The horse's eye consists of several parts which are all adapted to meet the horse's special needs.

The large, clear area covering the eye is called the cornea and is surrounded by the sclera, a narrow white rim between the eyelids, surrounding and supporting the entire eyeball (also known as the white of the eye).

The upper and lower eyelid are made up of cartilage covered by skin, which do not contact the eye because of a thin layer tears. The third eyelid, also known as a 'nictitating membrane' is not normally seen as it cleanses and lubricates the eye.

Structure of the Horse's Eye

Tears are secreted by the lacrimal gland, which measures two to three inches in diameter, via small ducts which open at the back of the upper eyelid. The lacrimal glands are located in the depression above the horse's eyes. Tears not only cleanse and lubricate the eye, but they also contain immune substances which help prevent eye infections.

The normal accumulation of tears is removed by evaporation, while excess tears are pooled in the inner corner of the eye and carried away by the nasolacrimal ducts down to the nose. The openings of these ducts may be seen by holding open the horses nostrils.

The stiff eyelashes on the upper eyelid comprise of four rows which criss-cross each other. The lower eyelid has a few straggly hairs, although some horses may have stiff lashes, which could scratch the surface of the eye.

The pupil is the opening in the center of the eye and is surrounded by the iris, a pigmented muscle. The pupil is round in young horses but after the age of five, the pupil assumes an elliptical, horizontal shape. The iris is usually brown or deep hazel with no white around the edges, except with Appaloosa's. Some horses have blue eyes, usually associated with horses with large white areas, such as Paints or Pintos.

Three chambers make up the inner eye. The anterior chamber, which is filled with a clear fluid, is located between the cornea and the iris. The posterior chamber sits between the iris and the lens and the vitreaous chamber is found behind the lens. This chamber supports the lens from behind as well as holding the retina in place at the back of the eyeball.

The spherical, large lens is held in place by suspensory ligaments, which are attached to the ciliary body. The ciliary body consists of muscle, blood vessels and connective tissue and produces a clear fluid in the front part of the eye.

The light enters the eye through the cornea and anterior chamber, into the pupil and lens. The iris expands and contracts, depending on the brightness of light. The light travels back to the retina, through the vitreous chamber. The retina is comprised of a layer of photoreceptor cells which converts light into nerve impulses the sends the messages to the brain, via the optic nerve.

The chornoid lies immediately behind the retina and consists of a layer of blood vessels which nourish the retina. Within this layer is special layer of cells called the tapetum lucidum, which acts like a mirror, reflecting the light back onto the retina, producing a double exposure of the photoreceptor cells. This is the layer which seems to make the horse's eyes reflect or shine in the dark.

Field of Vision

The horse's Field of Vision

A horse's eyeball is flattened from the front to back resulting in the distance from the cornea to the retina being different in all parts of the eye. The focal length is a few millimeters longer in the top part of the eye than it is in the bottom. This condition is called 'ramped retina'.

Unlike the human eye, the ciliary muscles are poorly developed in the horse's eye it cannot change the shape of the lens to bring objects into focus. The horse compensates by using the upper part of its retina, which has a longer focal length, to see objects nearby, while it uses the lower part of its retina to see objects in the distance.

Horse's can see two images, one out of each eye

As the horse brings its head to view objects in the distance he will also prick his ears forward. The muscles which cause the horse's ears to prick forward are the same ones which bring the horse's eyes together. The opposite occurs with objects close at hand.

The horse's field of vision is approximately 360°, although its binocular field of vision is only about 65°. When a horse focuses both his eyes on an object, this is called 'binocular vision'. 'Monocular vision' is when a horse sees a different image with each eye.

A horse has two blind spots, one in front which extends about four to five feet, and the other is directly behind the horse. As horse's approach obstacles, they may need lower their head in order to focus on the obstacle.

Domestic horse's seem to be near-sighted, while wild the horse seem to far-sighted. The horse's night vision is also far superior to the human's, allowing them to escape their predators.

What is Uveitis

Equine recurrent uveitis (ERU) is one of the most common eye disorders among horses, and the leading cause of blindness. Approximately 10% of the horse population suffer from this condition.

Uveitis is also known as "Moon Blindness", "Recurrent Uveitis", and "Periodic Opthalmia". Many people believe that the regularly occurring acute episodes were connected with the cycle of the moon. The cause of Uveitis is still raising many questions although many believe it is caused by direct trauma, bacteria, viruses, and parasites or possibly even allergies. Often a horse with Uveitis will test positive for leptospirosis, a type of bacteria which may be linked to Uveitis.

Uveitis may be confused with a corneal injury, so an immediate veterinary examination is crucial, especially in order to prolong the horse's vision.

Signs and Symptoms

Buffy's eye during an uveitis flare-up Notice the white, cloudy cornea

The iris, ciliary body and choroid make up the uveal tract in a horse's eye. Inflammation of one or all of these structures is called Uveitis. Signs of accute uveitis are a red, painful eye with squinting and possibly tearing. The eye will often appear cloudy. The pupil will be small and contracted and the eye will have a mushy feeling when you gently press it. Signs of chronic uveitis are less pronounced.

During a cycle of Uveitis, the horse is in acute pain. Tears streak his face while he tries to keep eye tightly shut. A horse which experiences ERU will eventually go blind.

Treatment

An immediate veterinary consultation is a must. Uveitis must be treated aggressively in order to preserve the horse's vision for as long as possible.

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