THE BRAIN 185 



ever, there is infiltration of the nerve tissue with blood with 

 resultant small foci of destruction. Such lesions always con- 

 tain many macrophages filled with red blood corpuscles. 

 (Plate XXIV, fig. 57.) 



In all brains, some degree of infiltration of the pia arachnoid 

 and some degree of perivascular infiltration (Plate XIX, fig. 

 48) is present. We have never found proliferative lesions in 

 the meninges. The perivascular and meningeal infiltration is 

 most marked in cases of considerable duration and probably 

 reaches a maximum at the end of the second week. As a rule, 

 the more numerous the proliferative lesions, the more extensive 

 are the perivascular and meningeal infiltrations, which ap- 

 parently represent a general response to the pathology of 

 typhus rather than a local response due to localization of the 

 virus of the disease. 



The perivascular reaction is widely distributed, and we have 

 failed in our attempts to correlate it with degree of menin- 

 geal infiltration or variation in the distribution of the prolifera- 

 tive lesions. The perivascular infiltration consists chiefly of 

 lymphoid and plasma cells, though macrophages (endothelial 

 leucocytes) and mast cells are not uncommon. A fibroblastic 

 proliferation has not been seen in our material. Polymorpho- 

 nuclear leucocytes, and rarely an eosinophile, are found. An 

 increase in the perivascular neuroglia is present in marked 

 degrees of infiltration. 



The most marked degree of perivascular infiltration is to 

 be found in the basal ganglia, pons, and medulla. 



The infiltration of the pia arachnoid is not diffuse. It oc- 

 curs in areas in all locations, most constantly and abundantly 

 over the cerebellum, pons, and medulla, and is independent 

 of proximity of either the proliferative lesions or perivascular 

 infiltration in the substance of the brain. The cells found 

 are chiefly macrophages (endothelial leucocytes), lymphoid, 

 and plasma cells. Small hemorrhages are also occasionally 

 found in the arachnoid and their ante-mortem occurrence 

 is shown by the phagocytosis of the blood corpuscles by 

 macrophages. 



