62 PRACTICAL DAIRY BACTERIOLOGY 



produce such milk the udder, flanks and under side of the 

 cow are carefully washed; the teat washed with water or 

 with a solution of 10 per cent, borax; sometimes a little of the 

 borax solution is injected into the milk duct. The milker washes 

 his hands and milking utensils, and puts on a clean milking 

 gown. Previously a flask with a narrow neck is plugged with 

 cotton and sterilized in the laboratory. This is taken to the 

 milking room, and the cotton removed only at the moment of 

 the milking. The first quarter of the milk is drawn in a sepa- 

 rate vessel, and then the mouth of the flask is placed close 

 to the teat, and the milk is drawn directly into it, after which 

 it is immediately closed again with the cotton plug. In this 

 way the chance of external contamination is practically elim- 

 inated, and the milk which is obtained contains only the bac- 

 teria from the udder itself. Such aseptic milk contains much 

 fewer bacteria than ordinary milk, but even with these pre- 

 cautions sterile milk is not obtainable. It usually contains, 

 however, only the harmless cocci, which have little effect upon 

 it, and it may be kept for a long time without souring or show- 

 ing other changes due to milk bacteria. Such a method of milk- 

 ing is, of course, impractical. A somewhat close approximation 

 to it is obtained by modern milking machines, which will be 

 referred to on a later page. 



EXTERNAL CONTAMINATION 



Most of the bacteria in ordinary milk come not from the 

 udder, but from external sources. These sources are several, 

 and may be considered in succession. 



The Cow. One of the chief sources of bacteria is the cow 

 herself. Her skin, even when kept in fairly good condition, 

 is never very clean and will always hold more or less dirt 

 and dust laden with bacteria. The cow in many poorly kept 

 dairies is, as is well known, rarely, if ever, cleaned: her flanks, 

 tail and skin become covered with a coating of manure, until 



