The immediate cause of diarrhœa is a congestion of the intestinal mucous membrane and a profuse secretion into the intestinal canal. When such congestion occurs in the small intestine alone, it may be counterbalanced by increased absorption in the large, so that the secretion must be excessive to produce liquid alvine discharges. When on the other hand it occurs in the large intestine or in both large and small, the product is likely to escape in the liquid form. In its turn the congestion of the intestinal mucosa may result from irritants in the bowels, from the presence in the blood of irritant agents which being secreted stimulate the intestinal glands to excessive secretion, and from reflex nervous action, starting from a distant point as in chilling or irritation of the skin or other organ. Among direct irritants of the intestinal mucosa may be named a full drink of cold water especially if the horse is trotted or galloped for twenty minutes immediately after;—soft, juicy, rapidly grown green food, to which the animal is unaccustomed, as the first grasses of spring;—cooked or pulped food or ensilage in hard worked animals;—many irritant and acid plants;—accumulations of hard feculent masses in the intestines;—irritation caused by intestinal worms especially the blood-suckers;—the presence in the intestines of undigested matters, and resulting fermentations, the result of diseased teeth and jaws and imperfect mastication, of disease of the salivary glands or ducts and imperfect insalivation, of a drink of water after a grain feed, washing a part out of the stomach in an undigested condition, of disease of the stomach, liver or pancreas interfering with their proper functions; unwholesome and fermenting food like spoiled grain, or fodder, or decomposed potatoes, apples, turnips, pumpkins, carrots, cabbages, etc.;—stagnant and putrid water;—tumors, ulcers, volvulus, invaginations, adhesions and other serious lesions of the bowels may act in the same way.