MECHANISM OF INFECTION IN TUBERCULOSIS 351 



only in its food. It is at least possible that some bacilli, in such 

 cases, find a resting place in the mouth and are carried thence 

 in currents of inspired air to the thoracic viscera, eventually 

 reaching the thoracic glands. 



Similarly it may readily be supposed that air-borne tubercle 

 bacilli, derived by children from their parents or their sur- 

 roundings, may sometimes be arrested in the saliva or dissolved 

 in the trachceal mucus and again coughed into the mouth, there 

 to be swallowed, so reaching the intestinal tract. 



The possibility that the intestine and its glands may be 

 infected in this manner is the more readily acceptable if it be 

 remembered that swallowed saliva does not excite the active 

 secretion of intestinal juices containing proteoclastic ferments 

 capable of digesting the bacilli. 



On the other hand, when taken in food, tubercle bacilli are 

 subjected to the full activity of digestive ferments. In the one 

 case, we have a relatively concentrated suspension of bacilli in 

 saliva permitted to act on the surface of the intestine un- 

 restrained ; in the other, bacilli are not only diluted to a great 

 extent by admixture with food but are subjected to the des- 

 tructive action of digestive juices. So far then from tuberculous 

 milk being a necessary source, it would appear that conditions 

 for infection of the gut in children are most favourable when 

 food is absent. 



It is not surprising, therefore, that though it is common 

 experience to find lesions in the digestive tract associated with 

 tuberculous infection of the lungs and thoracic glands, these 

 lesions are often neither so advanced nor so extensive as those 

 in the lungs. 



Human and Bovine Sources of Infection 



The exact nature of the mechanism of infection in young 

 children must remain uncertain if judged of on the evidence 

 afforded by considerations based on the incidence of the disease. 

 Some other means of estimating the characters of the infection 

 must be sought, if a clearer picture of the course of events is to 

 be obtained. 



Such a means apparently is at hand in certain differences 

 which have long been known to exist between the majority of 

 strains of tubercle bacilli derived from human sources on the 

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