This manifests itself chiefly as an inflammation of the pleura, and hence is identical with Tubercular pleurisy. The tubercle bacillus does not usually reach the pleura directly from the lung in phthisis pulmonalis, the pleurisy associated with phthisis being usually non-tubercular. On the other hand, the infection may reach the pleura by a local extension from the pericardium or peritoneum, from a tubercular lymphatic gland at the root of the lung, or from a tubercular abscess in connection with the vertebrae. It may also be conveyed by the blood.

Tuberculosis generally manifests itself at first as an acute or subacute inflammation,, accompanied by abundant fibrinous exudation, sometimes mixed with blood. The tubercles are buried under the exudation, which may be very tough, and they may escape detection unless the fibrine be peeled off". They are often best seen between the lobes of the lung, where the close contact of the surfaces hinders the deposition of fibrine, and here they are visible as closely-set white nodules. The inflammation may go on to suppuration, so that we may have an empyema.

The acute character usually subsides after a time, and, with chronic inflammation, new-formation of connective tissue occurs, forming firm adhesions in the midst of which the tubercles may be found. As in the case of tubercular pericarditis, the adherent and coalesced pleura will show two layers of tubercles, one belonging to the pulmonary and the other to the parietal layer.

There may be a partial extension of the tuberculosis into the lung, along the interlobular septa, but it is a very superficial process.