TKSTS 1'OK CKLLULAR CONTENT OF MILK. 127 



upon microscopic examination " an excessive amount of pus " 

 was found in a sample of herd milk the examination of the 

 individual cows was suggested. The presence of more than 5 

 pus cells per field of a one-twelfth objective in the milk of an 

 individual cow was regarded as justification for the exclusion of 

 the cow 7 from the herd. The original article describing the 

 test contained results of the examination of a series of cows, 

 one-third of which were condemned on the basis of the stand- 

 ard selected. 



Bergey (3, 4. 5) found it necessary to lower the standard 

 and regarded 10 cells per field as an indication of pus associ- 

 ated with inflammatory reaction of the udder caused by pyo- 

 genic organisms. 



Stewart's test. Stewart (28), of the Philadelphia Bureau of 

 Health, further modified the method and applied it to the 

 examination of large numbers of samples of mixed herd milk. 

 He describes the apparatus and method as follows : 



' ' This apparatus consists of a circular pan about 12 inches in diameter, 

 and -'4 -inch deep, containing twenty small glass tubes. The tubes con- 

 tain 1 cc. of milk and are filled by means of a small bulb similar to that 

 ordinarily used on medicine droppers. The end of the tube is closed by 

 a small rubber stopper, and the tubes are held in the pan by spring 

 clamps. This pan is fitted upon the ordinary Boekel water centrifuge 

 and covered with a lid which is held down by a thumbscrew. The pan 

 covered in this way furnishes a surface of very slight resistance to the 

 atmosphere during its revolution, somewhat on the principle of a child's 

 top. 



' ' By the old method the arms of the centrifuge containing the milk 

 encountered so much resistance in their revolution that the speed with 

 15 pounds water pressure was not more than 1,200 revolutions per minute, 

 while the speed obtained with the new apparatus is from 2,500 to 3,000 

 revolutions per minute with 15 pounds pressure. This rapid speed 

 causes sedimentation to occur in less than five minutes. When this is 

 completed the centrifuge pan can be lifted from the motor and the per 

 cent, of cream measured by a graduated scale marked upon the tube. 

 The heavier matter, as the insoluble dirt, pus cells and bacteria, is thrown 

 to t?he peripheral end of the tube, where it adheres to the rubber cork in 

 the lumen of the tube. To examine this sediment, the cork is carefully 

 removed and a spread made by rubbing the cork containing the sediment 

 over an area of a square centimeter on a 3-inch by 6-inch glass slide. 

 The proper area of the smear is obtained by placing underneath the slide 

 a scale of circles having an area of a square centimeter. After the 



