553 
are present in milk they will no doubt be present in greater concen- 
tration in cream and butter.® 
We can protect ourselves against the tubercle bacilli that are dis- 
tributed in milk by practicing home pasteurization ; but with butter 
this is not possible; and it is therefore desirable that the milk or 
cream used in the manufacture of butter should either be obtained 
from cows certainly free from tuberculosis, or that it should be 
pasteurized before it is used. 
SUMMARY. 
We have seen that tuberculosis is the commonest disease of both 
persons and dairy cows, and that persons and dairy cows are its com- 
monest victims ; we know that dairy products are indispensable, and 
that they are more commonly eaten in a raw state than other products 
from animals ; we have seen that tuberculosis is an insidious, chronic 
disease, and that tuberculous cows often expel tubercle bacilli long 
before they show signs of their diseased condition ; w T e have seen that 
milk is almost invariably contaminated with the material in which 
tuberculous cows most commonly expel tubercle bacilli from their 
bodies ; 6 we have seen that milk is so often infected with virulent 
tubercle bacilli that unless we know it to be derived from cows that 
are certainly free from tuberculosis it is not safe to use it in a raw 
state ; we have seen that tubercle bacilli in milk are transferred to the 
cream, butter, and cheese made from it, and may occur in these prod- 
ucts in greater concentration than they had in the milk from which 
they were derived ; we have seen that no better medium for the preser- 
vation of the life and virulence of tubercle bacilli can be found than 
the moist, bland, and opaque character of butter offers ; we have been 
told that the medical profession is well-nigh unanimous in the view 
that tubercle bacilli from the bovine source in dairy products are a 
serious menace to public health ; and we have seen that, in our fight 
for the suppression and eventual eradication of tuberculosis, we must 
seek to make harmless all the sources from which tubercle bacilli are 
expelled. Add to this that the available evidence regarding different 
types of tubercle bacilli shows that bovine types have been found in 
human lesions and human types in bovine lesions; that transition 
forms connect bovine types directly with human types ; that the most 
variable feature about a tubercle bacillus is the character that is used 
to classify it as a special type ; that tubercle bacilli of human types 
have been converted into bovine types and those of bovine types into 
a Bureau of Animal Industry Circular 127, 1908, pp. 4-5. 
1 The expulsion of virulent tubercle bacilli with the feces from the bodies of 
apparently healthy, tuberculous cows has been confirmed since this article was 
written by the British Royal Commission on Human and Animal Tuberculosis. 
See Third Interim Report, London, 1909. 
