NERVE TISSUE 



145 



The neurilemma cells of the peripheral nerve fibres are peculiar in that they do 

 not degenerate; instead of this their protoplasm increases, and their nuclei pro- 

 liferate. These cells are apparently concerned in the disintegration and absorp- 

 tion of the myelin. They also form protoplasmic bands which play an important 

 part in the regeneration of the nerve (see below). The same rule holds good for 

 dendrites as for axones as far as it has been possible to determine, namely, that 

 cut off from their cells of origin they undergo complete degeneration. 



At the time the law of Wallerian degeneration was established it was believed 

 that the central portion of the nerve and the cell bodies remained intact after 

 division of the nerve. More recently the method of JNIarchi (for nerve fibres) and 



Fig. 89. — Two Motor Cells from Ventral Horn of Dorsal Cord of Rabbit; fifteen 

 days after cutting major sacro-sciatic nerve. A, Cell in which the chromophilic bodies 

 appear disintegrated and nucleus eccentric; B, cell showing more advanced chromatoly- 

 sis, the chromophilic substance being present only in the dendrites and around the 

 nucleus in the form of a homogeneous mass; nucleus causes bulging of surface of cell. 



the method of Nissl (for neurone bodies) have shown marked degenerative 

 changes in the parts proximal to the lesion. The extent and rapidity of these 

 changes are dependent mainly upon three factors (i) the type of neurone — some 

 neurones being apparently more resistant than others to injury; (2) the 

 character of the injury — e.g., tearing the nerve causing the greatest reaction; 

 cutting the nerve, a reaction of less intensity; pinching the nerve, the least degree 

 of reaction; and (3), the location of the injur>', an injury near the cell body caus- 

 ing more effect centrally than one at a distance. In other words, the effect de- 

 pends upon the percentage of the neurone cut off. If the injury is very near the 

 cell body the latter may ultimately disappear, the proximal portion of the nerve 

 fibre which remains attached to it undergoing a final degeneration similar to 

 that of the distal severed portion. In the cell body there is an initial turgescence, 

 10 



