164 WOOLFOLK : 



infected, and this in turn infects all milk with which it is 

 mixed. A difference of opinion exists as to the proportion of 

 affected cattle which yield milk containing the tubercle bacilli. 

 Some have held that the udder must necessarily be diseased 

 before the bacilli can find their way into the milk ducts, and 

 as only a small proportion of the affected cows have the dis- 

 ease of the udder, the danger from this source was thought to 

 be slight. It seems likely, however, that the udder is affected 

 in a larger number of cases than has usually been admitted. 

 It requires a very long and careful examination to determine 

 positively that the udder is free from disease. Pearson, in the 

 examination of 1200 cows affected with tuberculosis in Penn- 

 sylvania, found the udder aifected in about 10 per cent. 



Numerous investigations have also shown that milk may 

 contain tubercle bacilli when there are no appreciable signs of 

 tubercular disease in the udder. 



The combined results of the ingestion and inoculation 

 experiments showed that the milk of 12 out of 56 reacting 

 cows, or 21 per cent., at one time or another during the exper- 

 ment contained virulent tubercle bacilli. The milk from 

 tubercular herds is a frequent source of tuberculosis in calves 

 and pigs. The calves born in tubercular herds are fed upon 

 the milk produced by such herds during their early life, and a 

 considerable portion of them are infected in that way. In 

 dairies where butter or cream is sold the skim milk is com- 

 monly fed to pigs and calves, and these animals are infected 

 to an enormous extent, records showing as high as 75 per 

 cent, of calves or pigs fed skim milk or centrifugal milk, 

 affected with tuberculosis. So well is the extent of this 

 known that packers in purchasing calves and pigs from dairj' 

 districts, not only discriminate against them, but in man}- 

 instances will not buy them only subject to the inspector's 

 post mortem examination, or in other words will only pa^' full 

 price for the calves and hogs that pass as good, and a dirty- 

 grease price for the condemned ones. 



There is another way in which milk plays a prominent 



