802 ' 



THE NERVOUS SYSTEM. 



FIG. 527. NEURONE DEGENERA- 

 TION AFTER INJURY. 



Teased nerve fibres from distal 

 portion of sciatic nerve of rabbit 

 three weeks after division of the 

 nerve. Osmic-acid stain. The large 

 and small black masses represent 

 disintegrated myelin and fat drop- 

 lets. 



DEGENERATION. 



NEURONE DEGENERATION. 



Degenerative changes may affect the entire neurone or any of its 

 parts. Such changes may result from direct injury to some part of the 

 neurone, to diminution or modification of its nutritive supply, to various 

 toxic conditions, etc. 



L Changes in the Neurone from Injury to One of its Parts. (a) CHANGES 



IN THE AXONE EESULTING FROM SEPARATION FROM ITS CELL BODY. 



That changes, presumably of a degenera- 

 tive character, occur in the peripheral end 

 of a nerve when its connection with the cen- 

 tral nervous system is broken has long been 

 known. This degeneration takes place as well 

 in the central as in the peripheral nervous 

 system, and is complete, involving every por- 

 tion of the axone distal to the point of section. 

 The changes are not progressive from the 

 point of lesion, but occur at nearly the same 

 time in all parts of the distal stump. These 

 changes consist in a breaking up of the 

 medullary sheath into segments (Fig. 527), 



which in turn disintegrate, forming variously shaped masses of myelin, 

 among which may be seen the axis cylinder, the whole being enclosed by 

 the ueurilemma. The method of Marchi (see page 837), which differen- 

 tiates between fat and myeliu, shows that coincident with the breaking 

 up of the myeliu there is an appearance of fat droplets (Fig. 528). These 

 fat droplets increase in number part passu with the decrease in myeliu, 

 but also ultimately disappear. During the progress of these changes in 

 the medullary sheath, the axis cylinder at first 

 segments and then undergoes dissolution. The 

 ueurilemma, on the other hand, appears to 

 take no part in the degenerative process. On 

 the contrary, it and its nuclei remain intact to 

 take part later in regenerative changes, should 

 these occur. If there is no reunion of the sev- 

 ered ends of the axone, the peripheral portion 

 completely disappears, its place being taken 

 by connective tissue. 



(b) CHANGES IN DENDRITES KESULTIXG 



FROM THEIR SEPARATION FROM THEIR CELL 



BODIES. If we consider, as does Van Gehuch- 

 ten, 1 that the peripheral arm of the spinal ganglion cell is a protoplasmic 

 process, making a physiological rather than a morphological differentia- 

 tion, we find that the same law holds good for dendrites as for axoiies 

 1 Van Gehuchten, "Anatomic du Systeme Nerveux de 1'Homme," p. 213. 



FIG. 528. NEURONE DEGENERA- 

 TION WFTER INJURY. 



Longitudinal section of distal 

 portion of the same nerve. Mar- 

 chi stain. The black dots repre- 

 sent droplets of fat resulting from 

 degeneration of the medullary 

 sheaths. 



