Basic Equine Eye Anatomy

If you know how the equine eye works, you’ll understand why injury affecting any part of its structure can create significant pain and severely affect vision.
Share
Favorite
Close

No account yet? Register

ADVERTISEMENT

Basic Equine Eye Anatomy
Dr. Robin Peterson Illustration

The equine eye is among the largest eyes of all land-based mammals, says Ann Dwyer, DVM, an equine practitioner at Genesee Valley Equine Clinic, in Scottsville, New York, who has a special interest in ophthalmology. It’s about 4 cm (1.5 inches) deep—essentially the width of a playing card. The back of the eye is actually a part of the brain, with its optic nerve (which sends visual information to the processing parts of the brain) looking kind of like a full moon and the retina capturing the images horses see. The nerve is positioned below a triangular reflective layer called the tapetum. The tapetum enhances night vision and is responsible for the “eyeshine” seen when you use a bright light to inspect the globes (eyeballs) at night.

The globe also contains a clear gel called the vitreous and a clear liquid called the aqueous humor. The equine eye holds about 26 cc of vitreous and 3 cc of aqueous, says Dwyer. Near the center of the eye is the clear lens, which sits just behind the iris—the colored part that surrounds the pupil, which is the opening that admits light toward the retina. In front of that is the cornea.

The cornea is a critical part of the eye, separating the inner workings of this organ from the outside world. At only 1 mm thick (less than the thickness of a playing card), the cornea has a distinctive anatomy made up of several unique layers, Dwyer says. The outer part of the cornea is made of a mosaic of flat epithelial cells that “fit together like a puzzle,” she says. Moving toward the inside is a layer called the stroma, made of connective tissue that’s layered like glass square tiles, alternately positioned at different 90-degree angles.

“This is a really complex structure that has to have a perfect geometrical form in order for it to be transparent,” says Dwyer.

All of this sits on a lower layer of the cornea, the endothelium, which is made of a single layer of cells that act like the eye’s defroster, keeping the outer layers relatively dehydrated. “If the cornea becomes cloudy because the endothelium is not functioning properly, it’s sort of like someone smeared butter across your glasses,” Dwyer says.

When injury affects any part of this delicate anatomy, it can create significant pain and severely affect vision, she says.

Share

Written by:

Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

Related Articles

Stay on top of the most recent Horse Health news with

FREE weekly newsletters from TheHorse.com

Sponsored Content

Weekly Poll

sponsored by:

Do you use slow feeders or slow feed haynets for your horse? Tell us why or why not.
295 votes · 295 answers

Readers’ Most Popular

Sign In

Don’t have an account? Register for a FREE account here.

Need to update your account?

You need to be logged in to fill out this form

Create a free account with TheHorse.com!