200 MILK AND THE PUBLIC HEALTH chap. 



The total amount of milk yielded by all 20 cows = (19 x 4:)x 

 + 2a; + -^ = 78-2a;, or roughly 78a;; .-. the number of leucocytes 



per cubic m.m. in the mixed milk = — ' ^ = 936. 



If the secretion of the diseased cow had been excluded the 

 number of leucocytes would have been 680, so that the inclusion of 

 the purulent secretion from this animal had only raised the leuco- 

 cyte count to 936. Such an increase is within the variations m.et 

 with amongst different mixed milk samples from healthy cows. 

 In other words, the addition of the fluid from these two affected 

 quarters might easily fail to be detected by the leucocj'te count. 



In only quite small herds is the estimation of the cellular 

 content of the mixed milk likely to give clear evidence of the 

 existence of mastitis in one of the cows yielding the milk. 



Value of the Estimation of the Sediment. — Numerous 

 investigators have attempted to judge the cleanliness of milk 

 samples from a determination of the amount of dirt or sediment 

 obtainable from them. The bacteria which gain access to 

 milk are contained in particulate matter, generally in manure, 

 so that there is some justification for estimating the deposit 

 in milk, and taking it as an index of the amount of undesirable 

 bacteria which had gained access to it. The test is also a non- 

 multiplying one, so is as valuable for vended milk as for byre 

 samples. The amount of sediment can also be fairly easily 

 and quickly estimated. 



On the other hand, against the use of this test as a measure 

 of the bacterial contamination of milk some weighty objections 

 can be adduced. In the first place, it is well known that only 

 a fractional part of the manurial matter added to milk is 

 recoverable after sedimentation. For example, Thomas ^ states 

 that the Chester city analyst, as the result of many control 

 experiments, in which a known weight of fresh cow dung was 

 added to a known volume of clean new milk, found that only 

 one-eighth part by weight of the cow dung originally present 

 was recoverable in the final results, the rest being represented 

 by water, extractives, and soluble salts which had become 

 incorporated with the liquid part of the milk. The fractional 

 part which can be recovered is a variable and not a definite 

 fraction of the whole. 



' Thr Councils Journal, June 1908. 



