196 PATHOLOGICAL HISTOLOGY IN GUINEA-PIGS 



is probably the result of typhus. Heaping up of the endothe- 

 lium of arteries and veins with perivascular accumulations of 

 the same type of cells (endothelial) as those in the skin lesions 

 are also frequently found. On the whole, vascular reactions are 

 more common in the lungs of guinea-pigs than in those of man. 



(e) The spleen: The spleen is usually markedly engorged, 

 the splenic follicles are large with conspicuous germinal cen- 

 ters, containing numerous mitoses. There is always an increase 

 in large cells in the pulp, with much phagocytosis. We regard 

 these cells as endothelial in origin. In the majority of the 

 spleens megakaryocytes and hematopoietic cells are absent. 

 In a few cases, occasional erythroblastic groups and mega- 

 karyocytes are found. In the splenic veins or sinuses are many 

 phagocytic mononuclear cells (endothelial leucocytes, macro- 

 phages) enclosing red blood corpuscles and nuclear remains of 

 other cells. Polymorphonuclear eosinophiles are present in 

 increased numbers in the pulp, and neutrophilic polymor- 

 phonuclears are present in small numbers. The peritoneal 

 cells on the surface of the spleen are usually swollen. The 

 spleens on which a thin transparent fibrin-like layer was noted 

 fail to show evidences of acute infection. The layer upon the 

 surface consists of a delicate, compact, fibrinous material cov- 

 ered on both surfaces with cuboidal cells. These cells are evi- 

 dently derived from the peritoneal epithelium, as the cells 

 upon the surface of the spleen are identical in appearance and 

 always contain many cells in mitosis. Mitotic cells are also 

 found upon the surface of the fibrin, with rarely a phagocytic 

 cell, and very rarely a polymorphonuclear leucocyte. The 

 histology of the spleens with this surface layer is not different 

 from spleens free from such coating, and, in the absence of 

 any reaction in the capsule, it is difficult to regard this material 

 as exudate from the spleen caused by secondary infection. 



Smear preparations have repeatedly shown no bacteria and 

 cultures on ordinary media were negative. 



(f) The liver: The lesions in the liver are insignificant but 

 similar to those in man. There is an increase in size of the 

 endothelium lining sinusoids (Kupfer cells) and phagocytosis 



