Relation of Disease-Bacteria to Milk. 87 



herds, in fact the great majority, are wholly free from all 

 taint. The disease has undoubtedly been most frequently 

 introduced through the purchase of apparently healthy but 

 incipiently affected animals. Consequently in the older 

 dairy regions where stock has been improved the most by 

 breeding, more of the disease exists than among the western 

 and southern cattle. 



Infectiousness of milk of reacting animals. Where the 

 disease appears in the udder the milk almost invariably con- 

 tains the tubercle organism. Under such conditions the 

 appearance of the milk is not materially altered at first, 



FIG. 21. Side view of a tuberculous udder, showing extent of swelling in 

 single quarter. 



but as the disease progresses the percentage of fat generally 

 diminishes, and at times in the more advanced stages where 

 the physical condition of the udder is changed (Fig. 21), 



