MILK HYGIENE 117 



Two methods are used for determining the number of 

 leucocytes in milk. 



Examination for leucocytes : Doane-Buckley method. Cen- 

 trifuge tubes, which are graduated at 0.5 cc. and 10 cc., to 

 aid in filling and in the subsequent removal of the supernatant 

 liquid, are employed. These are filled with the milk to be 

 examined. They are then heated to 70-75 C. for five to 

 ten minutes, and well shaken after heating. The fat in milk 

 is largely in aggregates of globules, which seem to frequently 

 inclose leucocytes. If the milk is heated, the aggregates of fat 

 globules are broken up and the leucocytes freed. If the un- 

 heated milk is used, a large number of leucocytes are to be 

 found in the cream. Since only the sediment can be exam- 

 ined, the leucocytes must be concentrated in it. 



The tubes of warm milk are centrifugalized in any of the 

 ordinary types of laboratory centrifuges, or a Babcock tester 

 may be used. Unless a high-speed centrifuge is available, it 

 is difficult to remove the smaller fat globules. If considerable 

 numbers are left, they seriously interfere . with the counting 

 of the leucocytes. These small fat globules may be removed 

 from the milk by the use of high-speed machines, or by cen- 

 trifugalizing the milk for eight minutes at 2000 revolutions 

 per minute, removing the upper layers as described below, 

 filling with distilled water, and centrifugalizing again for 

 three or four minutes. 



The cream and milk are best removed by the use of a 

 fine-pointed glass tube of 2 mm. internal diameter, attached 

 to an aspirating pump by thick-walled rubber tubing. It is 

 advisable to place between the pump and glass tube a bottle 

 to receive the milk. By touching the point of the tube to the 

 liquid the very upper layers may be removed. The fat that 

 adheres to the wall of the centrifuge tube should be wiped 



