The eye is one of the most fascinating and intricate parts of equine anatomy. As a lover of ophthalmology, I may be a bit biased, but I think most will agree that the eye is one of the first things that catches our attention when we approach a horse. We place so much emphasis on the appearance of the eye that phrases like “He has a kind eye” or “He has a worried eye” are commonplace in barns, at shows, and even at veterinary clinics.
The eye is an extremely complex structure with intricate anatomy. My goal is to help simplify a few of the basic structures of the eye so that you can better understand that there is more than just magic in how the eye works. First, a few fun facts!
- The equine eye is the largest eye of any land mammal—yes, including giraffes!
- Horses can see a total of 350 degrees around them
- Horses have “Dichromatic” color vision meaning they can see two colors—shades ofblue and gold.
Cornea—The cornea is the outermost surface of the front of the eye and should be completely clear! It is comprised of very few cells and no blood vessels. It receives its nutrition from the tear film. The entire cornea is only 1 – 1.5 mm thick at the thickest central area! The outermost layer, the epithelium, creates a watertight barrier to keep dirt, debris, and bacteria out of the eye. When this barrier is compromised, the underlying layers (the stroma and endothelium) are exposed to microorganisms like bacteria and fungi and infection can quickly take hold. The outermost layers of the cornea have more concentrated nerve endings than any other part of the body—this is why horse’s may show significant pain even with the smallest of corneal scratches. Interestingly, there are fewer nerves deeper in the cornea so a horse with a deep corneal lesion or rupture may appear more comfortable than one with a very small superficial scratch!
Iris– The iris is a part of the uveal tract. The uveal tract of the eye is made up of the structures that contain a blood supply and one of the most common equine disease “Equine Recurrent Uveitis” or “moon blindness” affects these structures. This disease is the leading cause of blindness in horses. The uveal tract includes the ciliary body, the iris, and the choroid. The ciliary body and choroid are structures that are not readily seen by the naked eye, but the iris is the colored portion of the eye that surrounds the pupil. The iris in the horse comes in two colors—brown (many shades) and blue. Its function is to control the size of the pupil in response to light to control the amount of light that reaches the back of the eye.
Lens– the lens is a crystalline structure whose role is to take the light entering the eye and focus it on the back of the eye. The normal lens is transparent and has no nerve or blood supply. The lens has complex metabolic and enzymatic functions which maintain its transparency—if any of these processes go awry, cataract formation may occur. Cataracts can be congenital (present at birth) or can result from inflammation secondary to trauma or to uveitis. They can range from small opacities in the lens that do not affect overall vision to a complete mature cataract that results in blindness.
“Fundus”- the fundus is a fancy term which simply means “the back of the eye” and is absolutely beautiful! (see below) The fundus includes the retina, optic disc (arrow in picture) which is the visible area of the large optic nerve that transmits images from the eye to the brain to be interpreted. The retina is the most metabolically active tissue within the body! The function of the retina is to convert the energy from light entering the eye to an electrical signal that will be sent to the brain. The upper part of the fundus, the fluorescent yellow/green area in the picture is called the tapetum. This enhances light sensitivity to allow horses to see better at night. When you shine a light at night and see the bright reflection in their eyes, it is actually their tapetum that you are seeing. Very cool!