Morbid Anatomy.—As in inflamma tion of serous membranes generally, there occurs hyperemia, proliferation and desquamation of endothelial cells, exudation of serum, and leucocytes on the surface of the pleura. The pleura loses its polish, partly on account of these changes and partly on account of fibrin, which forms a thin layer on its surface. In more severe cases the fibrin forms in thick, shaggy masses, of which the layers in contact with the pleura are more or less densely laminated. The fibrinous exudate forms on both surfaces, but more densely on the visceral pleura usually because infection generally takes place from the lung.
If the inflammation ceases at this stage we speak of it as a dry pleurisy. The exudate is partly absorbed and partly or ganized, adhesion of the opposed pleural surfaces resulting. It is probable that in mild cases all the exudate is absorbed, leaving no adhesions or other traces of inflammation. This is difficult of dem onstration, but we know that even severe peritonitis may leave no permanent hesions, and from analogy we have reason to believe that similar results occur in pleurisy.
There are, at least, three distinct forms of pleurisy: (1) a form in which "fibrinoid degeneration" occurs along with marked filtration of the pleura by cells; (2) a form in which there is a general increase of the connective tissue of the pleura without marked filtration by cells and without "fibrinoid degenera tion" of the connective-tissue fibrils; (3) a form in which the whole thickness of the inflamed pleura consists of highly vascular and very young cicatricial tis sues. Endothelium may be found cover ing the "false membrane," and the con nective tissue of the pleura itself may undergo the change termed "fibrinoid degeneration." But these changes are not commonly seen. In the greater number of human cases examined all trace of the normal endothelium was lost. W. S. Lazarus Barlow (Brit. Med. Jour., Sept. 3, '98).
In the early stages of tuberculous pleurisy the lesions are not different from those of simple inflammation, excepting for the presence of the bacillus; and, even later, distinct tubercles often do not form. A. N. Peron (Presse 'bled., Feb. 19, '98).