198 PATHOLOGICAL HISTOLOGY IN GUINEA-PIGS 



(macrophage, large mononuclear phagocyte). Porymorpho- 

 nuclear leucocytes also are found in these lesions. The mural 

 thrombi and proliferative perivascular reactions are more 

 common in the tunica and epididymis than in the testis. 

 Lesions of the seminiferous tubules were not found. 



Diffuse perivascular infiltrations are not a conspicuous 

 feature of typhus in the guinea-pig; the cells occurring in the 

 testes and adnexa are largely lymphoid cells and plasma cells 

 with occasional polymorphonuclear eosinophiles, neutrophiles, 

 and a rare mast cell. NeilFs (1917) brief description of the 

 testicular lesions in guinea-pigs in our opinion overstates the 

 degree of involvement. 



2. RlCKETTSIA IN GlJINEA-PlG TISSUES 



Our successes in demonstrating rickettsia in guinea-pig 

 tissues have not been equal to those with human tissue, 

 partly owing to the fact that we have devoted but little time 

 to the attempt and partly because the character of the lesions 

 is milder in that they present fewer guides to the search. 



Scattered paired coccoid bodies often can be found in swol- 

 len endothelium in blood vessels of the skin of the scrotum, 

 the testes, and the brain. (Plate XXX, fig. 76.) The more 

 minute form of paired bodies in large numbers in endothelial 

 cells of blood vessels in the same tissues are also usually to be 

 found if sufficient search is made in well-stained preparations. 

 The capillaries and vessels of pre-capillary size of the brain, 

 with early lesions, are the most favorable fields in the search 

 for rickettsia, and here only have we found the compact 

 groups of paired organisms, such as are found in human blood 

 vessels. (Compare Plate XXVIII, fig. 69, from a guinea-pig's 

 brain with Plates XXVII and XXXIII, figs. 65 and 87, from 

 human material.) 



If we are correct in our interpretations of the various forms 

 of rickettsia in human tissues, to the effect that the minute 

 forms represent the early forms, the short duration of the 

 disease in guinea-pigs would explain the rarity of the clusters 

 of the larger forms of rickettsia. 



