STRUCTURE OF NERVE CELLS. 60 1 



The corresponding changes in sensory nerve cells after section of 

 the peripheral nerve, have been studied by Lugaro, 1 R. Fleming, 2 Cox, 3 

 and others ; partial atrophy and chromatolysis in the spinal ganglion 

 cells having been noted by them. 4 Lugaro failed to find any definite 

 changes in the posterior ganglion cells on cutting the root between the 

 ganglion and the spinal cord, or at most a very slight amount of 

 chromatolysis. This is in accordance with the observation that in tabes, 

 with complete atrophy of the fibres of the posterior columns of the cord, 

 the spinal ganglion cells may be intact, van Gehuchten, 5 with Nelis, 

 has pursued similar investigations upon the effects on the cells of the 

 ganglion of the vagus, of cutting that nerve peripherally to the ganglion. 

 These observers find that changes, quite comparable to those above 

 described for motor nerves, proceed in the cells of this ganglion, but 

 that the process of disintegration is not replaced by a process of 

 reparation, so that at the end of three months the cells have almost all 

 disappeared. The complete disintegration of some motor cells, as a 

 result of division of motor nerves, has also been affirmed by NissL I 

 have found a similar change to take place with the cells of Clarke's 

 column of the spinal cord, after section nearer the bulb of the afferent 

 (cerebellar) tracts. The cells on the side of the section undergo 

 chromatolysis, atrophy, and ultimately almost disappear. 6 



In all cases the nerve fibres cut off from the cell body undergo disin- 

 tegration and disappear in a few days. In the case of the medullated 

 nerves, their section also involves a breaking up and gradual removal of 

 the substance of the medullary sheath, accompanied by multiplication 

 of the nuclei which are embedded in this sheath. This is the well- 

 known Wallerian degeneration of nerve fibres, 7 and the first signs of it 

 appear about twenty-four hours after section of the nerve, in the 

 commencing fragmentation of the medullary sheath. This in two or 

 three days is broken up into segments of variable size, and these, as the 

 degeneration progresses, become gradually resolved into drops and 

 droplets of myelin. Simultaneously with or immediately before these 

 alterations in the myelin sheath, a change in the chemical consti- 

 tution of the fibrils of the axis cylinder appears, as shown by an 

 alteration in their capacity for staining, 8 and it is about this time that 

 the peripheral part of the nerve ceases to be excitable by electrical 

 stimulation, although for the first few hours its excitability was 

 increased. The fibrils next become wavy and varicose, and soon break 

 up into isolated portions (granules), which break up still further, so 

 that they are soon indistinguishable from the perifibrillar substance. 

 Similar changes occur for a certain distance along the central part of 

 the cut nerve. The degenerated axis cylinder may show complete 



1 Op. cit. ; Riv. dipatol. nerv., Firenze, 1896, vol. i. 



3 Edin. Med. Journ., 1897, N.S., vol. i. p. 174 ; and Brain, London, 1897, vol. xx. p. 56. 



3 Monthly Internal. Journ. Anat. and Physiol., 1898, vol. xv. p. 241. 



4 Cox (loc. cit. ) finds that the changes occur in some cells early, in others considerably 

 later, and points out the possibility of there being two kinds of cells in the ganglia, as 

 Dogiel (Anat. Anz., Jena, Bd. xii.) had also supposed on other grounds. 



6 Op. cit. 



e "Proc. Physiol. Soc.,'"' March 18, 1899, Journ. Physiol., Cambridge and London, vol. 

 xxiv. 



7 Augustus AValler, " Experiments on the Section of the Glosso-Pharyngeal and Hypo- 



flossal Nerves, and Observations on the Alterations produced in the Structure of their 

 rimitive Fibres," Phil. Trans., London, 1850, pt. 2, p. 423; "Nouvelle method e pour 

 1'etude du systeme nerveux, etc.," Convpt. rend. Acad. d. sc., Paris, 1851, tome xxxiii. p. 606. 



8 Monckeberg and Bethe, loc. cit. 



