614 DR JAMES W. DAWSON ON 



differ both from white fibrous tissue and yellow elastic tissue. He thinks that there 

 is strong evidence that in various toxic conditions the capillaries of the central 

 nervous system are prone to undergo definite change, while those of other organs 

 escape. The explanation of this selective action is to be found in the fact of their 

 higher structural differentiation, which has been obtained at the cost of a peculiar 

 vulnerability to the action of certain toxins. 



The adventitia is immediately bounded on its outer side, without the presence 

 of any peri- vascular space, by a condensed layer of glia — the glia peri- vascularis 

 limitans. This glia layer is a continuation inwards of the glia superficialis limitans 

 which was carried inwards by the in-growing vessels of the pia when they invaginate 

 the embryonic nerve tube. The inner layers of the pia form the adventitia coat of 

 these penetrating vessels, and this intimate relation between glia and adventitia is 

 maintained till the finest capillaries are reached. In these the glia limitans may 

 be formed by a very fine protoplasmic reticulum in which the glia " Fuss " of 

 immediately adjoining cells are inserted. 



It was formerly supposed that there was no evidence of the existence of vaso- 

 motor nerves in the intracerebral arteries, but Lapinsky has demonstrated that 

 medullated fibres reach the media and are distributed to the muscular fibres as non- 

 medullated fibres. 



The disposition of the areas in relation to the blood-vessels has led to the 

 supposition that these play an important role in the genesis of the areas. This 

 topographical relation is frequently obvious even macroscopically, especially in areas 

 in the cerebral white matter, and numerous illustrations point to its microscopic 

 proof. This dependence on the vessels is brought to light only where the areas are 

 isolated : in later stages, through coalescence, the original relationship is no longer 

 recognisable. A parallelism between the sclerosis and the area of distribution of 

 a vessel would argue that the development of the process depends upon the condi- 

 tion of the vessel or of the " noxa " circulating within it. 



To the alterations in the vessel walls in sclerotic areas numerous writers have, 

 therefore, ascribed an essential significance, while others have regarded them as 

 accessory and subordinate. Few writers have noted their entire absence ; Taylor, 

 in eight cases, could find no trace of vessel change, and Erben, in a careful investi- 

 gation of five cases, found none that were not common to the whole central nervous 

 system. Variations in the findings have been explained by the different stages in 

 the development of the sclerotic process. In early stages engorgement and dilatation, 

 cell infiltration of the walls, dilatation of the lymphatic sheaths, and capillary 

 haemorrhages have all been noted ; at a later stage, marked nuclear increase in the 

 adventitia and, in the actual sclerotic areas, condensation and hyaline change of the 

 vessel walls. Probably none of the histological changes have been so variously 

 interpreted as those related to the vessels. 



