PATHOLOGICAL HISTOLOGY 163 



dependent of secondary infection and their number is deter- 

 mined by the abundance of degenerated cells, endothelial cells 

 and connective tissue cells of the intima which become affected 

 by the rapid accumulation of the reacting cells. 



Even in fairly advanced cases, ninth to eleventh day, acute 

 lesions of the intima may still be found in capillaries and 

 small arteries and veins without marked peri vascular reaction. 

 (Plate XIII, figs. 37 and 38.) Capillaries, venules, and 

 arterioles with greatly swollen endothelium may be traced 

 through many serial sections and cells containing rickettsia 

 are more frequent than in the earliest days of the rash. The 

 inference from an intensive study of the skin of all stages of 

 the disease is that following the initial lesions due to the locali- 

 zation of rickettsia in the vascular endothelium, the subse- 

 quent spread of the organism excites a less marked cellular 

 response, and may produce only a moderate reaction of the 

 endothelium in situ. 



Hemorrhages into the corium about capillaries and vessels 

 of pre-capillary size are found on and after the eighth day. 

 (Plate XII, fig. 35.) The mechanism of their occurrence seems 

 to be a solution of continuity of the endothelium, due to de- 

 generation and necrosis after the formation of thrombi in 

 larger vessels has occurred, indicating that a mechanical factor 

 - stasis in veins is a necessary element. The occurrence of 

 thrombi in the deeper vessels only at a later stage than the 

 involvement of the superficial vessels suggests a centripetal 

 extension of the infection. This we have actually shown to 

 occur in particular cases by the study of the necroses over 

 bony prominences where the condition is one of infarction 

 involving skin and subcutaneous fat, due to the extension of 

 thrombi facilitated by a moderate degree of stasis. Repair of 

 the thrombi in the blood vessels occurs by organization and 

 may be found as early as the fifteenth day. Repair of the 

 perivascular nodules is difficult to follow. We have noted 

 proliferation of the fibroblasts (connective tissue cells) at 

 the periphery of the nodules on the eighteenth day and 

 partial disappearance of the infiltrative cells with permeation 



