96 BACTERIA IN MILK 



Although the majority of the evidence available favours the 

 hypothesis that the source of intramammary contamination 

 is external it is difficult to establish this entirely on account of 

 the impossibility of putting the ducts and cisterns in a sterile 

 condition. Once infection of the udder has occurred, the 

 organism, finding the mammary secretion an excellent pabulum 

 for development, persists and the small quantity of milk re- 

 maining from one milking contaminates the next, the process 

 being repeated until the cow becomes dry. That the amount 

 of milk allowed to remain in the udder has a very material 

 influence upon the bacterial count of the milk obtained at the 

 next milking is shown by the experiments of Stocking, 12 who 

 found as the average of ten experiments 6542 bacteria per c.cm. 

 in milk obtained after thoroughly stripping the udder as against 

 11,324 per c.cm. when this was neglected. The importance of 

 this factor is now well recognised in large dairies using milking 

 machines, for it is invariably the custom to take out the last 

 strippings by hand, owing to the impossibility of obtaining 

 this milk by means of the machine. This hand-milked secretion 

 often contains more bacteria than the portion immediately 

 preceding it, due, Stocking suggests, to more vigorous manip- 

 ulation of the udder dislodging bacteria from the ducts and 

 which remained there during the earlier part of the milking. 

 The contaminated milk left in the ducts is, of course, mostly 

 discharged in the fore milk and a decreasing count is obtained 

 as milking proceeds. Stocking 12 reports the following results 

 in this connection as the averages of four experiments : 



Bacteria per c.cm. 



Streams 1 and 2 10,143 



Streams 5 and 6 2,347 



Streams 9 and 10 272 



Streams 13 and 14'. 382 



Strippings 204 



The influence of the rejection of the contaminated fore 

 milk was shown by the following figures: 



