TUBERCULOSIS 193 



able that few people or domesticated animals can be placed un- 

 der continued exposure to the germs without ultimately hav- 

 ing the disease. Predisposing conditions undoubtedly have 

 much to do with its development — close confinement, lack of 

 exercise, lack of ventilation and sunshine, and injudicious in- 

 breeding, for instance, but these alone cannot cause it. 



Modes of entrance. — Germs of tuberculosis enter the body 

 (1) through the lungs, with inhaled air; (2) through the 



Pig. 66. — Bovine Tuberculosis. (M. H. B.) 



A famous show steer, in prime ' ' condition. ' ' Liable to be as badly dis- 

 eased as the preceding. 



stomach and intestines, with food and drink; (3) through 

 broken skin or mucous membrane by inoculation; (4) by infec- 

 tion in utero, i.e., before birth. The latter is of very rare 

 occurrence and of small importance. 



Structures affected.— Tuberculosis may affect almost any tis- 

 sue or organ of the body. Milk from tuberculous cows may 

 be tuberculous; but the danger is especially great when the 

 udders are affected. Statistics indicate that about one cow out 

 of every four and six tenths (1:4.6) of all tuberculin reacting 

 cows, give virulent milk. 



The extent of tissue diseased may vary from a small lesion 



