598 THE NERVE CELL. 



arrangement of their nervous system was at one time regarded as 

 affording one of the surest bases for its foundation ; and we must 

 find some other reason than want of continuity of the fibrillae 

 for the explanation of the fact that, in preparations made by the 

 chromate methods of Golgi, the nerve cells always appear distinct from 

 one another and never actually united. But if we hold with Leydig 

 (see below, note 4) that the fibrillre are not the true conducting material, 

 but merely supporting structures, and that the conducting function 

 is carried on by the perifibrillar substance, which, it is possible to 

 suppose, does not share the continuity of the fibrillar we can, whilst 

 accepting the conclusions of Apathy regarding the continuity of the 

 nerve fibrils, still hold the neurone theory, so far as concerns the peri- 

 fibrillar substance. According to Mbnckeberg and Bethe, 1 the peri- 

 fibrillar substance is discontinuous at the nodes of Kanvier. 



There is some reason to believe that nerve fibrils are tubular. The fact 

 that they are prone after death and under the influence of reagents to 

 become varicose, is an indication of such a structure. Monckeberg and 

 Bethe 2 account for the varicosities by supposing that the interfibrillar sub- 

 stance may form droplets around the fibrils. But in sections of nerve fibres, 

 the axis cylinders of which have not undergone any marked shrinking in the 

 process of preparation, the fibrils, when examined or photographed with a very 

 high magnifying power, have a distinctly tubular appearance. 3 



It is generally assumed, the existence of fibrils in the nerve element being 

 presupposed, that these are the parts of the element along which the con- 

 duction of nerve impulses actually takes place. 4 If this is so, we can conceive 

 that the nervous impulses might pass along such minute tubes, in the form of 

 a chemical change propagated from molecule to molecule, or as a physical 

 change in the form of waves, started either by external mechanical means 

 (tactile nerves, auditory nerves), or by contraction of the body of a nerve cell 

 (motor nerves (?), olfactory nerves (?), visual nerves). The fact that nerve 

 fibres are practically unfatiguable is in favour of a purely physical explanation 

 of their peculiar manifestation of activity ; moreover, no chemical change and 

 no development of heat 5 can be shown to occur in them as a result of their 

 excitation. This is not the case with the body of the nerve cell, for the grey 

 matter of the nerve centres does tend to become acid and also warmer 6 after 

 excitation. The fatigue of nerve centres is in fact a matter of every-day 

 experience, and it has also been shown by Mosso 7 that such fatigue, produced 

 by mental strain, is accompanied by the development of fatigue products, 

 which cause, through the circulation, fatigue effects on the voluntary 

 muscles. It must, however, be conceded that both the unfatiguability and 

 the absence of demonstrable chemical change in nerve fibre does not necessarily 

 prove that no chemical changes are occurring ; for such changes may be so 

 slight as to be entirely beyond our means of observation, owing to the very small 



1 Loc. cit. According to Engelmaim, this discontinuity at the nodes of Ranvier involves 

 the nerve fibrils of the axis cylinder {Arch. f. d. gcs. Physio/., Bonn, 1880, Bd. xxi.). 



2 Loc. cit. 



3 See, for example, Fig. 116 in my " Essentials of Histology," London, 1898. For the 

 recent literature of the structure of nerve cells, see further Flemming, " Morphologie 

 der Zelle" in "Ergebnisse der Anatomie u. Entwickl.," Wiesbaden, 1898, Bd. vii. ; 

 Goldscheider and Flatau, "Norm. u. path. Anat. d. Nervenz. u.s.w.," Berlin, 1898; 

 Nissl, "Nerven u. graue Substanz," Milmhen. med. Wchnschr., 1898, Nos. 31, 32, 33. 



4 Leydig holds that it is more probably the interfibrillar substance which is actually 

 the conducting part of the nerve fibre. "Der reizleitende Theil des Nervengewebes," 

 Arch. f. Anat. u. Entwcklngsgesch., Leipzig, 1897, S. 431. 



5 H. D. Rolleston, Journ. Physiol., Cambridge and London, 1890, vol. xi. p. 208. 



6 Mosso, Croonian Lecture, Phil. Trans., London, 1892, vol. clxxxiii. B. p. 299. 



7 Trav. du lab. dc physiol., Turin, 1890. 



