128 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



of the individual. Dickinson, however, in 1869 * showed that in 

 amputations of long standing the motor cells in the anterior horn 

 of the cord decrease in number and the fibers in the central stump 

 become atrophied. This observation has been corroborated by 

 other observers, and it is now believed that after section of a nerve 

 chronic degenerative changes ensue in the course of time in the 

 central fibers and their cells, resulting in their permanent atrophy. 

 We have, in such cases, what has been called an atrophy from 

 disuse. A fact that has been discovered more recently and that 

 is perhaps of more importance 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 is cut the corresponding cells of origin 

 may show distinct histological changes within the first twenty-four 

 hours. These changes consist in a circumscribed destruction of 

 the chromatin material in the cells (chromato lysis), 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. 63). 

 The cell also becomes swollen and the nucleus may assume an 

 excentric position. These retrogressive changes continue for a 

 certain period (about eighteen days). After reaching their maxi- 

 mum of intensity the cells usually undergo a process of restitution 

 and regain their normal appearance, although in some cases the 

 degeneration is permanent. According to other observers a number 

 of the cells in the spinal cord and spinal ganglia undergo simple 

 atrophy after section of their corresponding nerves, and some of 

 the nerve fibers in the central stumps may also show atrophy, 

 while others undergo a genuine degeneration, which, however, 

 comes on much later than in the peripheral stumps. It seems 

 evident that the behavior of the cells and fibers on the central side 

 of the section is not uniform; atrophy rather than degeneration is 

 the change that is prominent, and this atrophy in some neurons 

 occurs early, while in others it is apparent only after a long interval 

 of time. An explanation of this variation in the reaction of the 

 nerve cells and their disconnected central stumps cannot yet be 

 given. On the peripheral side of the section, as stated above, the de- 

 generative changes are complete and affect all of the fibers. t 



* " Journal of Anatomy and Physioloogy," 3, 176, 1869. 



t Nissl, "Allgemeine Zeitschrift f . Psychiatric," 48, 197, 1892. Also Bethe, 

 loc. tit., and Ranson, " Retrograde Degeneration in the Spinal Nerves," The 

 Journal of Comparative Neurology a.nd Psychology, 1906, xvi., 265. 



