NATURE OF THE NERVE IMPULSE. 127 



Degenerative Changes in the Neuron on the Central Side 

 of the Lesion. — According to the Wallerian law of degeneration, 

 as originally stated, the nerve fiber on the central side of the injury 

 and the nerve cell itself do not undergo any change. As a matter 

 of fact, the central stump immediately contiguous to the lesion 

 undera;oes typical degeneration and regeneration similar to that 

 described for the fibers of the peripheral stump. The immediate 

 degenerative changes in the fibers in the central stump were supposed 

 to extend back only to the first node of Ranvier, — to affect, there- 

 fore, only the internodal segment actually injured. Later it was 

 found that the degeneration may extend back over a distance of 

 several internodal segments. This limited degeneration on the 

 central side must be considered as traumatic,— that is, it involves 

 only those portions directly injured by the lesion. The central 

 end of the fiber in general was supposed to remain intact as long 

 as its cell of origin was normal. It was thought at first that after 

 simple section of a nerve trunk, in amputation, for instance, the 

 nerve cells and central stumps remain normal throughout the life 

 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 (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. 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. It seems evident that the behavior 

 of the cells and fibers on the central side of the section is not 

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



