INFECTION OF THE UDDER. 703 



way. The apparent rarity is explained, however, by the fact that 

 an effectual closure mechanism exists in the teat duct, and that 

 the milk seldom remains pent up in the udder for any long time. 

 Numerous experiments, in which pure cultures of particular bacteria 

 have been injected into the galactophorous sinuses, show, however, 

 that inflammation may start here. Kitt even produced mastitis 

 by painting the teat with a pure culture. 



It has been much disputed whether inflammation of the udder 

 can be caused by not milking the animal (overstocking). Whilst this 

 was formerly regarded as a frequent cause of mastitis, experiment 

 has shown that retention of the milk alone never produces disease 

 of this description. The retention of milk as such, certainly does 

 not produce mastitis, but it favours its production, because of the 

 readiness with which decomposition and the growth of bacteria 

 occur in the retained milk and cause inflammation. 



This explains why mastitis is most common during hot weather ; 

 Strebel says most cases occur between March and September. Other 

 factors may also be at work, such as the animal's condition, the 

 attention it receives, and the time during which the calf is with it ; 

 but this much is unquestionable, that warm weather favours decom- 

 position of the milk, and the multiplication of bacteria. 



(3) Finally, infection through the blood stream appears possible 

 (hematogenous mastitis), though it is probably very rare. 



It also seems possible that when the disease is widely spread, 

 or conditions are favourable for the transmission of the specific 

 microbe, mastitis may take an epizootic form ; the infecting material 

 being easily carried from animal to animal during milking. It is 

 not astonishing, considering the varying virulence of pathogenic 

 microbes, that many different forms of udder infection occur, so 

 that sometimes only slight irritation, sometimes severe inflammation, 

 or sometimes gangrene results. 



In view of the intimate relationship that exists between the 

 parenchyma and stroma of the gland, it is scarcely surprising that 

 acute inflammatory processes often extend from one to the other, 

 that in inflammation of the parenchyma the stroma, and in disease 

 of the stroma the parenchyma, often suffers. The former occurs 

 the more readily, because the blood-vessels of the udder ramify 

 Avithout exception in the stroma. 



