Editor’s Note: This is an excerpt from Understanding Equine First Aid by Michael A. Ball, DVM.

The eye of the horse is simple in its structure, but it handles complex functions. What something looks like to a horse is unknown. Can the horse see color? Does it see an image in the same way humans do? Despite all of our scientific experiments, hypothesis, speculation, and interest, what exactly a horse sees and how it perceives things will, most likely, always remain a mystery.

The equine eye functions to collect and “focus” incoming light. The reflected light carries the image and transmits that focused image to the brain. The parts of the eye include the cornea, iris, aqueous chamber, pupil, lens, vitreous chamber, retina, and optic nerve. Well before an ocular emergency occurs, you should familiarize yourself with the easily visible outer structures of a horse’s eye. Observe the eye with the aid of a penlight or other light source. Evidence of pain is a clear symptom of most ocular diseases. However, sometimes it is not the first sign, so paying some attention to the eye and becoming familiar with what looks normal might help you head off some problems. For example, when the eye has a bluish-white, filmy appearance, it’s a sign of trouble!

The cornea is composed of several layers, with the transparent outer layer acting as a protective barrier and the inner cell layer functioning to move water out of the cornea. The outer surface or epithelium only allows limited penetration of water. If the outer protective epithelium is damaged (scratched, torn, etc.) or the inner layer becomes diseased or damaged, the in-between layer will take on water and develop a bluish-white patch of edema