The various alterations in the structure of the heart, arising out of acute myocarditis and the different kinds of degeneration, naturally lead to a weakness and a diminution in the resisting power of the muscular walls, which favours the occurrence of rupture. The determining causes are: violent exertion, falls, excessive excitement, tympanitis, an overloaded condition of the stomach or intestines, etc. Any one of these causes, by obstructing the passage of blood in the larger vessels, increases the pressure on the walls of the heart beyond its power of resistance. It is stated by Zuill that the tear is usually located in the walls of one of the auricles. In our experience it has most frequently occurred in the left ventricle. The occurrence is usually almost immediately fatal; it is said that in falling at the moment of the rupture the horse sometimes utters a piercing cry. When the rupture is slight the ordinary symptoms of internal hemorrhage are exhibited. The animal staggers, and, if not supported, falls, the visible mucous membrane of the nostril, mouth, and eye become white and bloodless; there is difficulty of breathing, loss of consciousness, and convulsions, and death occurs at varying periods, from a few minutes to several hours. Obviously treatment even in prolonged cases is not likely to be of any use, although it is sometimes effectual in cases of internal hemorrhage from rupture of the vessels of the liver. Here large doses of opium, with gallic or tannic acid, has sometimes arrested the flow of blood, and the animal has partially recovered, but a rent in the walls of the heart is necessarily irremediable.