492 NUMBER AND SIGNIFICANCE OF LEUKOCYTES IN MILK 



small, round, glass plate. Each of the spaces seen in the field of 

 the microscope represents accordingly ^nrVir ^ a cubic millimeter. 



If the four hundred squares ruled on the inner plate were alike and 

 undivided into groups the eye would easily lose track and an accurate 

 count would be difficult. For this reason the first one of each group 

 of five squares both from right to left and from above downward has 

 an additional ruling going through the centre of the square. These 

 lines, however, are ignored in counting, and their only object is to 

 enable the observer to see when he has counted the cells in five, ten, 

 fifteen, or twenty squares, or any multiple of five squares. 



Calculation of the Result. Suppose that the sediment contained 

 in 1 c.c. of what was left in the tube is taken and that 242 leukocytes 

 have been counted in two hundred of the squares; this would mean 

 an average of 1 .21 leukocytes per square. Since each square represents 

 3Tnnr cu bic millimeter, each cubic millimeter of the sediment would 

 contain 1.21 X 4000, or 4840 leukocytes. In blood work it is customary 

 to indicate the number of leukocytes present in 1 cubic millimeter of' 

 blood; but in milk, 1 cubic centimeter is used as the standard. Since 

 1 cubic centimeter is equal to 1000 cubic millimeters, it is necessary 

 to multiply 4840 by 1000. This gives 4,840,000, which indicates the 

 number of leukocytes present in the 10 c.c. of milk, as 1 c.c. of 

 the sediment examined contained all of the leukocytes in the 10 c.c. 

 originally used. To obtain the number of leukocytes in terms of 

 1 c.c. it is, therefore, necessary to divide 4,840,000 by 10, which gives 

 484,000. In other words, it is found that 242 leukocytes present in 

 two hundred squares means that the milk examined contained 484,000 

 leukocytes per 1 c.c. 



The simple mechanical rule for finding the number of leukocytes 

 is therefore: After repeated centrifuging, leave enough of the clear 

 fluid with the sediment to make the total of sediment and clear fluid 

 equal to 1 c.c. Shake well; prepare counting chamber; count the 

 number of leukocytes in 200 squares and multiply the sum obtained 



bv 2000 - (*?-XJP?) 

 oy zuuu - V 200 x 10 ' ' 



If only 0.5 c.c. was left in the centrifuge tube then the sum of 

 leukocytes found in 200 squares is multiplied by 1000. 



When the leukocytes are very numerous it is sufficient to count 

 them in 100 squares only; when they are scanty it is better to count 

 them in 400 squares. In the former case the sum must be multiplied 

 by 4000; in the latter by 1000, in order to obtain the figures for 1 c.c. 

 of milk. 



Campbell has made numerous comparative tests, and he finds that 

 heating the milk increases the leukocytes in the sediment no matter 

 what method of sedimentation and estimation is employed. He also 

 points out that more recent knowledge concerning the leukocyte 

 contents of milk compels the dismissal of former standards as unre- 

 liable and inequitable. 



Variability in the Leukocyte Count in Milk from Healthy Cows 

 Russell and Hoffmann found that the milk of some of the cows 



