Ranson, Retrograde Degeneration. 383 



pear until fifteen days after Wallerian degeneration has begun 

 and that the cells of origin of these tracts disappear. 



OBSERVATIONS ON THE WHITE RAT. 

 Introduction and Summary. 



In a previous paper the writer (3) called attention to the 

 complete degeneration of the splenium of the corpus callosum 

 in a young rat after deep incision of the occipital lobe, and 

 reference was made to a similar observation by von Gudden (5). 

 If this degeneration had occurred according to Waller's law, 

 about half the normal number of fibers should have been pres- 

 ent on each side of the lesion. But in this case all the fibers 

 had disappeared. This means that the fibers underwent seri- 

 ous alterations in both directions from the point of injury. This 

 observation taken by itself only shows the absence of the mye- 

 lin sheath in the proximal portion of the injured fiber, as no 

 stains were used which would demonstrate the presence or ab- 

 sence of naked axis cylinders. Since there were no medullated 

 fibers in the corpus callosum at the time of the operation the 

 absence of myelin might be interpreted as due to an arrest of 

 development. But further observations made on older rats 

 have shown that a true degeneration closely resembling the 

 Wallerian type may occur in the proximal portions of severed 

 fibers. 



The chief difference between the changes in the proximal 

 and the distal portions is that the latter pass more rapidly 

 through the stages of fragmentation, solution and absorption, 

 these changes affecting the whole extent of the severed portion 

 at the same time ; while in the proximal portions the changes 

 occur somewhat later and may involve only the part of the fiber 

 nearest the point of injur)'. Thus forty five days after a wound 

 is made in the medullated corpus callosum of young rats (21 to 

 70 days of age) the ordinary Wallerian degeneration has run 

 its full course and the resulting debris is entirely absorbed. But 

 at this time fragmentation of myelin may be distinctly seen in 

 the proximal portions of the severed fibers, affecting especiall}' 

 the part in the vicinity of the lesion. 



