DESCRIPTION OF TISSUES 195 



The series of thirty-six guinea-pigs include the brains of 

 guinea-pigs inoculated directly from man, from monkeys, 

 with the intestinal tracts of lice and from guinea-pigs. We 

 have never seen lesions of the choroid plexus in guinea-pigs. 

 The lesions in the guinea-pig are more easily found in the 

 cerebral cortex than in the pons, medulla, or cerebellum. 



In the guinea-pig (No. 219) referred to on page 145, which 

 died of paralysis, proliferative lesions are very numerous; 

 eighty-one are present in a single cross section of the cortex 

 of the cerebrum. (Plate XVIII.) Small perivascular hem- 

 orrhages are also present in this case, a rare finding in the 

 brains of typhus guinea-pigs. 



The histo-pathology of the remaining organs is like that in 

 man, though much less striking. 



(b) The skin: In the skin of the scrotum and ears we have 

 found vascular lesions and perivascular infiltrations like those 

 in man; but the proliferative lesions leading to nodule forma- 

 tion is not so prominent a feature. Thrombi are rare in the 

 skin. The endothelium forms large groups of cells, which re- 

 main attached to the vessel wall; and many of the cells in such 

 masses are phagocytic. A concentric thickening of the intima 

 due to proliferation of the endothelium to the extent of two 

 or more layers of epithelium-like cells also occurs. The peri- 

 vascular nodules often contain mitotic figures and contain 

 very few polymorphonuclear leucocytes. They appear to be 

 almost pure collections of macrophages, i.e., endothelial 

 leucocytes. 



(c) The heart: Proliferative lesions are rare; but occur ad- 

 jacent to blood vessels in the myocardium. Occasional small 

 perivascular infiltrations with lymphoid cells, plasma cells, 

 macrophages, and granular leucocytes can usually be found in 

 a single section through the ventricles. 



(d) The lungs: Rarely mural thrombi of arteries and veins 

 and occluding thrombi of capillaries have been found. These 

 lesions are not accompanied by striking reactions in the ad- 

 jacent tissues. The accumulations of phagocytic endothelial 

 cells in the capillaries of the alveolar walls in restricted areas 



