22 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



its way into the milk. It may not appear to be extensive con- 

 tamination, but every hair and dust particle which finds its way into 

 the milk-pail carries with it a large number of bacteria, and whilst 

 it may be possible, by muslin straining, to remove the hairs and 

 microscopic particles, the conveyed bacteria will remain. No one 

 having in his mind Tyndall's " raft-theory " can watch an ordinary 

 milking process without being convinced that it is by such rafts 

 that countless germs, most of which are fortunately innocent and 

 harmless, are introduced into the milk. Further reference is made 

 subsequently to this question of dust in the air as a vehicle of 

 organisms. The accompanying Plate is made from a photograph 

 of a Petri dish of nutrient medium exposed at a spot in Central 

 London where milk is delivered daily. A close examination of 

 the colonies will show the particle of dust stranded on the summit 

 of the colony, itself consisting of the progeny of the bacteria carried 

 on to the plate by the particle of dust acting as a raft or vehicle, 

 as suggested by Tyndall. 



But even yet, it is to be feared, we have not exhausted the cow- 

 element In the pollution of milk. Its diseases, its uncleanliness, 

 and its diet, do not complete its contribution. There still remain 

 the open milk-ducts of the udder. The mammary gland of the 

 cow, as is well known, is divided into four quarters, to each of 

 which a teat is attached. The teat is composed of spongy elastic 

 tissue surrounding the main milk-duct, which differs from the 

 secretory ducts in function and in size, and is indeed the final 

 channel through which the milk passes from the udder. Now 

 this milk-duct is practically open to the air, and from its valvular 

 shape, it will contain at the close of milking, however thorough the 

 manipulation, some small amount of milk adherent to its walls. 

 The temperature of such retained milk is at or near blood-heat. 

 Bacteria in the air or from the soil have no difficulty whatever in 

 gaining admittance to such residual milk, and under such favouring 

 circumstances can and do multiply with enormous rapidity. It is, 

 in fact, an excellent incubator. At the next milking the first 

 or fore-milk contains this residual, highly-contaminated, milk, 

 which for some hours at least has been lying in the milk-duct. 

 The result is that the fore-milk always contains an excess of 

 bacteria, which, in the case of an experiment by Schultz, were found 

 to amount to 80,000 micro-organisms per cc, while the last drawn 

 milk was practically sterile ; and in a similar experiment by Russell 

 under ordinary circumstances, 2800 bacteria per cc. were found in 

 the fore-milk, whilst the average of the total yield was only 330 



