Horse Sense The ciliary muscles in a horse’s eye can make only small changes to...

Horse Sense

The ciliary muscles in a horse’s eye can make only small changes to the shape of the lens, so a horse can’t change the shape of the lens to focus on objects at different distances as humans do. Instead, a horse relies on the fact that its eyes aren’t spherical. As Figure V.1 shows, different points at the back of the eye are at somewhat different distances from the front of the eye. We say that the eye has a “ramped retina”; images that form on the top of the retina are farther from the cornea and lens than those that form at lower positions. The horse uses this ramped retina to focus on objects at different distances, tipping its head so that light from an object forms an image at a vertical location on the retina that is at the correct distance for sharp focus.

Certain medical conditions can change the shape of a horse’s eyeball; these changes can affect vision. If the lens and cornea are not changed but all of the distances in Figure V.1 are increased slightly, then the horse will be

A. Nearsighted.

B. Farsighted.

C. Unable to focus clearly at any distance.