Another mode in which violent effort injures the lungs is by the rapid and continued inhalation of great quantities of air during rapid breathing, so as to dilate the lungs suddenly to their fullest extent. Sometimes from irregular distribution of the ærial current or from the want of tone in a particular part of the lung that gives way under the pressure and the air cells become overdistended or ruptured. This condition is especially met in the more rapid paces. It is well exemplified in the results of the deep breathing after cutting the vagi nerves. In either case the result will be more certain if the effort is made upon a full stomach or with the functions of the vagus nerve impaired by a previous faulty diet. That broken wind is a frequent concomitant or sequel of _chronic bronchitis_ is undeniable, and theoretically nothing is more likely to cause dilatation and rupture of the air cells and consequent impairment of the innervation and contractility of the lung than violent fits of coughing, while the bronchial tubes have thickened and friable walls, or are partially plugged by tenacious mucus. Broken wind is mainly a disease of old horses, though I have seen several cases in five-year-old animals, and Bouley records a case in a two-year-old colt out of a badly broken winded mare. This would seem to indicate an hereditary proclivity, and there is no doubt that the shallow, narrow, weak chest predisposing to this as to many other pulmonary complaints is transmitted from parent to offspring. _Nature of the Disease._ Emphysema of the lungs is the most constant structural change met with in the bodies of animals which have suffered from broken wind. This condition of the horse’s lung appears to have been noticed by the early Greek writers. It was advanced as the cause of broken wind by Riding in 1704 (Pathologie Veterinaire), by Floyer in England in 1761 (Treatise on Asthma in Man), by Vitet in France in 1783 (Medicine Veterinaire, Lyon), by Freutzel in Germany, and Bracy Clark in England in 1795. It was only, however, after the admirable discoveries of Lænnec that the question was systematically investigated by Delafond, who has furnished the most comprehensive data on the subject. Out of fifty-four broken winded horses dissected by him no less than forty-five had the lungs extensively emphysematous. This emphysematous lung differs according to whether the emphysema is _vesicular_ or _interlobular_.