TUBERCULOSIS. 687 



These tui)ercles, produced by the presence of colonies of bacilli, 

 are due to the defensive reaction of the invaded tissues, which gradu- 

 ally undergo change and are destroyed in a direction radiating from 

 the centre towards the periphery. The tubercle in itself has no very 

 specific character — only the bacillus. 



The elementary lesion may remain isolated, but very frequently 

 it is closely surrounded by other similar tubercles, and becomes- 

 enveloped in a common inflammatory area. A large portion of an 

 organ may appear as if riddled with tubercles of different age and 

 size, w^hile the interstitial connective tissue reacts and forms fibrous 

 separating partitions. The general appearance is that described under 

 the term " diffuse tuberculous infiltration." 



At a still more advanced stage in the development of the disease 

 conglomerations are produced, consisting of tuberculous masses the 

 size of a hazel-nut, a walnut, an egg, a man's fist, or even larger. 

 These lesions, irrespective of size, undergo caseous degeneration from 

 the centre towards the periphery. 



In exceptional cases the tubercles remain fibrous. More frequently, 

 particularly in animals of the bovine species, they become infiltrated 

 with lime salts. Caseous degeneration not only invades the centre of 

 the tubercles but also the peripheral layers, and sometimes the whole 

 of a conglomerated mass. 



Steadily pursuing their course of pathological development, the 

 tuberculous masses become softened and are transformed into tuber- 

 culous abscesses, which open towards any free passage, leaving behind 

 sometimes ulcerations, sometimes caverns of varying sizes, or blind 

 simple or bifurcated fistulae. 



Kecent experiments by Nocard and Eossignol (1900) prove con- 

 clusively that a certain time (alw^ays more than a fortnight) elapses 

 between the moment of entry of the contagion into the organism 

 and that at which its effects become manifest by furnishing a 

 reaction to tuberculin. Calcification or softening of the lesions, 

 moreover, never occurs in less than fifty days. 



According to the organs studied, these tuberculous lesions assume 

 certain appearances, which in each locality seem almost always to be 

 identical. 



Thus, as regards the larynx, trachea, and bronchi, the tubercles 

 develop in the depths of the mucous membrane, rapidly undergoing 

 caseous transformation, softening and purulent degeneration, and pro- 

 ducing numerous isolated or confluent ulcerations in the air passages. 



According to the case and the kind of animal affected, the lung 

 presents either disseminated tuberculous formation, tuberculous in- 

 filtration, tuberculous conglomeration, or cavern formation. 



