=Purulent Fluid=, as found in the cavity of the chest, consists of a white or cream-colored, opaque, and homogeneous fluid, combined with more or less albuminous matter, in the form of shreds and flakes, yet destitute of the granular matter of the sero-purulent fluids, and not separating into a fluid and solid portion when at rest, as is the case with the latter. It is a fact well established, that genuine purulent matter may be formed in the pleural cavity, as well as in other serous cavities, without ulceration of any portion of the surface, or discharge of an abscess into the same, it being the result of a more advanced stage of the process which gives rise to the serous or plastic effusions. It may be secreted directly from the capillaries of the inflamed surface, or, in some instances, it would appear to be derived from the organized false membranes, which have taken on a suppurative action. =Pneumothorax.= Air may enter the pleural cavity, by perforation of the walls of the chest from external injury, or, as is more common, by the destruction of the pulmonary portion of the membrane, from the bursting of a distended air-cell, or from softening of tubercular deposits, or bursting of an abscess. If there are but few or no adhesions, the accumulation of air in the cavity may be accompanied by a more or less complete collapse of the lung, as in hydrothorax. This condition, during life, is not readily distinguished from emphysema, both being accompanied with similar oppression in breathing, distension of the chest, and displacements of the heart, and with increased clearness on percussion. Serous or sero-purulent effusions will frequently accompany the presence of air in the cavity, and thus give rise to many of the peculiar physical signs which may have been noticed during life, as metallic tinkling, a splashing sound on shaking the chest, etc. Section VIII. THE LUNGS AND BRONCHIAL TUBES.