THE TUBERCLE BACILLUS 193 



or droplets, fingers or food, it passes either through the tonsils 

 or the mucous membranes of the upper respiratory tract or is 

 carried to the intestines and passed through the tissues there. 



Infection by drinking milk from tuberculous cows has been 

 clearly demonstrated. It has also been shown that such infection 

 does not necessarily come from cows with tuberculous lesions of 

 the udder, but may be conveyed in milk from cows showing no 

 lesions of the udder whatever. Perhaps in all such cases dried 

 feces falling into the milk from the skin of the cow is responsible 

 for the presence of the tubercle bacillus. Human infection by 

 this means must necessarily pass by way of the tonsils or the ali- 

 mentary tract. The majority of cases of cervical adenitis and 

 abdominal tuberculous in young children are undoubtedly con- 

 tracted in this manner. 



It may be stated, then, that the two chief modes of infection are 

 by inhalation and by ingestion of the tubercle bacillus. In the 

 former the organisms are for the most part derived from human 

 beings; in the latter, milk and milk products from tuberculous 

 cows or food contaminated from human cases are responsible. 



Heredity. In the strict sense of the word tuberculosis is not 

 considered hereditary. It is extremely unlikely that spermatozoa 

 or ova infected by tubercle bacilli would undergo normal develop- 

 ment. It is generally conceded that a hereditary tendency or 

 disposition to the disease may be transmitted from the parent 

 to the offspring, although what the tendency is has not been clearly 

 defined ; it may be a feeble constitution, or a structural peculiarity 

 or possibly an inability on the part of the body cells to generate 

 defensive antibodies when infection occurs. Congenital infection, 

 though rare, does occasionally occur, in which case tubercle bacilli 

 pass from the mother to the fetus by way of the placenta. Ex- 

 trauterine infection is much more likely to be the cause of tuber- 

 culosis in infants. Animal experiments have shown that the young 

 of infected mothers are usually infected only when suckled by the 

 tuberculous parent; when nourished by a healthy foster mother 

 they remain normal. The fact that tuberculosis seems to persist 

 in certain families may be due solely to the intimate associations 



