The Equine Eye: Ocular Emergencies (Book Excerpt)

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

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.

The pink tissue surrounding the eye is called the conjunctiva. You should look at this tissue and learn what its normal color looks like (it should be pink). When this tissue is inflamed, it becomes red and swollen, which can be an early warning sign of an eye problem.

A unique structure of the equine eye is the third eyelid. The third eyelid, or nictitans, is in the inside lower corner of the eye socket and in the normal horse generally is tucked away out of sight. When the horse blinks, the third eyelid sweeps across the eye like a small windshield wiper helping the tear film keep the cornea surface clean. There is a constant quantity of tears being secreted onto the eye. The tear film serves to lubricate and moisten the corneal surface and provides a certain degree of immune protection. Immediately behind the cornea is a fluid-filled chamber called the anterior (front) chamber. The fluid in this chamber is normally crystal clear and allows a clear view of the iris. If the fluid between the cornea and iris becomes cloudy, it is a sign of inflammation within the eye. The iris is essentially everything brown surrounding the black pupil.

Horses' eyes do not have the wide variety of colors found in humans' eyes, but occasionally you will come across a horse with a bluish-white iris or other variations. If you carefully examine the eyes of a healthy horse, you will see dark brown "punching bag-shaped" structures on the upper edge of the iris. They are part of the normal anatomy, but if they enlarge to obstruct the pupil they have become abnormal.

The iris is what controls the size of the pupil in response to the dimness or brightness of the ambient light. To check it, look into the horse's eyes in dim light, then shine your flashlight close to its eyes. The iris contains many small blood vessels, so trauma to the eye might make it bleed. The presence of blood is often seen as a red haze in the anterior chamber of the eye. Technically, the iris is classified as part of the uvea (pronounced U-Vee-A). Uveitis (pronounced U-Vee-itis), which is also called periodic ophthalmia or moon blindness, is an inflammation of the uvea. The iris is part of the eye that is affected by moon blindness.

The lens is a clear structure of gelatinous consistency that sits directly behind the pupil. Its function is to focus the incoming light. The cornea and lens function together in focusing light and image on the back of the eye. This image is, for interest sake, upside down and backwards, similar to how the lens of a camera obscura works. It is the brain that flips the image around so that it makes sense. The lens should be clear, so if there is a white opacity behind the pupil it is likely to be a cataract. An opacity of the lens is called a cataract.

Behind the lens is a clear, jelly-like substance called the vitreous. Finally on the very back surface of the eye is the retina. It is the retina that collects the focused image and transmits it to the brain via the optic nerve. Most of the image perceived by the right eye is processed by the left side of the brain and vice versa; the optic nerves cross over and go to opposite sides of the brain.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

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