Pleural tubercle occurs in all the forms in which we have stated that tuberculosis attacks serous membranes; that is to say -

(a), as a rapid metamorphosis (either complete or partial) of a pleuritic coagulum; this is most common on the parietal lamina; -

(6), as tuberculous formation in a pseudo-membranous coagulum that is either becoming organized in some manner or other, or that has become organized to a certain stage; as, for instance, tubercle on and in layers of areolar, fibro-areolar, and other exudations; -

(c), as primary acute tuberculosis in the form of the most minute miliary tubercles.

The second kind of tuberculous formation is very frequently associated with secondary inflammation of the pseudo-membrane serving as the base to the tubercle, and in these cases we may frequently observe hemorrhagic exudation.

Pleural tubercle is, in most cases, the result of a dyscrasia that has already revealed itself in tuberculosis of a parenchymatous tissue, most probably of the lungs or bronchial glands; sometimes, however, it occurs as the first in the series of successively developed tuberculosis, and not unfrequently it is associated with quiescent or even retrograde pulmonary tuberculosis, indicating a tumultuous recrudescence of the general disease, and from that period being frequently combined with general tuberculosis.

Pleural tubercle not unfrequently softens and gives rise to tuberculous abscesses in the different pseudo membranous structures in which it is deposited. These abscesses may penetrate the pleura, and even the thoracic walls, with or without caries.