Have you learned anything interesting today?

[VictorR]

Vomiting (emesis) is a complex physiological event that requires a closely coordinated sequence of reflexive motions. When you are going to throw up, you draw a deep breath, your vocal cords close, your larynx rises, and the soft palate shifts to close off your airways. Then your diaphragm contracts downward, which loosens pressure on the lower esophagus and the sphincter where it enters the stomach. Next, the muscles of the abdominal wall contract spasmodically, which puts sudden pressure on the stomach. With the upward “doors” open, the contents have a clear exit pathway. All of these separate actions happen involuntarily, of course, controlled by distinct “vomiting centers” in the brain.

Horses, however, have a number of key physiological differences to ensure that any food they ingest takes only a one-way trip. For example, the muscles of the equine lower esophageal sphincter are much stronger than in other animals, making it nearly impossible to open that valve under backward pressure from the stomach. Also, the equine esophagus joins the stomach at a much lower angle than in many animals, so when the stomach is distended, as with gas, it presses against the valve in such a way that holds it even more tightly closed. And, located deep within the rib cage, the equine stomach cannot be readily squeezed by the abdominal muscles. Finally, horses have a weak vomiting reflex—in other words, the neural pathways that control that activity in other animals are poorly developed in horses, if they exist at all.

Q&A: Why can’t horses vomit?