The collection of air or gas in the cavity of the pleura has already been noticed as coexisting with liquid effusion in some cases of advanced pleurisy. It may arise from other causes, among which may be noted: 1. When a mass of pulmonary tubercle connected with a bronchial tube has opened into the pleural sac. 2. When a communication has been established between the pleural cavity and the alimentary canal, as in combined rupture of the stomach and diaphragm, or of the double colon and diaphragm. 3. When a rib is fractured and the broken end penetrates the lung tissue and opens into one or more small bronchial tubes. 4. When a wound has been inflicted penetrating the walls of the chest and forming a valvular orifice through which air is drawn inward during each inspiratory act, but out of which it cannot pass when the thorax collapses. The amount of gas present may be extremely slight, or in a case such as that from a valvular wound it may cause complete collapse of the lung, filling up the entire half of the thorax and bulging into the opposite half. The _symptoms_ are often very obscure. If with liquid the metallic tinkling after rising, in small animals the splashing when shaken and the other sounds of auscultation and percussion will point it out as described under _pleurisy_. In the case of a broken rib the distortion, swelling and tenderness, will lead to suspicion. A penetrating sound will be sufficiently evident, and in the case of tubercle previous cough and ill-health will have been manifest. The specific signs of uncomplicated pneumothorax are: 1st, A drum-like resonance on percussion over the seat of the gas, usually at the upper part of the chest; 2d, A partially suppressed or distant respiratory murmur over the same area; 3d, A muffled or suppressed sound of the cough; 4th, Sometimes, especially if the gas is abundant, prominence of the chest on that side; 5th, There are also more or less distress and anxiety, difficult breathing, quick, weak, rapid pulse, and other signs of illness. Some cases of this kind recover spontaneously or with the liquid effusion with which they are associated; in traumatic cases the wound is sometimes sealed up by a pleuritic exudation which here becomes a curative process; while in some examples of valvular wound of the lung or walls of the chest death may ensue in a period varying from a few minutes and upwards to weeks. _Treatment_ is limited to the prevention of the ingress of air through an external wound where that exists; the employment of opiates and other agents to moderate attendant suffering; to measures calculated to moderate the intensity of resulting pleurisy, and, in cases where there is imminent danger from accumulation of gas, to the puncture of the chest and the careful withdrawal of the gas by aspiration. If necessary sterilized air may be made to replace the aspirated gas. PYO-PNEUMOTHORAX, EMPYEMA.