190 PATHOLOGY OF TYPHUS IN MAN 



notes we referred to this appearance, for a considerable .period 

 of our research, as gray stippling of endothelial cells. We now 

 conclude that the appearance is due to masses of the most 

 minute form of rickettsia because of our ability to definitely 

 stain them and because of their frequent association in the 

 same cell with the larger more deeply stained form (Plate 

 XXIX, figs. 71, 72, and 73). Cells massed with diffusely dis- 

 tributed rickettsia occur in the endothelium adjacent or sur- 

 rounding thrombi as well as in vessels showing no lesions 

 other than swollen endothelium. In contracted arteries and 

 veins these swollen cells project into the lumen and are 

 often filled with uncountable numbers of paired granule-like 

 rickettsia. 



Occasionally, we have been able to see short straight or 

 curved rod-like forms in association with the other forms 

 (Plate XXXII, fig. 83), but for the present we distinguish 

 with certainty two forms only, a larger, relatively deeply 

 staining, paired, lanceolated form and a smaller, lightly 

 stained, paired, coccoid form in globular masses or diffusely 

 distributed in endothelial cells. 



In searching for rickettsia in lesions it is necessary to 

 practice much patience as the vagaries of staining and possibly 

 of distribution make their demonstration a matter of fortune 

 unless a system of searching is followed. (Plate XXXIV, figs. 

 88 and 89.) Our method has been to locate lesions in vessels 

 and then to follow each vessel in both directions from the 

 lesion through the short series of sections which it has been 

 our practice to make from each block of skin. The best results 

 in staining are probably obtained by partially differentiating 

 the sections in alcohol and completing the differentiation after 

 mounting in oil of cedarwood by exposure to sunlight. 



The finding of rickettsia in the mononuclear cells of the 

 perivascular " nodules" and the evidence that these cells 

 multiply in the nodules as shown by mitoses lead us to the 

 conclusion that this reaction is almost wholly a proliferative 

 one and caused by the presence of the parasite of typhus in 

 these cells. It also seems evident that the rickettsia are carried 



