_Hydrothorax_ or _water in the chest_ is common to all domestic animals. It is as we have seen one of the most ordinary results of _pleurisy_, and may persist long after that disease has disappeared. It likewise occurs independently of inflammation as a dropsical effusion. Thus when the return of blood by the bronchial, pulmonary or intercostal veins, is hindered by any cause such as tumors in the bronchial glands or subvertebral region a passive effusion may take place through the coats of the vessels. In imperfection of the mitral valves the regurgitation of blood in the pulmonary veins during each cardiac systole equally causes such transudation. Chronic disease of the kidneys (Bright’s disease) with the retention of effete and injurious materials in the blood leads to dropsy of the chest as in other parts of the body. Again in many debilitated conditions parasitic and otherwise, with a tendency to general dropsy the chest participates and a collection of fluid takes place in the pleuræ. The nature of the contained fluid will vary according to the conditions in which it has been effused. If the result of inflammation there are the different stages already indicated: _first_, of a yellow citrine color or red from contained blood; _second_, grayish and muddy either from contained pus or other changes taking place in the fluid; and _third_, clear limpid and translucent as seen in the later stages. If merely a dropsical effusion the fluid is watery clear and translucent or with a slight straw color. The inflammatory effusion contains fibrine or fibrinogenous elements, is associated with the formation of false membranes, and though it may remain fluid so long as it is retained in the chest, it coagulates rapidly when withdrawn. The dropsical effusion rarely contains fibrine, and then only in very small amount, and it does not coagulate when drawn off from the chest. The inflammatory effusion usually contains a greater proportion of common salt, phosphates or albumen than exist in the blood, and floating granules, particles and cell forms, none of which conditions characterize the dropsical effusions. The most prominent feature of the inflammatory effusions is thus seen to be their power of coagulation, by virtue of the contained fibrine, when exposed to the air. _Symptoms._ When a sequel of pleurisy it is manifested by the symptoms already mentioned under that head as indicating the occurrence of effusion.