Horses have fairly good eyesight, thanks to their very peculiar eyes. At a diameter of roughly 5 centimeters (2 in), they are the largest of any land mammal. When measured in volume, a horse’s eye is up to nine times largerthan that of a human. Legend has it that this means horses see things as bigger than humans, which is why they sometimes startle easily, but this is not true.
The horse eye has three eyelids—two ordinary ones and a third called the nictitating membrane—which is located in the inner corner of the eye and occasionally sweeps the eye, lubricating and cleaning it if need arises. Horses can’t properly focus their eyes like humans do. Instead, the lower parts of their retina see objects at a distance, and the upper ones are for closer viewing. This means that if you want to know where a horse is looking, you should pay attention to how it’s holding its head. If you see a horse standing on a field and it’s doing that thing where it’s standing with its head held up high and ears pricked forward, it’s not just showboating—the animal is probably just looking at something interesting in the distance.
