30 Dairy Bacteriology. 



teria, yet in the secreting cells of the udder of a healthy 

 cow, germ life does not seem to be present. Only when 

 the gland is diseased are bacteria found in any abun- 

 dance. In the passage of the milk from the secreting 

 cells to the outside, it receives its first infection, so that 

 when drawn from the animal it generally contains a 

 considerable number of organisms. 



A study of the structure of the udder shows the man- 

 ner in which such infection occurs. 



Structure of the udder. The udder is composed of se- 

 creting tissue (gland cells) that is supported by fibrous 

 connective tissue. The milk is elaborated in these cells 

 and is discharged into microscopic cavities, from whence 

 it flows through the numerous channels (milk sinuses) 

 that ramify through the substance of the udder, until 

 finally it is conveyed into the milk cistern, a common re- 

 ceptacle holding about one half pint that is located just 

 above the teat. This cavity is connected with the out- 

 side by a direct opening (milk duct) through the teat. 

 During the process of milking, the milk is elaborated 

 rapidly in the gland cells, and their contents upon rup- 

 ture of the milk cells, flow down into the cistern. The 

 normal contraction of the muscles at the lower opening 

 of the outer duct prevents the milk from passing out 

 except when pressure is applied, as in milking. The in- 

 ner walls of the milk duct and cistern are always more 

 or less moist, and therefore afford a suitable place for 

 bacteria to develop, if infection once occurs, and con- 

 ditions are favorable for growth. 



Manner of invasion. Two possible sources of inva- 

 sion of the udder by bacteria may exist. If bacteria are 

 present in the circulating blood, there is the possibility 

 of organisms passing directly through the tissues into 



