156 UNIVERSITY OF WISCONSIN STUDIES 



and at sufficiently high speed to throw down most of the leu- 

 cocytes without injuring them, e. g., 2,000 r. p. m. for ten min- 

 utes. 



Spreading of sediment. "When the tubes are taken out 

 of the centrifuge, the fat layer at the top of the tube is 

 loosened, the milk and stain mixture poured out, the cork re- 

 moved, and the sediment 011 it carefully spread on a glass 

 slide, as one would make a blood smear. It must always be 

 borne in mind, however, that water cannot be used to spread 

 the smear, as this violates the principles involved and in- 

 variably causes the cell nuclei to take up the stain, even in 

 raw milk. 



Microscopical examination of smears. The smears are 

 first examined with the low power (16 mm. lens). In the 

 raw milks the background, or the entire microscopic field, 

 is stained blue, the depth of the stain depending upon the 

 thickness of the film : in this blue background appear numerous 

 clear areas. The smaller of these are fat globules. The larger 

 ones may be large fat globules or clusters of the same, but 

 usually they are the leucocytes of the milk. Occasionally 

 there are the deeply stained mononuclear leucocytes. The 

 general impression one gets from these preparations is a blue 

 field, which tends to be uniform, and in which there are a 

 number of holes or clear places. 



The smears of the pasteurized milk, on the other hand, do 

 not have the background as deeply stained as do the smears 

 from the raw milk, although in thick portions it may be quite 

 blue. The nuclei of the leucocytes here are always stained and 

 their color is deeper than that of the background. .The area 

 immediately surrounding the cells usually takes the stain 

 deeply and shades off into the color of the background, forming 

 what I have called a "dark halo". 



Because of the lighter color of the field, the fat globules 

 do not stand out as they do in the raw milk. The most 

 prominent objects here are the leucocytes with their darkly 

 stained nuclei. 



The cells are noticeably smaller in the pasteurized than 

 they are in the raw milk. 



