236 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Red corpuscles occur frequently, especially in tuberculous and 

 malignant exudates, which may be definitely hsemorrhagic. It is 

 necessary to distinguish corpuscles belonging to the exudate, 

 which are intimately mixed with it, from those derived from the 

 puncture, in which case the blood is most marked at the beginning 

 or end of the flow. 



Endothelial cells axe very important, and it is necessary to be 

 able to recognise them at once. In certain passive exudates 

 (due to cardiac and renal disease) these cells occur as large flat 

 plates, exactly as if the endothelial coat had been scraped off the 

 pleura (Plate X., Fig. 2). They are then seen to be much larger 

 than the largest of leucocytes, and to have a diameter three or four 

 times that of a red corpuscle. Each has a nucleus (sometimes 

 more) which does not usually stain very deeply, protoplasm which 

 stains more faintly still, and one or more nucleoli which stain 

 very deeply in wet preparations, less so in dry ones. 



These cells are often grouped into" placards" (Plate X., Fig. 2), 

 the edges of adjoining cells fitting into one another like those of the 

 counties on a map. The groups of cells thus formed are always 

 flat, and careful focussing up and down shows that they consist 

 of a single layer of cells — an important fact, as it distinguishes 

 them from masses of cells of a malignant growth. 



Endothelial cells are very phagocytic, and ingest bacteria, red 

 corpuscles (Plate X., Fig. 3), leucocytes (Plate IX., Fig. 2), etc. 

 They often undergo fatty degeneration (Plate X.^-iFig. i) or 

 general degeneration, shown by their very faint staining ; complete 

 solution of the protoplasm may occur, and the nucleus be set free. 

 It may then be mistaken for a lymphocyte. 



Where inflammation takes place in a serous membrane the 

 first thing that happens is that the endothelial cells are set free, 

 so that they are always found with the leucocytes in the early 

 stages of pleurisy or peritonitis. If the inflammation is severe 

 they are destroyed, and the fluid at a later date does not contain 

 them. If the inflammation is less intense they are stimulated to 

 growth, and the young proliferating forms are often very similar 

 to the large lymphocytes and hyaline leucocytes. They vary 

 greatly in size, forming a continuous series between a cell as large 

 as the large lymphocyte to one as large as the plates described 

 above. They are round or oval, not mutually adapted, as in the 

 endothelial cells which have desquamated in passive exudates ; 

 sometimes two hemispherical cells may be found in apposition 



