Predisposing Influences. 523 



obtained of the steppe-breeds: Bulls 4.40% tuberculous, oxen 16.84%, cows 5.397o; 

 while the colored breeds gave the following results: Bulls 3.92%, oxen 6.98% and 

 cows 22.61%. 



External influences which reduce the vitality of the animals 

 and the resisting powers of the tissues in general favor the 

 development of tuberculosis. This applies particularly to damp 

 and poorly ventilated dirty stables and to lack of exercise in 

 the open air. These two factors alone tend to produce super- 

 ficial breathing and catarrhal conditions of the respiratory tract 

 and thus they produce a local predisposition, which is increased 

 by the collection of secretions in the bronchial tubes. In addi- 

 tion to this close stabling affords more abundant opportunity 

 for infection from diseased animals, especially when frequent 

 purchases are made from suspicious sources. 



The predisposing influence of catarrhal conditions of the air pas- 

 sages is particularly striking in cattle that are kept in the vicinity of 

 sulphur and arsenic mines and smelters. In these animals the inhahition 

 of the smoke from the works, which contains arsenious acid, causes a 

 catarrh of the air passages and then pulmonary tuberculosis (which used 

 to be known as smelter smoke pneumonia) usually spreads very rapidly 

 (Siedamgrotzky, Johne, Pfeifer). In this connection the experiments of 

 ("adeac & Malet deserve mention. They succeeded in producing pul- 

 monary tuberculosis in rabbits by the inhalation of dust contaminated 

 with tubercle bacilli only in such cases where the respiratory tract was 

 previously irritated with fumes of bromine; also the experiments of 

 Plate according to which irritation of the intestinal mucous membrane 

 of guinea pigs vdth croton oil favored infection of the intestines with 

 tubercle bacilli in 80% of the animals. 



Insufficient and improper feeding with food deficient in 

 nutrients (distillery slops and factory refuse) and over-exer- 

 tion or excessive milk production are also predisposing factors 

 by weakening the organism. 



Tuberculosis frequently develops after acute inflammation 

 of the lungs. The diseased lung tissue and the incompletely 

 absorbed exudate constitute a favorable medium for the develop- 

 ment of tubercle bacilli. In other cases an attack of acute 

 disease weakens the body and prepares the soil for a renewed 

 and successful attack of an existing, but dormant, tuberculous 

 process. Finally the shedding of the milk teeth probably favors 

 in young animals, especially in pigs, infection through the 

 buccal cavity and the development of tuberculosis of the cervical 

 lymph glands. 



Pathogenesis. The principal ports of entrance for the tubercle 

 bacilli are doubtless the respiratory and digestive tracts. It is an open 

 question, however, as to which channel is of greater importance in nat- 

 ural infection. 



The fact that in man as well as in animals the lungs and the ad- 

 jacent lymph glands are, in the vast majority of eases, affected either 

 alone or in conjunction with other organs or parts of the body had 



