130 MICROSCOPIC TESTS OF MILK. 



2. CONCENTRATION OF CEI,U*LAR KI.KMKNTS. 



Ten cc. of milk are placed in an ordinary sedimentation tube, and after 

 heating as above directed, and subsequently shaking, the milk is centri- 

 fugalized twenty minutes at 1,200 revolutions per minute. A hand 

 centrifuge may be employed for this purpose, but where available a steam 

 turbine Babcock milk tester may be found more practicable. 



Note. This speed, maintained for the time mentioned, is sufficient to 

 sediment practically all the cell elements suspended in the milk. In our 

 experience we have found the number of cells in supernatant milk to 

 average only 3)4 per cent. 



3. PREPARING THE SAMPLE FOR EXAMINATION. 



After centrifugalizing, the cream and the supernatant milk are removed, 

 with the exception of the last half cc. by aspirating with an exhaust pump 

 and wiping the walls of the tube' with a cotton swab. After thoroughly 

 mixing the sediment with a glass rod, enough of the emulsion is placed 

 in an ordinary blood counter (Thoma-Zeiss pattern) to fill exactly the 

 cell. The preparation is then allowed to stand for a minute or two to 

 permit the cellular elements to settle to the bottom of the cell, while the 

 few fat globules in the liquid rise to the surface. This method permits 

 of the differentiation of the cells from the small fat globules, so that a 

 distinct microscopic observation can be made. 



EXAMINATION OF MATERIAL. 



The preparation is examined in an unstained condition. 



NOTE. Most observers have usually stained the sediment prior to ex- 

 amination, but we have found with the above treatment that the cells 

 may be enumerated quite as well in an unstained as a stained condition. 



The count is made w T ith a 1-inch eve-piece and | objective. Where the 

 number of cell elements exceeds 12 or 15 per microscopic field, above 

 referred to, one-fourth of the entire ruled area of the counter, equivalent 

 to 100 of the smallest squares of the cell, are counted. Where the cell 

 elements are less abundant, one-half of the cells in the entire area (two 

 to four hundred squares) are enumerated. The average number of the 

 cells per smallest square is then obtained, which, when multiplied by 

 200,000, gives the number of cells per cubic centimeter in the original 

 milk. (If multiplied by 4, 000, 000, we have the number of cells per cubic 

 centimeter in the sediment examined. As the sediment represents the 

 concentration of the cells into one-twentieth of the original volume of 

 milk taken, 10 cc. to one-half cc., this number should be divided by 20 

 to give the number of cells per cubic centimeter in the original milk.) 



NOTE. The above factor of 4, 000, 000 is obtained as follows: The cubic 

 content of the blood counter represents one-tenth of a cubic centimeter. 

 This volume is divided by means of the ruled scale into 400 small cubes, 

 each equal to one four-thousandth of a cubic millimeter, or one four- 

 millionth of a cubic centimeter. 



