458 MILK 



ment is questionable. Other difficulties present themselves, such 

 as the length of time required to finish vaccination and the neces- 

 sity of keeping the vaccinated animals separated' from each other. 

 Furthermore, in spite of vaccination, some animals become in- 

 fected with tuberculosis and the immunity is always short-lived, 

 lasting for a period of from one to two years. As a practical 

 method for combating bovine tuberculosis, vaccination has proved 

 a failure up to the present time. 



It is clear that from an economic standpoint eradication of 

 bovine tuberculosis is much to be desired. Another aspect of 

 the question is the relation of the disease to human health. Milk 

 is one of the most important articles of diet, especially in infancy. 

 It has been shown that milk is frequently contaminated with 

 living tubercle bacilli of the bovine type. The infectiousness of 

 bovine tubercule bacilli for man is, therefore, a question of ut- 

 most importance. Tuberculosis is the commonest disease of man 

 and of cattle alike. It has been stated that Theobald Smith was 

 the first to prove conclusively that there are differences between 

 bacilli of the human and bovine types. Human bacilli gain 

 access to the system through inhalation, although infection of the 

 alimentary tract from contaminated food may play an important 

 role. Tubercle bacilli enter chiefly with food, except in the case 

 of accidental, external infections contracted by veterinarians, 

 butchers, and others occupied with slaughtering cattle. 



It was formerly believed that inhalation was by far the most 

 common means of infection. For some years this view has lost 

 ground, and proofs are accumulating which lay stress upon infec- 

 tion through the digestive tract. It has been shown that tubercle 

 bacilli can pass through the intestinal mucosa and through the 

 mesenteric glands. Intestinal tuberculosis is relatively rare in 

 adults. In infancy the mesenteric glands are easily affected. 

 Consequently, intestinal tuberculosis is much more common 

 among infants than among adults. Moreover, it has been shown 

 that dust particles do not penetrate the lungs, as a rule, beyond 

 the first branches of the bronchi. Tuberculosis of the lungs 

 commences in the capillaries, and this has been considered proof 

 of the hematogenous origin of the disease. 



Transmission of tuberculosis through the air is now consid- 

 ered to be less frequent than it. was formerly believed to be. Of 

 course, tubercle bacilli are emitted with sputum of patients, but 

 the sputum is a sticky, gummy substance which is difficult to 

 break up and which settles quickly. By the time sputum is so 

 completely dried and pulverized that it can be carried into the 

 air as dust, the bacilli have been exposed to sunlight and desicca- 

 tion for a long time. The majority of bacilli are probably de- 



