6o6 A MANUAL OF PHYSIOLOGY 



essentially similar phenomena take place also in the fibres 

 of the central nervous system. 



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 com- 

 pletely gone. If portions of the nerve distal to the lesion 

 are examined at different periods after section, a remarkable 

 process of degeneration (commonly spoken of as Wallerian 



FIG. 188. DEGENERATION OF SPINAL NERVES AND THEIR ROOTS AFTER 

 SECTION. The shading shows the degenerated portions. 



degeneration) is seen to be going on. In the medullated 

 fibres this begins on the second or third day with a swelling 

 of the axis-cylinder, which breaks up into detached pieces 

 (fragmentation). The medullary sheath also separates into 

 clumps and droplets of myelin. The nuclei under the neuri- 

 lemma proliferate and insinuate 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 nuclei can still 

 be recognised. This process goes on simultaneously along 

 the whole nerve, from the cut end to the periphery, and 

 more rapidly in warm than in cold-blooded animals. In a 



