The serous cavities contain very little fluid normally, but considerable quantities are frequently present as a result of pathologic conditions. The pathologic fluids are classed as transudates and exudates. _Transudates_ are non-inflammatory in origin. They contain only a few cells, and less than 2.5 per cent. of albumin, and do not coagulate spontaneously. The specific gravity is below 1018. Micro-organisms are seldom present. _Exudates_ are of inflammatory origin. They are richer in cells and albumin, and tend to coagulate upon standing. The specific gravity is above 1018. Bacteria are generally present, and often numerous. The amount of albumin is estimated by Esbach's method, after diluting the fluid. Bacteria are recognized by cultures, animal inoculation, or stained smears. Exudates are usually classed as serous, serofibrinous, seropurulent, purulent, putrid, and hemorrhagic, which {266} terms require no explanation. In addition, chylous and chyloid exudates are occasionally met, particularly in the peritoneal cavity. In the chylous form the milkiness is due mainly to the presence of minute fat-droplets, and is the result of rupture of a lymph-vessel. Chyloid exudates are milky chiefly from proteids in suspension, or fine débris from broken-down cells. These exudates are most frequently seen in carcinoma and tuberculosis of the peritoneum. Cytodiagnosis.--This consists in a differential count of the cells in a transudate or exudate, particularly one of pleural or peritoneal origin. [Illustration: FIG. 117.--Cytodiagnosis. Polymorphonuclear leukocytes and swollen endothelial cells from acute infectious non-tuberculous pleuritis (Percy Musgrave; photo by L. S. Brown).] The fresh fluid, obtained by aspiration, is centrifugalized for at least five minutes; the supernatant liquid is poured off; and cover-glass smears are made and dried in the air. The smears are then stained with Wright's blood-stain, to which one-third its volume of pure methyl-alcohol has {267} been added. Cover the smear with this fluid for one-half minute, then dilute with eight or ten drops of water, and let stand about two minutes. Wash gently in water, and {268} dry by holding the cover-glass between the fingers over a flame. Mount in balsam and examine with a one-twelfth objective. [Illustration: FIG. 118.--Cytodiagnosis. Lymphoid cells from pleural fluid; case of tuberculous pleuritis (Percy Musgrave; photo by L. S. Brown).] [Illustration: FIG. 119.--Cytodiagnosis. Endothelial cells from transudate or mechanical effusion (Percy Musgrave; photo by L. S. Brown).] Predominance of polymorphonuclear leukocytes (pus-corpuscles) points to an acute infectious process (Fig. 117). Predominance of lymphocytes (Fig. 118) generally signifies tuberculosis. Tuberculous pleurisy due to direct extension from the lung may give excess of polymorphonuclears owing to mixed infection.