The lesions of tuberculosis vary greatly in appearance, according to the organs affected, though the method of development is always identical.
The primary lesion corresponds to what has been termed tuberculous granulation, or anatomical tubercle properly so called; this, the macroscopical, pathological entity, assumes the form of a small prominent centre, semi-transparent, greyish, opaque or yellowish, according to its age.
These tubercles, produced by the presence of colonies of bacilli, are due to the defensive reaction of the invaded tissues, which gradually 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, while 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 tuberculous abscesses, which open towards any free passage, leaving behind sometimes ulcerations, sometimes caverns of varying sizes, or blind simple or bifurcated fistulæ.
Recent experiments by Nocard and Rossignol (1900) prove conclusively that a certain time (always 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 producing numerous isolated or confluent ulcerations in the air passages.