BACTERIA AND DISEASE 28 1 



is non-vascular, owing to the fact that new capillaries can- 

 not grow into the dense nodule, and the old ones are all 

 occluded by the growth of the nodule. 



From the local foci of disease the tubercle process spreads 

 chiefly by three channels : 



(a) By the lymphatics, affecting particularly the glands. 

 Thus we get tuberculosis set up in the bronchial, tracheal, 

 mediastinal, and mesenteric glands, and it is so frequently 

 present as to be a characteristic of the disease. This is the 

 common method of dissemination in the body. 



(b) By the blood-vessels, by means of which bacilli may be 

 carried to distant organs. 



\c] By continuity of tissues, infective giant-cell systems 

 encroaching upon neighbouring tissues, or discharge from 

 lungs or bronchial glands obtaining entrance to the gullet 

 and thus setting up intestinal disease also. 



It has been abundantly proved that the respiratory and 

 digestive systems are principally affected by Koch's bacillus. 

 Wherever the bacilli are arrested, they excite formation of 

 granulations or miliafy tubercular nodules, which increase 

 and eventually coalesce. The lymphatic glands which col- 

 lect the lymph from the affected region are the earliest 

 affected, always the nearest first, and then the disease ap- 

 pears to be appreciably stopped on its invading march. 

 Each lymphatic gland acts as a temporary barrier to pro- 

 gress until the disease has broken its structure down. It 

 remains local, in spite of increase in number and importance 

 of the foci of disease, as long as the bacilli have not gained 

 access to the blood stream. 



Toxins and Tuberculin. Koch, Crookshank, and Herroun, 

 Hunter, and others have isolated products from pure cult- 

 ures of the tubercle bacillus. These have comprised chiefly 

 albumoses, alkaloids, and various extractives. Koch's ob- 

 servations led him to suppose that in pure cultures of tubercle 

 a substance appeared having healing action on tuberculosis, 



