INFECTIOUS MASTITIS IN GENERAL 13 



dorsal position is not physiological to the cow. Thus 

 usually the bacterial infection takes place in a direction 

 opposite to that of gravitation, never in the direction with 

 it, which usually is an obstacle to infection. 



Finally the milk stream passes only in one direction, 

 outward from the udder through the teat canal by sucking 

 or milking. A retrogressive stream does not occur. The 

 udder is mechanically distended by the milk and collapses 

 like an empty glove after milking, without the influence 

 of any in-sucking force. Thus the infection must enter 

 against the stream, never with it. 



Biological causes for sterility of milk in the udder. 

 In spite of the before mentioned obstacles to infection one 

 or more bacteria may gain entrance, but these may be taken 

 up and destroyed by the ever-present leucocytes which are 

 more or less numerous. The leucocytes play a very impor- 

 tant part inasmuch as they inhibit the invasion of the 

 milk by bacteria. [The normal healthy udder of the cow 

 harbors bacteria throughout its whole extent (Ward). 

 The healthy udder always gives milk which is entirely bac- 

 teria free (Kitt., vol. i, 2d ed., p. 227).] 



Predisposing factors to infection through the teat 

 canal. These factors are of two types : {a) physical nature, 

 and ( b ) biological nature. 



Predisposing Causes of a Physical Nature. — Arrest of 

 milk secretion is produced by neglecting to milk or from 

 poorly regulated milking hours. For the same reasons 

 the cisterns can become so distended with milk that the 

 pressure finally overcomes the tension of the sphincter 

 muscle and the teat canal fills with milk which continu- 

 ously drops from the end of the teat. A rapidly growing 

 or motile bacillus can now easily gain entrance against 

 the stream, grow in the milk in the teat canal, and finally 

 infect the entire milk. 



A paralysis of the sphincter muscle following age, as 



