158 DAIRY BACTERIOLOGY 



will be necessary in dairies to filter the contents of each can or 

 churn through a separate cotton wool disc, and also to examine 

 the empty cans carefully, for most of the dirt usually remains in 

 them. As has been previously pointed out, the chief impurity of 

 milk is cow manure, which contains 80 per cent, of water, and 

 soluble matter, both of which are completely incorporated with the 

 milk owing to the shaking up which occurs in transit ; at the same 

 time, the bacteria which constitute an appreciable proportion of 

 the solid matter, are distributed throughout the milk. The above 

 estimate applies to normal dung ; if the cows are suffering from 

 diarrhoea, the dung will contain a still larger proportion of soluble 

 matter, and consequently the filtration test will show less dirt. 

 Further, the more liquid the dung the greater will be the propor- 

 tion of dangerous bacteria introduced into the milk. The estima- 

 tion of dirt therefore furnishes no measure of the bacterial contents 

 of the milk, and it must also be remembered that very dirty 

 milk which has been well cooled may contain fewer bacteria than 

 less dirty milk which has been inadequately cooled. In reality, 

 therefore, the only sure indication afforded by the dirt test is 

 whether the milk has been properly cleaned or not, either by 

 straining, filtering or centrifuging. 



TROMMSDORFF'S LEUCOCYTE TEST 



The particles of dirt and foreign bodies which are found in 

 suspension in milk are removed much more completely by centri- 

 fuging than by sedimentation or filtration. Thus, of the white 

 blood corpuscles or leucocytes which normally do not separate out 

 when the milk is allowed to stand for a relatively short time, 

 3 to 50 per cent, are separated by centrifuging, or even more if the 

 milk is warm 1 . The heating should, however, not exceed 70 C., 

 for otherwise precipitation of the proteins may occur. As was 

 first shown by Barthel, the centrifuge slime therefore consists very 

 largely of leucocytes 2 . By direct microscopical counts, normal 

 milk has been shown to contain J to 1 J million leucocytes per cubic 

 centimetre 3 . The number of leucocytes increases as the yield of 

 milk decreases, being particularly high at the beginning and 

 towards the end of the lactation period. It is still higher in cases 

 of udder disease, and on this fact Trommsdorff has based his test, 

 the object of which is to ascertain whether or not the milk has 

 been derived from healthy cows. The test is carried out as 



1 Campbell, U.S. Dept. Anim. Industry, Bull. 117, p. 19. 



" Revue generate du lait," 1901, vol. 1, p. 193. 



3 Prescott and Breed, " Journal of Infectious Diseases," 1910, vol. 7, 

 p. 632 ; Breed and Stiger, ibid., 1911, vol. 8, p. 361. 



