796 



NERVE 



A spinal nerve is composed of efferent fibres whose cells of origin 

 are in the grey matter of the anterior horn, and afferent fibres 

 whose cells of origin are in the posterior root ganglion. When such 

 a nerve is cut below the junction of its roots, muscular paralysis 

 and impairment of sensation at once follow in the region supplied 

 by the nerve; but for a time the nerve remains excitable to direct 



stimulation. The excitability 

 gradually diminishes, and in a 

 few days is completely gone. 

 If portions of the nerve distal 

 to the lesion are examined at 

 different periods after section, a 

 remarkable process of degenera- 

 tion (commonly spoken of as 

 Wallerian degeneration) is seen 

 to be going on. In the medul- 

 lated fibres this begins on the 

 second or third day with a 

 swelling of the axis-cylinder, 

 which breaks up into detached 

 pieces (fragmentation), and as- 

 sumes a granular appearance. 

 The medullary sheath also under- 

 goes fragmentation at the lines 

 of Lantermann, and a little later 

 separates into clumps and drop- 

 lets of myelin. The nuclei under 

 the neurilemma increase in size, 

 proliferate by mitosis, and in- 

 sinuate themselves between the 

 fragments of the medullary 

 sheath and axis-cylinder, which 

 ultimately disappear, leaving the 

 nerve- fibre represented only by a 

 kind of mummy of connective 

 tissue, in which the neurilemma 

 with its abnormally numerous 



Fig. 277. Degeneration of Nerve-Fibres 

 after Section (Barker, after Thoma). 

 I, normal fibre; II, degenerating fibre; 

 III, further stage of degeneration; 

 S, neurilemma ; m, medullary sheath; 

 A, axis-cylinder; L, Lantermann's line 

 or cleft; R, node; mt, drops of myelin; 

 a, remains of axis-cylinder ; w, prolifera- 

 ting cells of neurilemma. 



nuclei can still be recognized. 



The fragmentation of the myelin 

 sheath is not dependent upon the proliferation of the nuclei, since it 

 occurs also in nerves removed from the body and kept under con- 

 ditions in which the nuclei do not proliferate (Feiss and Cramer). 

 The protoplasm around the nuclei of the neurilemma also increases in 

 amount, and undergoes other changes, which will be more particularly 

 referred to in describing the regeneration of nerve. The degenerative 

 process begins near the cut end, and extends gradually to the peri- 



