MECHANISM OF INFECTION IN TUBERCULOSIS 343 



So relatively innocuous are the bacilli of tuberculosis and so 

 slight is the local disturbance to which they give rise that they 

 are not held up in the lung, as are the virulent organisms which 

 cause pneumonia. Becoming ingested by the cells which line 

 the air-chambers (alveoli) they are passed on into the lymphatics 

 of the lung and find their way to the glands lying at the 

 lung root, which act as filters for the lymphatic system. There 

 apparently they remain, either to be destroyed or perhaps, as in 

 the case under notice, to give rise to slow degenerative changes 

 in the gland substance ; the result is the formation of that curious 

 cheese-like or " caseous " material which is so characteristic of 

 tuberculous lesions. 



The process of infection continues but the lymph-gland 

 filters prevent bacilli coming from the lungs from entering 

 the blood stream. As time goes on and the gland substance 

 is destroyed, bacilli either periodically find their way directly 

 into the blood coursing through the glands or pass perhaps 

 by way of the efferent lymphatic channels to the main lymph 

 ducts and thus reach the blood. The main lymph ducts in 

 such cases as this do occasionally show tuberculous lesions. 



Under normal conditions the bacilli are destroyed in the 

 blood stream. 



So far accurate investigation supports the picture we have 

 drawn. A day comes, however, when general resistance to 

 tuberculous invasion is lowered, either through chill or hunger 

 or by some such shock as the baby under notice received. 



Or perhaps tubercle bacilli in the bifurcation gland give 

 rise to such destruction of tissue that the wall of a small artery 

 or vein in the gland becomes eroded or infected and tubercle 

 bacilli pass directly into the blood stream. 



It is remarkable that Poirier has shown that veins from the 

 bifurcation glands— or, as he calls them, the inter-tracheo- 

 bronchial group— pass directly into the back of the great Inferior 

 Vena Cava, the main vein from the lower limbs and trunk and 

 thus enter the heart by the shortest possible route. 



In either case, bacilli enter the blood stream in greater 

 numbers to find there far less resistance to their multiplication 

 or dissemination than under the conditions of health. Passing 

 into the venous blood from the bronchial glands directly into 

 the right auricle of the heart, they are hurried on in the heart's 

 blood stream into the right ventricle and pumped through the 



