Till-: BACILLUS OF TUBERCULOSIS 299 



severity, and that there is, as a rule, no great difference in degrees of 

 virulence for animals in the tubercle bacilli obtained from different 

 sources. As is seen from the results of post-mortem examinations in 

 which, according to the completeness of the examinations, the remains 

 of old tuberculous processes have been found in the lungs of about 

 one-third to one-half of all the bodies examined, many cases of pul- 

 monary infection must occur without showing any visible evidence of 

 disease, and heal of their own accord. The possibility of favorably 

 influencing in many an existing tuberculosis by treatment also proves 

 that, under natural conditions, there is a varying susceptibility to the dis- 

 ease. Clinical experience teaches, likewise, that good hygienic con- 

 ditions, pure air, good food, freedom from care, etc., increase immunity 

 to phthisis. Animal experiments have shown that not only are there 

 differences of susceptibility in various animal species, but also an 

 individual susceptibility in the same species. The doctrine of indi- 

 vidual susceptibility, therefore, is seen to be founded on fact, although 

 the reasons for it are only partially understood. 



INFECTION BY INGESTION OF MILK AND MEAT. Phthisical sputum, 

 however, is not held responsible for the occurrence of all human tuber- 

 culosis. Milk also serves as a conveyer of infection, whether it be the 

 milk of nursing mothers suffering from consumption or the milk of 

 tuberculous cows. The transmission of tubercle bacilli in the milk of 

 tuberculous cows has been abundantly proved. Formerly it was thought 

 that in order to produce infection by milk there must be a local tuber- 

 culous affection of the udder; but it is now known that tubercle bacilli 

 may be found in the milk when an internal organ is infected, and when 

 careful search fails to detect any udder disease. The milk of every cow 

 which has any well-developed, internal, tuberculous infection must there- 

 fore be considered as possibly containing tubercle bacilli. Rabino- 

 witsch and Kempner proved beyond all question that not only the milk 

 of tuberculous cattle, which showed no appreciable udder disease, but 

 also those in which tuberculosis w r as only detected through tuberculin, 

 frequently contained tubercle bacilli. Different observers have found 

 tubercle bacilli in the milk of from 20 to 60 per cent, of tuberculous 

 cows. When we consider the prevalence of tuberculosis among cattle 

 we can readily realize, even if the bovine bacillus with difficulty infects 

 human beings, the danger to which children are exposed from this 

 source of infection. Thus, taking the abattoir statistics of various 

 countries, we find that about 10 per cent, of the cattle slaughtered 

 were tuberculous. A less probable source of infection by way of 

 the intestines is the flesh of tuberculous cattle. Here the danger is 

 considerably less, from the fact that meat is usually cooked, and also 

 because the muscular tissues are seldom attacked. In view of the 

 finding of the bovine type of bacilli in a considerable percentage of 

 the few cases tested of tuberculous children, the legislative control 

 and inspection of cattle and milk would seem to be an absolute neces- 

 sity. As a practical and simple method of preventing infection from 

 suspected milk the sterilization (by heat) of the milk used as food must 



