624 DR JAMES W. DAWSON ON 



types — wedge-shaped and round ; and that a remarkable symmetry can be recognised 

 in their localisation — a symmetry which can easily be underestimated when the 

 areas are at different stages of development. 



2. Secondary Degeneration. 



The absence of secondary degeneration has been, till recently, accepted as one of 

 the cardinal characteristics of the histological picture. Charcot related this feature 

 to the preservation of the axis cylinders in the sclerotic tissue, and Schultze has 

 shown that complete demyelination of the nerve fibre at a circumscribed level 

 occasions no secondary degeneration even in the myelin sheath itself, and that this 

 follows only a considerable alteration of the axis cylinder itself. 



The question of secondary degeneration is one not easily decided. A glance over 

 the numerous illustrations of the spinal cord at successive levels will suffice to show 

 that the zones of sclerosis are not followed in the usual sense of the term by 

 secondary degeneration. It is probably frequently present, however, especially in 

 late stages, but the ordinary difficulties of proving or recognising it in cases where 

 numerous islets of sclerosis are present are increased by numerous factors. We have 

 already referred to the frequent presence of diffuse changes which unite sclerotic 

 areas or, on longitudinal section of the cord, are continued upwards or downwards 

 as " Markschattenherde " : it is probable that some at least of these changes must be 

 related to a commencing secondary degeneration. The destruction of the axis 

 cylinders is, again, only relative in any one area, and in consequence, the secondary 

 degeneration will affect only a certain percentage of the nerve fibres of one column 

 or even of one bundle : its recognition as a secondary degeneration is thus rendered 

 more difficult. Further, the sclerosis below or above any one level passes over the 

 boundaries of a system degeneration and may even affect the whole transverse 

 section of the cord — the element in this change due to secondary degeneration would 

 be impossible to trace. And, finally, more acute processes arising in the affected 

 columns, without any loss of axis cylinders in area.s above or below, may be explained 

 by an analogous process in the area under consideration. It may also be noted that 

 numerous, though only microscopically evident, areas may frequently be found in 

 the course of one and the same conducting tract. 



Changes outside the areas, such as the diffuse alterations of the myelin sheath, 

 diffuse commencing alterations of the glia cells, similar dilatation and engorgement 

 of blood-vessels and sieve-like dilatations of the adventitial lymph spaces and of the 

 peri-vascular glia meshes, as the single expression of a not otherwise provable 

 change, have all been referred to in the previous study. In late stages the blood- 

 vessels throughout the whole central nervous system, and especially in the con- 

 volutions, showed varying slight changes, similar to those within the areas. Anton 

 and Wohlwill have found the vessel infiltration sharply limited to the areas, but 

 dilatation and engorgement throughout the whole cerebral white matter. 



