564 DR JAMES W. DAWSON ON 



sheath stain and counterstained with picro-fuchsin,* under a low magnification we 

 recognise at once three of the outstanding characteristics of an old sclerotic area 

 (fig. 335) : the disappearance of the myelin sheath of the nerve fibre, the prolifera- 

 tion of the glia, and the alterations of the blood-vessels. By means of a glia stain 

 (fig. 333) and a diffuse stain (fig. 336), these changes are confirmed and their 

 details revealed ; by an axis cylinder stain the fourth histological characteristic is 

 represented — the persistence of numerous axis cylinders (figs. 334, 424) ; and, by 

 the Marchi method, it is seen that this old sclerotic area contains no degenerating 

 nerve fibres nor granular cells containing the products of the degenerated myelin. 



The shape of this area is brought out clearly by Weigert's stain as an elongated 

 oval with its long diameter in the long axis of the cord (fig. 31). The continuity, 

 therefore, of the nerve fibres is broken by this oval area, and the absence of the 

 myelin at the sides is more or less sharply defined from the surrounding tissue, the 

 margins forming more or less sinuous lines, but the limits of the area, on longitudinal 

 section, are never straight or even curved outlines, for individual nerve fibres or 

 bundles of such pass into the borders. The myelin sheath of these fibres stains 

 irregularly and weakly, and the termination is usually broadly broken off, or it may 

 be swollen or narrowed (fig. 406). 



Numerous fine glia fibrils course in regular, undulating lines parallel to and close 

 to one another (figs. 3, 334). The neuroglia nuclei lie oftei) in short rows or in 

 groups, but more often, as in this area, isolated with the long diameter in the long 

 axis of the fibrils, and, as a rule, quite independent of them. The number of the 

 nuclei is smaller than in the normal tissue, but in structure and form they are 

 slightly larger, clearer, and more oval. Between the glia fibrils and surrounded by 

 them are found numerous axis cylinders, which are mostly thin and fine but thicker 

 and more homogeneous in structure than the glia fibrils (fig. 334). They do not 

 run in such regular lines, and frequently show as faint diffusely-stained bands. The 

 impression is often given that in place of the myelin sheath there has been a sub- 

 stitution of glia fibrils, forming a glious sheath to the axis cylinder. Lapinsky has 

 wrongly interpreted this process of substitution as a metamorphosis of the medul- 

 lated sheath — a conception which accentuates the enveloping character of the glia 

 fibril proliferation. 



The blood-vessels stand out clearly in this sclerotic tissue. Those cut trans- 

 versely show, with picro-fuchsin stain, thickened, homogeneous, pink-stained walls, 

 and on longitudinal section the numerous longitudinally-running small vessels 

 (fig. 342) have a similar structure, with few cell elements even in their adventitia. 

 The lumen of the smallest capillaries is often obliterated and that of the larger 

 vessels narrowed. Weigert picro-fuchsin sections show beautifully the arrangement 

 and distribution of these smaller vessels and bring out the abundant vascular supply 

 of the various tissues. 



* Such sections will in future be designated Weigert sections. 



