The Equine Eye
from issue: 30-4
The Equine Eye
by Heather Smith Thomas of Salmon, ID
Horses don’t see like we do. Equine vision is better than ours in that the horse has a much broader field of vision, and better night vision. But in other ways the horse’s vision is poorer; he can’t focus as easily and clearly on close objects.
The horse’s head is large, with eyes set wide apart at the sides of his head; he seldom sees an object with both eyes at the same time and generally sees a different picture with each eye. In the wild, this double vision was a big advantage, making it difficult for a predator to sneak up on him. He can see movement on both sides and to the rear, and be prepared to flee from danger. He can focus both eyes to the front to watch something, but it takes more effort. Only when making a concentrated effort to look straight ahead does the horse have depth perception as we know it.
Wide angle vision is easy with the horse’s large and prominent eye; when grazing with his head down he can see to the front, both sides, and behind (except for the small areas blocked from view by his legs). Using both eyes, he has a visual field of nearly 340 degrees, almost a complete circle. The average horse sees a 215 degree range with each eye. Humans, by contrast, have a field of vision encompassing only 180 degrees (half circle) using both eyes.
The horse sees a fairly wide view with his head lifted, but there is a blank spot behind his body, and the lower part of his visual field is a blind zone. He can’t see anything closer than about 3 feet in front of his forefeet. With his head up, he can’t see where his feet are landing, but can see the ground where they are about to land.
Because the eyes are set at the sides of his head, they would seem likely to suffer injury from bumping. Yet the horse’s eye is seldom injured, partly because he has a long flexible neck and is very conscious of his eyes, swinging his head out of the way of most obstacles and dangers. A cow, by contrast, with her thick, tough eyelids, automatically closes her eye to protect it from bumping. The horse prefers to keep his eyes open, so he slings his head up or to the side to avoid a bump.
Also the eye is encircled by bone which forms a strong, rigid arch (the supraorbital process) above and in front of the eyeball, to protect it. Behind and above the eyeball is a fat-filled cavity which acts as a cushion. This is the supraorbital fossa, the dent between the eye and ear, which moves in and out as he chews. This dent contains a pad of fat continuous with the fat behind the eyeball. If the eye is bumped, the pressure on the eyeball pushes it back into this cavity, sheltering the eye in the bony orbit. A hard blow will cause the eye to retreat backward instead of bursting, and the fat bulges out into the supraorbital fossa temporarily as it is displaced by the retreating eyeball.
This cavity also serves another useful purpose. If a fly or piece of dirt touches the eye surface, the eye can draw back a little into the fatty cavity and a third eyelid membrane quickly wipes over the front of the eyeball.
This third eyelid, the membrane nictans, is a firm sheet of elastic cartilage which can move easily over the round eyeball. It also keeps the eye moist by spreading tears over its surface. The third eyelid is attached to the pad of fat behind the eyeball. Any action which causes the muscles of the eye to pull the eye back into this fatty cavity also causes the third eyelid to move upward and flick across the eye. This happens when the horse feels an insect or foreign object touch his eye, or when he yawns. It also happens if he has tetanus, because this disease causes the retractor muscle at the back of the eyeball to contract in spasms, shooting the third eyelid across the eye.
The horse’s eye is an irregular ball, covered at the front by a thin mucous membrane called the conjunctiva. This pink membrane forms a continuous covering over the inside surface of the eyelids, the third eyelid and the cornea’s edges. It covers the cornea as a single layer of transparent cells. Light enters at the cornea, which is a thick, tough, clear tissue over the front of the eye. Surrounding the cornea is the sclera, or white of the eye. Not much of this is visible, except when the horse rolls his eyes. The sclera is where ocular muscles that move the eyeball are attached.
A salty solution, called the aqueous humor, fills the front chamber of the eye, between the cornea and the lens behind it. This salty fluid flows freely through the pupil — the hole in the muscular diaphragm or iris (colored part). The iris surrounds the pupil and limits the amount of light that reaches the retina at the back. If the light is too bright, the iris constricts and makes the pupil smaller. The foal or young horse has a round pupil, but it becomes more elongated horizontally as he grows older. By the time the horse is 5 or 6 the pupil is elliptical.
The lens directs light rays onto the retina at the back of the eye. The retina is the back lining, containing the light-receiving cells. The image is transmitted to the brain by the optic nerve; when the brain receives the nerve impulses generated by the light receiving cells, it translates them into visual images.
The horse can see quite well in the dark because of a special triangular shaped layer (tapetum lucidum) which lies behind the retina at the upper part of the back of the eye. This mirror-like structure is an iridescent pigment layer which reflects light back onto the retina. It concentrates the light and intensifies the image, giving the horse better vision when light is dim. The glistening tapetum lucidum is what makes an animal’s eyes shine in the dark when a light is directed at them. Light beamed at a horse’s eye in the dark will give back a green reflection.
Because his eyes are so sensitive to light, the horse must keep out extra glare during bright sunlight. The horizontally shaped pupil keeps out more daytime glare than the round pupil of a human eye, but is not as effective as the vertical slit in a cat’s eye. But the horse compensates by having built-in sunglasses. These consist of several tiny, round appendages hanging from the upper edge of the pupil, like an awning or sunshade. These black or chocolate colored pea-like appendages are the corpora nigra and appear on the edge of the iris when the horse is 5 or 6 years old, increasing in size for many years and shading the eye. Sometimes a few smaller ones project upward from the lower edge of the pupil, but most hang from the top. They partially block the pupil opening when it is constricted, absorbing excessive light.
Most animals and humans focus their eyes by changing the shape of the lens, using the muscles that surround it. These muscles can make the lens rounder for close-up vision, or flatter for looking far away, to properly bend the light rays to give the clearest image. By contrast the horse doesn’t have highly developed lens muscles and generally changes the position of his eye by raising or lowering his head to get the best focus.
Often the flighty reactions of a horse to harmless objects is due to the way he sees things. He may react strongly to any movement at the outer edges of his vision because his instinct tells him it may be a predator. And because his eyes may not focus rapidly when shifting his gaze to different distances, he may be easily startled by something that suddenly looms up in front of him or comes up behind him. Many horses spook at something unusual along the path until they have a chance to examine it more closely and satisfy themselves it is harmless.