586 DR JAMES W. DAWSON ON 



(b) Cerebral area : e.g. in the central white matter. 



(i) Nerve fibres cut in various directions (figs. 5, 6, and 361-369). 



It is when we come to trace the evolution of such an area that we recognise how 

 complicated is the resultant sclerosis. The nerve fibres, instead of running parallel 

 in one direction, here run in every possible direction — not only in straight lines, but 

 in curves. It is a web in which, in addition, the threads in bundles and groups of 

 bundles cross each other and also curve round to cut across the longitudinal, trans- 

 verse, and obliquely-running threads. Those who have studied, by means of the 

 various elective stain's, the structure of an area, e.g. of the central white matter, 

 have realised how difficult it is to define the plexus formed even when only one of 

 the structural elements is stained. When, however, in consequence of the degenera- 

 tion of the myelin sheath, or of both myelin sheath and axis cylinder, their place is 

 taken by a dense fibrillar tissue, which more or less follows the varying direction of 

 the lost fibres, it will be seen how aptly such a tissue may be described as an 

 inextricable tangle, in which the finest meshes are invisible under low power. 



Here also the first stage is one of reaction of all the tissue elements : at times it 

 seems as if the glia cell enlargements were distinctly the primary change ; at other 

 times it is equally clear that a myelin sheath alteration, or even its complete dissolu- 

 tion, has preceded the glia cell change ; and again it occurred that both changes are 

 coincident. Heidenhain's iron-hsematoxylin stain is of great service in allowing 

 both the change in the myelin sheath and the change in the cell elements to be 

 recognised in the same section, and frozen sections were found likewise of great help 

 in coming to a decision as to which was the primary tissue element involved. It 

 was distinctly evident that both changes preceded any recognisable alteration in the 

 vessel walls. The frequent centrally-placed vessel and all the capillaries and pre- 

 capillary vessels were dilated and engorged. In Van Gieson's stain the smallest 

 capillaries are brought out distinctly, for the outer membrane of the endothelial cells 

 stains intensely with fuchsin — and, in addition, a fine double contour, frequently 

 present, gives the impression of a distinct adventitia or elastic coat even to these 

 fine capillaries. At this stage, also, there is an increase in the small normal glia 

 nuclei of the cerebral white matter, and an indication of a commencing enlargement 

 of the nucleus. 



At a later stage the large glia cell proliferation (figs. 13 and 434-436) is the most 

 outstanding characteristic : the rapid appearance in such large numbers of those 

 large protoplasmic cells with homogeneous protoplasm and long-branching processes 

 is closely related to the numerous dilated vessels everywhere present in the affected 

 tissue. It would seem that it is the glia cells, which normally lie within and close 

 to the peri-vascular glious sheath, which first enlarge and proliferate, and several of 

 their broad processes end in the outer wall of these vessels. Closely following this 

 proliferation both of the largo and small glia nuclei, appear the first fat granule cells, 



