Besides the changes connected with exudation and effusion, and organization or suppuration in the exuded products, _gangrene_ sometimes results. A case of this kind is related by _Percivall_. The _duration_ of pleurisy may thus extend from two days in very acute cases to several weeks, or even months if we estimate it by the continuance of _hydrothorax_ in the chronic cases. _Post Mortem Appearances._ These consist mainly in the presence of false membranes lining the pleura and hanging in cobweblike shreds into the cavity of the chest, and of the liquid effusion which fills up the chest at its most dependent part. The pericardium also contains fluid in many cases. The periods at which exudation takes place, and when the principal changes take place in the exuded materials have been well investigated by Dupuy, Hamont, Delafond and St. Cyr. They induced pleurisy by injecting irritant liquids into the chest, and noted the regular sequence of changes. Dupuy injected two drachms of oxalic acid dissolved in three ounces of water. Symptoms of pleurisy at once came on, with the friction sound characteristic of its early stages. Next day friction sound had ceased and evidence of effusion existed. The same experiment repeated on several horses showed that if killed at any period subsequently to this, considerable exudation had already taken place. In one horse in which the disease was of 50 hours’ standing the chest contained 43 pints of citrine-colored fluid, and abundance of yellow, thick, false membrane enveloping the costal and pulmonary pleuræ. Hamont injected seven ounces of a weak solution of tartaric acid into the left pleural sac, repeated the injection next morning and destroyed the horse twenty minutes afterward. The chest opened immediately showed a small amount of liquid on the affected side, and the pleura injected and reddened. Delafond made twenty-two experiments with the same general result. Percivall found recent adhesions between the lungs and side so early as seventeen hours after the commencement of the pleurisy. Andral injected rabbits with acetic acid and in nineteen hours found in the injected pleura soft, thin, false membranes traversed by red anastomosing lines, and in certain cases a serous or puriform fluid. W. Williams found a false membrane formed twenty-four hours after the injection of the irritant.