THE NERVOUS SYSTEM. 807 



REGENERATION. 



Regeneration of nerve tissue in the sense of an actual reproduction 

 of neurones probably never occurs in the adult human nervous system. ' 

 As has been noted, extreme chroniatolysis may be succeeded by complete 

 recovery. In the case of axonal chroniatolysis, the degenerative process 

 usually reaches its maximum in about three weeks. Regeneration of 

 chromophilic bodies then begins. This reconstructive process is slow, 

 the cells often requiring mouths to return to a normal condition. At 

 some stage of the process there is usually an over-production of chro- 

 matic substance, and the cells appear darker than normal. 



Primary union between the ends of divided nerve fibres does not 

 occur. Complete degeneration of the distal portion always precedes the 

 regenerative process. In this reconstruction the nuclei of the neurilenima 

 seem to play an important part. They increase in number, and there is - 

 also an increase in the protoplasm which surrounds them. With union -" 

 of the divided ends the axones of the central stump may grow out again __ 

 if the ganglion cells be intact, and ultimately resume function. The 

 rnyelin first reappears as droplets which coalesce and finally form a 

 complete sheath. This reconstructive process in the nerve fibre is ex- 

 tremely slow, often requiring many mouths for its completion. 



REPLACEMENT NEUROGLIA HYPERPLASIA. Under various condi- 

 tions in which there is destruction of the parenchyma, as, e.g., in ascend- 

 ing and in descending degeneration in the spinal cord, there occurs a 

 compensatory increase in the interstitial elements. This new tissue is at 

 first cellular, most of the cells being of the spider variety. Later there 

 is an increase in the ueuroglia fibres, and the tissue often becomes dense 

 and hard. What part the mossy cells or other less common types of 

 neuroglia cells take in the proliferative process is as yet unknown. 

 With the increase in fibres there are often shrinkage and the formation of 

 dense fibrous tissue. This replacement hyperplasia in neuroglia seems 

 quite similar in nature to replacement connective-tissue hyperplasia in 

 other organs, and is often considered inflammatory. 



DEGENERATIONS AFFECTING SYSTEMS OF NEURONES. 



General Considerations Concerning- Neurone Systems. 



Before considering the subject of systemic degeneration we may refer briefly to 

 the situation in the nervous system of certain of the more important groups of neurones 

 and the paths which their axones take. 



That the cell bodies of neurones are grouped in the gray matter of the brain, cord 

 and ganglia, and in the end organs of certain nerves of special sense, has already been 

 mentioned. This grouping of neurones serves definite physiological ends. Their cell 

 bodies form centres or nuclei, while their axoues are collected into bundles or fibre 

 tracts. Thus, in the region of the fissure of Rolando are grouped the centres of those 

 neurones which have to do with voluntary motion, and what is known as cerebral local - 



1 For review and literature on "Regeneration," see Barker, "The Nervous System," 

 p. 245 e t seq. 



