Vomiting, or the ejection of the stomach's contents by the nostrils and mouth, is comparatively rare in the horse; so much so that many persons believe that it never occurs. In the chapter on choking we have already spoken of apparent vomiting, which was in reality only the ejection of the contents of the tube leading to the stomach. Two reasons are commonly advanced for the rarity of vomition in the horse: first, an insusceptibility to nausea; and second, a peculiar valve-like arrangement of the parts at the entrance to the stomach. The first reason is not an entirely satisfactory one, as nausea is often exhibited by horses; and the second needs more proof. Without arguing the case we will pass on to the known causes and treatment where treatment is possible.

1. Engorgement of the stomach with indigestible food, followed by fermentation, elimination of gases, and consequent distention. This has been known in several instances to be followed by repeated vomition, leaving the patient in a prostrate condition, but not always resulting in death.

2. When the oesophagus at its terminal portion has been permanently dilated - an argument in favour of the valve theory.

3. In cases of rupture of the walls of the stomach or of the diaphragm. The late Professor Robertson entertained a fourth reason, of which, however, we can have no proof - namely, the closure of the pyloric orifice and consequent prevention of the contents of the stomach passing out into the bowel. Post-mortem examinations have repeatedly proved that vomition may occur when any of the three first-named causes exist.