576 DR JAMES W. DAWSON ON 



direct action on the nerve fibres by direct compression of the proliferating glia, but 

 that there is an indirect action upon their nutrition. By accumulating around the 

 vessels the glia, if it does not close their lumen, is said at least to limit their ex- 

 pansion, and diminish the blood-flow, and interfere with the circulation in the 

 peri-vascular lymphatics. In this simple atrophy of the myelin sheath the de- 

 generated products, owing to the slowness of the process, are removed just as 

 formed, in the form of very fine granules or in solution, without requiring the 

 presence of true granular cells. These cells are said to appear only when the 

 process is more rapid : they may occur at the periphery of an area of true, primary 

 sclerosis, where the affected tissue causes a reaction in the normal tissue and an 

 excentric spread of the process occurs. 



In the description of an area of old sclerosis in the cord (p. 563) an oval area in 

 the posterior columns was chosen, because here the glia fibrils normally constitute a 

 very uniform fine network and the pathological increase of the fibrils appears to take 

 place much more regularly in a direction parallel to the normal nerve fibres. In the 

 lateral columns, however, the gradual increase of glia in a sclerosed area, on trans- 

 verse section, can be followed up much more easily because of the more definitely 

 reticular structure of the normal glia in this situation. The glia trabecules, in the 

 lateral column, run out transversely to the long axis, both from the marginal glia 

 zone and from the lateral grey matter. In the substance of the white matter they 

 break up into a reticulum which is much coarser and more transverse than in the 

 posterior columns, and the resultant meshes enclose the fibres and groups of fibres, 

 which in their turn are larger than the average fibres of the posterior columns. 



Fig. 343 shows an area in the lateral column, in which there is a commencing 

 thickening of the glia trabeculse and of the finest fibres forming the reticulum. This 

 gradually increasing thickening (fig. 344) can be followed up till the finest glia 

 meshes are almost obliterated and a dense fibrillar feltwork takes their place 

 (fig. 345). With glia and cell stains it is seen that the septa which are formed by 

 the glia fibres around the groups of nerve fibres become thicker and denser and 

 richer in fibrils : they, as it were, force the groups of nerve fibres more and more 

 apart and divide them into smaller groups, and the fibrils penetrate amongst indivi- 

 dual nerve fibres. There is thus produced a feltwork of glia tissue, composed of glia 

 fibrils, becoming more and more dense. As this condensation increases, even under 

 low power it can be recognised that the ring of myelin around individual nerve fibres 

 is becoming thinner, and finally the brilliant yellow-green ring (Van Gieson's stain) 

 disappears altogether and leaves a naked axis cylinder. Sometimes this condensed 

 glia has a granular, but more often a homogeneous, appearance, as if the individual 

 fibrils had fused. Simultaneous with the thickening of the alia trabeculse and 

 reticulum the glia nuclei in the nodes of the reticulum and within the trabeculse 

 have enlarged and proliferated, becoming large protoplasmic cells with long-branch- 

 ing processes (fig. 347). In the glia septa, large and small, in which the blood-vessels 



