36 SOURCE OF BACTERIA IN MILK. 



microorganisms are present in the milk at the moment of 

 secretion, the mammary gland itself will be found to be in- 

 fected with local centers of disease. It is really a matter of 

 little practical importance how this case is settled. If an 

 animal is attacked by pathogenic disease there is quite likely 

 to be local udder infection, which may frequently be so slight 

 as to be incapable of detection, and even if the udder is not 

 infected there is always a chance for secondary contamina- 

 tion of the milk. If an animal is attacked by a contagious 

 disease her milk is thus quite likely to contain the bacteria, 

 even though the scientific evidence seems to indicate that the 

 bacteria are not secreted through the gland. 



BACTERIAL CONTAMINATION OF NORMAL MILK. 



Whatever the condition of the milk the moment it is 

 secreted it is always contaminated with bacteria by the time it 

 reaches the milk vessel, or even before, for there are a 

 variety of sources from which bacteria are sure to find an 

 entrance to the milk during or immediately subsequent to its 

 passage through the milk ducts. The sources from which 

 these bacteria are derived are several. 



i. The Milk Ducts. If the mammary gland of the cow 

 is examined it .will be found to have a structure shown in 

 Fig. 10. In this figure the upper part of the gland is seen 

 to be moderately solid. The lower part of the gland is filled 

 with irregular ducts and spaces, and at the upper portion of 

 the teat there is rather of a large space called the milk 

 cistern. It has been demonstrated in a variety of ways that 

 these ducts and spaces in the milk gland are more or less 

 infected with bacteria (26). The milk first drawn at each 

 milking, called " fore milk," is quite certain to be filled with 

 bacteria in considerable quantity even at the moment of leav- 

 ing the milk teat (20), whereas at the close of the milking, 

 after the various ducts have had an opportunity of being 

 washed out by the stream of milk, the number of bacteria 



