Pleurisy sometimes occurs simultaneously on both sides; in these cases both sides may be attacked at the same time, or the pleurisy on one side may succeed that of the other.

Pleurisies with long-standing effusion may give rise to cachexia, general dropsy, hydrothorax on the opposite side, hyperaemia and oedema of the lung of the same side, asphyxia, dilatation of the right side of the heart, venosity, more or less obliteration of the lung, and thus not un-frequently to the eradication of a pre-existing tuberculosis.

There is a typhous pleurisy, which, in the strict limitation of the term, is almost always associated with typhous pneumonia.

We have already mentioned, that one of the terminations of empyema consists in suppuration of the pleura; but this, and a similar destruction, may sometimes arise from the pulmonary parenchyma itself, as well as from without it; in the latter case, abscesses lying on the surface of the thorax, accumulations of tuberculous pus in the sternum and its vicinity, or in the vertebral column, softening encysted exudations on the peritoneum diaphragmaticum, perforating abscess of the liver and mammary gland, ichorous cancer, etc, may be regarded as exciting causes. Perforation of the costal pleura occurs for the most part where there is a tolerably thick and resistant layer of exudation deposited by a previous pleurisy, and adhesion thus established with the lung; there is then no discharge of pus into the thoracic cavity, but it not unfrequently happens that it finds its way through fistulous passages into the adherent lung, and leads to its ulcerous destruction.