522 NERVE. 



sectional difference, local warming in the neighbourhood of the cross 

 section up to 35° C. increases, whilst local cooling diminishes the amount 

 of the electrical derivation. It is not clear how far such increase and 

 diminution is dependent upon the alteration in the resistance of the 

 moist conductor, with its consequent variation in electrical distribu- 

 tion. There are, however, other considerations in connection with 

 excitatory electromotive changes which afford a basis for the belief 

 that temperature also acts upon the source of the electromotive change 

 in such a way that warmth increases the susceptibility of the axis 

 cylinders to the alterations produced in the neighbourhood of the cross 

 section. 



(c) Connection iviih nerve cells. — The functional and structural altera- 

 tions which follow the severance of their cellular connection cause very 

 marked changes in the nerve fibres as regards the production of the 

 resting electromotive differences. As might be assumed, the onset of 

 the degenerative processes is associated with diminished demarcation 

 difference. It is interesting to note that such difference, although 

 diminished in amount, can be observed in degenerated mammalian 

 nerve after the functional attribute of conductivity has disappeared. 

 The interruption of continuity, which is the cause of the latter, occurs 

 at intervals along the whole course of the fibres, but histological 

 examination in this earlier stage of the process shows that any cross 

 section may comprise fragments of axis cylinders with their medullary 

 sheaths. The existence of the cross sectional difference of potential 

 shows that even in their altered condition such fragments are still 

 susceptible of the change which is the basis of the resting difference. 



The electromotive source under these circumstances is presumably localised 

 in the fragments of swollen axis cylinders which happen to be included in the 

 area of the cross section, for, when complete degeneration has been attained, no 

 cross sectional difference is found. Thus, if a portion of a nervous column be 

 completely degenerated, the normal amount of the resting difference of such a 

 tissue is very much diminished. Examples of this are the following : the differ- 

 ence in the normal spinal cord of the cat on the lower dorsal region was found to 

 average "03 I) ■ after section of all the lumbar posterior roots three months 

 previously, the difference was only "012 D ; after hemisection above the 

 investigated area three months previously, it was "018 D, and "02 D after 

 degeneration through section of one posterior column below the investigated 

 area. 1 In these cases the lower values are chiefly due to the substitution of 

 some nerve fibres by a layer of indifferently conducting tissue, thus increasing 

 the moist envelope ; but it is possible that the remaining fibres, although 

 apparently normal in structure, may have lost their susceptibility to the pro- 

 duction of the electromotive effect, and that in addition to the increased con- 

 nective tissue envelope, there is a diminution in the amount of the electromotive 

 change produced in the apparently sound fibres. 



(d) Arrest of the circulation. — The arrest of the circulation, or the 

 removal of the nerve from the body, is followed by changes in the 

 amount of the difference, the consideration of which involves the question 

 as to the duration of the change in freshly excised nerves. 



The prominent feature exhibited by such nerves is a steady decrease |j 

 in the amount of the resting difference, this being much more marked I 

 in warm-blooded than in cold-blooded animals. In the case of the! 

 sciatic nerve of the frog, the difference may in two to four hours sink i 



1 Gotch and Horsley, Phil. Trans., London, 1891, vol. clxxxii. B, p. 267 ct seq. 



