Between the mouth and stomach of all mammals, there is a tube called the esophagus. When the esophagus enters the stomach, it is surrounded by a muscle which allows all mammals to keep food inside, but if necessary, it relaxes to return the food as rumination (in the case of cows) or as vomit (for example in dogs, cats, humans); but on the horse that muscle is so strong that it is impossible for food to return, so they can’t vomit, and they cannot burp either.
For horses, having that muscle so developed is a great inconvenience, since vomiting is a defense system of the body that seeks to eliminate something that is not in good condition in the stomach.
