ACTION ON TISSUES— TUBERCLE. 9 



7. Returning to the tubercle bacillus, as an example of an organism 

 acting first chemically on the tissues by its products, and then 

 upon the cells weakened by this reaction, it may be observed that 

 a single tubercle bacillus coming in contact with a healthy mucous 

 surface is unable to retain its footing for a long enough time to allow 

 of the performance of any pathogenic function. In the lung, such a 

 solitary bacillus, even if allowed to remain, could find no food from 

 which to elaborate its own peculiar products, and until this could be 

 done, there would be no changes in the cells with which it is in con- 

 tact. If, however, a mass of caseous material from some tubercular 

 focus be introduced into an air vesicle or a small bronchus, the con- 

 ditions are altogether changed. Such a mass, containing the bacillus, 

 its food, and its products, is brought directly into contact with the 

 epithelial surface, and unless the epithelium is endowed with the 

 greatest vitality, it cannot withstand the attacks first of the products 

 of the bacillus, and, second, of the bacillus itself. 



That this is not merely hypothetical, is evidenced by the fact that 

 in a case of acute tuberculosis of the lung the following distribution 

 of the bacillus may be observed. 



Taking the centre of a lobule, usually the bronchus, as the point 

 from which to commence the observation, it will be noted that in 

 this bronchus, and in the air vesicles immediately surrounding it, 

 caseation has become a well-marked process. In the caseous mass 

 bacilli are found (in the specimens under consideration) in enormous 

 numbers. Passing from the caseous centre, it will be found that 

 where traces of the original structure exist, the bacilli are still 

 present, but are not so numerous as in the centre. Extending the 

 observation to the margin of the tubercular mass, it will be noticed 

 that there is an area of active cell proliferation, not confined to the 

 endothelium of the lymphatics, but appearing also in the epithelium 

 of the air vesicles. In this mass of proliferating tissue there are 

 absolutely no bacilli at first ; a few may be distinguished, here and 

 there, near the proliferating area, but where the proliferation is most 

 marked the bacilli are absent. In a very large number of sections 

 examined this was invariably the case. This can be best accounted 

 for on the theory that the bacilli during their growth and develop- 

 ment secrete or excrete a chemical substance which alters the 



