to sensations 

 over a great extent of the body. This is due, however, to a particular state 

 of the central organs, and not to any direct communication among the sensory 

 fibres. X ' ,* ,,*< ,^ 



III.. The motor influent is?prcpagate< ohiy inte ceftf-riftig^Mirection, never 

 in a retrograde course. Jtjnay originate in a spoiltaneeu^ change in the cen- 



