THE BRAIN 181 



larly valuable in deciding the relationship of vascular lesions 

 to the proliferative lesions. Our observations are in the main 

 confirmatory of the authors named above. We are, however, 

 not willing to accept without reservation the dictum that 

 cells of mesenchymal origin do not invade the brain substance 

 in the formation of lesions. The identification of wandering 

 cells in the central nervous system, in spite of a voluminous 

 literature, still presents great difficulties as reliable criteria for 

 the recognition of the various types of cells have not been 

 formulated. We believe that the wandering mononuclear 

 phagocyte (macrophage, endothelial leucocyte, etc.) does enter 

 nervous tissue, and is an important constituent of the typhus 

 " nodule" in the brain. 



Our study of human brains from early typhus deaths and of 

 guinea-pig brains secured from all stages of experimentally 

 transmitted typhus shows that the first evidence of typhus is 

 found in capillaries and pre-capillaries in changes to be observed 

 in the endothelium. Swollen endothelial cells and endothelial 

 cells exhibiting degenerative changes in nucleus and cyto- 

 plasm, often accompanied by small platelet and fibrin thrombi, 

 are found in the brains of guinea-pigs killed on the first or 

 second day of fever. (Plates XXII, XXVIII, and XXXI, figs. 

 53, 59, and 77.) Similar vascular lesions in company with fully 

 developed lesions are frequent in human brains from patients 

 dying before the tenth day of typhus, as various stages of the 

 evolution of the brain lesions are constantly found together 

 both in human and animal material. (Plates XXI, XXII, 

 and XXIII, figs. 51, 54, and 56.) 



Capillaries, when obliterated by the proliferation of endothe- 

 lial cells, often appear bloodless and therefore of doubtful 

 identity; the presence of one or several blood corpuscles 

 taken up by the detached endothelium has frequently served 

 to settle doubt as to the presence of a damaged blood vessel. 

 Escape of red blood corpuscles is frequent and small perivascu- 

 lar hemorrhages are constantly seen in both guinea-pigs 

 (Plate XXII, fig. 53) and human brains (Plate XXIV, fig. 

 57) about vessels of pre-capillary size. 



