I ACTION OF THE BACILLUS 331 



a whole lung. Some authorities look upon giant cell formation as 

 a sign of chronicity of the process. Further, in some of the lower 

 animals, the giant cells become packed with tubercle bacilli, while 

 in man it frequently occurs that few or none at all are found. 

 When the giant cells do contain bacilli they are usually arranged 

 in one of four ways : (a) polar, (6) zonal, (c) mixed, or (d) at the 

 periphery of the giant cell. The breaking down of the nodule is 

 partly due to the bacterial poisons, and partly to the nodule being 

 non-vascular, owing to the fact that new capillaries cannot grow into 

 the dense nodule, and the old ones are occluded by the growth of 

 the nodule. 



At first the disease is local, owing to the unfavourable action of 

 the blood, to phagocytic action, or to the fewness of the number 

 of bacilli absorbed. From the local foci of disease the tuberculous 

 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 so frequently present as to be a 

 characteristic of the disease. This is the common method of 

 dissemination in the body, and by this channel the virus of 

 tuberculosis is carried along with the stream of lymph and infects 

 progressively the lymph vessels and glands. It may also be pro- 

 pagated along the lymphatics in an opposite direction to the lymph 

 stream. 



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

 carried to distant organs. But this channel is comparatively 

 rare. Blood is not a favourable medium for the tubercle 

 bacillus. 



(c) By continuity of tissues, that is by infective giant cell systems 

 encroaching upon neighbouring tissues, or discharge from lungs or 

 bronchial glands obtaining, for example, entrance to the gullet and 

 thus setting up intestinal disease. 



It has been abundantly proved that the respiratory and digestive 

 systems are those principally affected by the tubercle bacillus. 

 Wherever the bacilli are arrested, they excite formation of granula- 

 tions or miliary tubercular nodules, which increase and eventually 

 coalesce. The lymphatic glands which collect the lymph from the 

 affected region are earliest affected, always the nearest first, and for 

 a time the disease may appear to be appreciably stopped on its 

 invading march. Each lymphatic gland acts as a temporary barrier 

 to progress 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 body generally. 



Channels of Infection, — The common methods of invasion by 



