194 NERVE-CENTRES [CH. XVI. 



valuable results have been obtained. We will, however, be content 

 with alluding to only one pathological condition, namely, that pro- 

 duced by extremely high fever (hyperpyrexia) ; in this condition 

 chromatolysis is very marked and is produced by the coagulation of 

 the proteins of the cell-protoplasm by the high temperature. 



The question has arisen whether the Nissl granules are present 

 as such in the living cell, or whether they are artifacts produced by 

 the fixative action of strong alcohol. But, whichever view is correct, 

 the method is a valuable one, and Nissl's views on this question 

 appear to be indisputable : they are briefly as follows : Healthy cells 

 fixed and stainejj in a constant manner will appear the same under 

 constant optical conditions, and the appearances then seen form the 

 equivalent of such healthy cells during life. It follows that if the 

 cells prepared by the same method and examined under the same 

 conditions show a difference from the equivalent or symbol of healthy 

 cells, the difference is the measure of some change that has occurred 

 during life. The view most generally held is that the granules are 

 artifacts, and that the actual Mssl substance in the living nerve-cell 

 is a fluid plasm of rich nutritive value to the fibrils. 



Chromatolysis alone is not indicative of cell destruction, and a 

 cell may recover its function afterwards. The integrity of the 

 nucleus and of the fibrils between which the Nissl substance lies is 

 much more important to the actual vitality of the cell. 



When a nerve-fibre is cut across, the distal segment undergoes 

 Wallerian degeneration ; this is an acute change. But the nerve-cell 

 and the piece of the nerve-fibre still attached to it do not remain un- 

 affected. If regeneration of the fibre, and restoration of function 

 takes place, no change is observable. But if regeneration does not 

 occur (and it never takes place in the central nervous system), the 

 cell and its processes undergo a slow chronic wasting ; one of the 

 earliest signs of this disuse atrophy is chromatolysis. 



Classification of Neurons according to their Function. 



In addition to the anatomical classification of the nerve-cells 

 already given, they may be grouped into four chief classes on a 

 physiological basis : 



1. Afferent neurons. 



2. Efferent neurons. 



3. Intermediary neurons. 



4. Distributing neurons. 



1. Afferent neurons. Originally the cell bodies are situated at the 

 periphery, and are connected with a process or afferent fibre which 

 passes to and arborises among the nerve-cells of the central nervous 



