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.