THE INFLUENCE OF TEMPERATURE. 521 



brates as the lobster give -048 D, 1 in Anodon and in Meclone -026 D. 2 

 In vertebrates, the olfactory of the pike gives '02 D, whereas niedullated 

 nerves from this fish of the same cross sectional area gave only -01 D. 3 

 A still greater contrast is that between the sciatic nerve and the 

 spinal cord ; the latter gives a very large cross sectional difference of 

 potential, -046 D in the cat, and '029 D in the monkey. These latter 

 differences are not due to the larger cross sectional area, since, on com- 

 paring the cord of a small monkey with a sciatic nerve from a large 

 animal of the same species, the cord difference was found to be three 

 times that of the nerve. 4 



Finally, in many nerves, the difference varies with the locality 

 of the cross section. A striking illustration of this is afforded by 

 comparing the lumbar posterior roots with the whole sciatic nerve. 

 In the cat the seventh lumbar root gave '026 D, whereas the whole 

 sciatic nerve gave only -018 D. 4 It appears, further, that in many 

 purely afferent nerves a more central cross section is more effective than 

 a more peripheral one, so that, on connecting the two cross sections by 

 appropriate contacts, the more central is galvanometrically negative to 

 the more peripheral, and a similar difference is found between two 

 points on the longitudinal surface, when equidistant from their respective 

 cross sections. 5 On the other hand, in some purely efferent nerves, this 

 relationship is reversed. Thus, in the efferent electrical nerves of 

 Torpedo, the increased negativity of a peripheral cross section over a 

 more central one amounts to "002 D. 6 



It is not improbable that these results are, to some extent, associated with 

 the varying susceptibility of the nerve fibres to injury through external 

 agencies owing to differences in the sheaths, etc., of the different regions. 



If the phenomena in question are present, the one in all afferent, the other 

 in all efferent fibres, then, in a mixed nerve, the results must vary in pro- 

 portion as the afferent fibres are more or less numerous than the efferent 

 ones. As a matter of fact, such a mixed nerve as the sciatic gives results 

 which vary with the position of the cross section. 



The large electromotive effect obtained with non-medullated nerves shows 

 that the medullary sheath plays little or no part in the production of the 

 difference. If the sources of the change are localised in the axis cylinders, 

 the absence of such an additional moist envelope would serve to bring the 

 experimental contacts into far closer proximity with the electromotive source, 

 and the large difference would be explicable. 



It is probable, however, that although the absence of the sheath must be a 

 prominent factor in producing larger effects, it is the electromotive characters 

 of the axis cylinders themselves which are the most potent factor. Hence 

 we are led to infer that different axis cylinders vary as regards the amount 

 of the electromotive change, which can be brought into prominence by their 

 cross section. 



(b) Temperature. — In any nerve which shows the resting cross 



J Fredericq, Arch. f. Physiol., Leipzig, 1880, S. 68,71. 



2 Biedermann, Sitzmigsb. d. k. Akad. d. Wissensch., Wien (3), Bd. xciii. ; Fnchs, ibid., 

 1894, Abth. 3. 



3 Ktiline, Untersuch. a. d. physiol. Inst. d. Univ. Heidelberg, Bd. iii. S. 149. 



4 Gotch and Horsley, Phil. Trans., London, 1891, vol. clxxxii. B, p. "267 et seq. 

 5 Schitf, "Lehrbuch d. Muskel- u. Nerven-Physiol.," 1859; Mendelssohn, Arch. f. 



Physiol., Leipzig, 1885, S. 381. 



6 du Bois-Reymond, loc. cit. ; "Living Torpedoes in Berlin," ''Biological Memoirs," 

 Oxford, 1887, p. 464. 



