NATURE OF THE NERVE IMPULSE. 121 



has been discovered more recently and that is perhaps of more im- 

 portance is that the nerve cells do undergo certain definite although 

 usually temporary changes immediately after the section of the 

 nerve fibers arising from them. It has been shown that when a 

 nerve fiber is cut the corresponding cell may show distinct histo- 

 logical changes within the first twenty-four hours. These changes 

 consist in a circumscribed destruction of the chromatin material 

 in the cells (chromatolysis) which in a short time extends over 

 the whole cell, so that the primary staining power of the cell is 

 lost (condition of achromatosis) (see Fig. 58). The cell also be- 

 comes swollen and the nucleus may assume an excentric position. 

 These retrogressive changes continue for a certain period (about 

 eighteen days). After reaching their maximum of intensity the cells 

 usually undergo a process of restitution and regain their normal 

 appearance, although in some cases the degeneration is permanent. 

 After section of the nerve fibers, therefore, two processes of degenera- 

 tion may occur in the central cells : one temporary, that reaches its 

 maximum in two to three weeks, and from which the cell recovers 

 completely, as a rule, in about three months; and one permanent, 

 which comes on only after many months or even years and is to 

 be regarded as the result of prolonged inactivity.* 



*Nissl, " Allgemeine Zeitschrift f. Psychiatrie," 48, 197, 1892. Also 

 Bethe, loc. cit. 



