202 



NERVE-CENTRES 



[CH. XVII. 



piece of evidence in favour of the latter view. 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 stained 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. 



Chromatolysis is the term applied to designate the disappearance 



FIG. 214. Nissl's granules. A. Normal pyramidal cell of human cerebral cortex. B. Swollen cede- 

 matous pyramidal cell from a case of status epilepticus. Notice difluse staining, and absence of 

 Nissl's granules ; the nucleus is enlarged and eccentric. The lymph space around the cell is 

 dilated. C. Pyramidal cell of dog after ligature of vessels going to brain and consequent anaemia. 

 Notice great swelling of the nucleus, and advanced chromatolysis, most marked at the periphery 

 of the cell. 700 diameters. (After Mott.) 



or disintegration of the Mssl granules. The process generally begins 

 at the periphery of the cell and in the dendrons, but in advanced cases 

 the whole cell may be affected. We have already alluded to the fact 

 that chromatolysis occurs in various abnormal states, and the diminu- 

 tion of the chromophilic nucleo-proteid indicates a diminution of the 

 vital interaction of the highly phosphorised nucleus with the sur- 

 rounding cell protoplasm. 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 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 



