Discover which animals can't vomit.
Rats and horses can't vomit.
For most of us, vomiting is an unpleasant and necessary response to various stimuli (eating or drinking too much, ingesting poison, listening to Rush Limbaugh). Vomiting is not much fun, but at least most people usually feel better afterward. Some animals, such as horses and rats, are physically unable to vomit.
In order to understand why horses and rats can't vomit, it's important to understand how vomiting -- or emesis, for the technically minded readers -- works. When the body registers something in the stomach that shouldn't be there, the brain sends signals to the muscles surrounding the stomach telling them to contract, forcing its contents upward and outward. At the same time, the esophagus relaxes, allowing that heaving food to be ejected from the body.
Rats, though, don't have the same brains as humans (thankfully), and are lacking the part of the brain that orders the stomach to vomit. But even if they did, it would do little good: Rats have a powerful seal separating the esophagus from the stomach that they are unable to open. In the same way, horses' digestive systems are a one-way street, which means that when they overeat they simply have to feel miserable -- or, in extreme cases, die.
Horses and rats aren't the only animals that are considered nonemetic. Rabbits, guinea pigs and the Japanese quail also share that honor.