St. Cyr in a series of 43 experimental and casual pleurisies in horses, found that in a very few hours there was marked local congestion and swelling of the pleura speedily followed by the formation of soft, pulpy, friable false membranes, largely amorphous and granular but impregnated with many cells and nuclei. These adhere feebly to the pleura but may accumulate with prodigious rapidity so as to cover in three or four days the whole pleural surface on one or both sides. The attendant serous effusion was bloody, turbid, or lactescent. The pleural surface under the false membrane was highly vascular and studded with fragile, red conical elevations projecting into the membrane. Exceptionally the sub-serous connective tissue became the seat of exudation as well. From the sixth to the ninth day the false membranes began to become vascular and from the tenth to the fourteenth day commenced to organize into the connective tissue. With the advent of this stage, the inflammatory action tended to subside, and the reabsorption and repair to ensue. _Pleuritic effusion._ This varies greatly at the different stages of the disease. As effused it has a composition resembling that of the blood:— Water, 911 to 924 Albumen, 63.33 to 82.50 Fibrine formers, 2.16 to 12.50 Extractive matter. Salts. The progressive changes from the hæmorrhagic effusion to the limpid hydrothorax and their relation to the different stages of the disease and the subsidence of the inflammation are of the greatest importance in deciding questions of responsibility, when the animal has recently changed hands. St. Cyr has classified his cases in the following instructive table: ─────────────────────┬─────────────────────────────────────────┬─────── Duration of the │ │ Disease. │ Effusions. │Total. ─────────────────────┼──────┬─────────────────┬────────┬───────┼─────── │ Port │ │Muddy or│ │ │Wine. │Sero-sanguineous.│Grayish.│Limpid.│ ─────────────────────┼──────┼─────────────────┼────────┼───────┼─────── From 1st to 7th day.│ 9│ 6│ 3│ │ 18 „ 8th to 15th day. │ 2│ 3│ 4│ 6│ 15 „ 16th to 30th day. │ │ 1│ 1│ 5│ 7 After 30th day. │ │ │ │ 3│ 3 ─────────────────────┼──────┼─────────────────┼────────┼───────┼─────── │ 11│ 10│ 8│ 14│ 43 ─────────────────────┴──────┴─────────────────┴────────┴───────┴───────