![]() The lacrimal apparatus produces tears, providing nutrition and moisture to the eye, as well as helping to remove any debris that may have entered. It lies on the inside corner of the eye, and closes diagonally over it. Unlike humans, horses also have a third eyelid ( nictitating membrane) to protect the cornea. The upper eyelid is larger and can move more than the lower lid. The opening between the two lids forms the palpebral tissue. The eyelids are made up of three layers of tissue: a thin layer of skin, which is covered in hair, a layer of muscles which allow the lid to open and close, and the palpebral conjunctiva, which lies against the eyeball. ![]() The adnexa of the eye, including the third eyelid (seen in the left corner) Horses are capable of having dichromatic (differently-colored) eyes.Īs in humans, much of the genetics and etiology behind eye color are not yet fully understood. The eyes of horses with the Champagne gene are typically greenish shades: aqua at birth, darkening to hazel with maturity. Heterozygous or single-dilute creams, such as palominos and buckskins, often have light brown eyes. Homozygous cream dilutes, sometimes called double-dilutes, always have light blue eyes to match their pale, cream-colored coats. In the case of horses with white markings, one or both eyes may be blue, or part-blue. The white spotting patterns most often linked to blue eyes are splashed white, frame overo, and sometimes sabino. Blue eyes are not uncommon and are associated with white markings or patterns. Homozygous cream dilutes ("double-dilutes") have pale blue eyes, while the blue eyes associated with white markings (bottom) are a clearer, deeper color.Īlthough usually dark brown, the iris may be a variety of colors, including blue, hazel, amber, and green. The lens is made up of onion-like layers of tissue. The lens of the eye lies posterior to the iris, and is held suspended by the ciliary suspensory ligament and the ciliary muscle, which allows for "accommodation" of the eye: it allows the lens to change shape to focus on different objects.The cornea (clear covering on the front of the eye) is made up of connective tissue and bathed in lacrimal fluid and aqueous humor, which provides it nutrition, as it does not have access to blood vessels. The sclera (white of the eye) is made up of elastin and collagen. The fibrous tunic consists of the sclera and cornea and protects the eye.The iris lies between the cornea and the lens, and not only gives the eye its color, ( see "eye color," below) but also allows varying amounts of light to pass through its center hole, the pupil. The tapetum lucidum reflects light back onto the retina, allowing for greater absorption in dark conditions. It forms the tapetum lucidum when it crosses over the fundus of the eye, causing the yellowish-green eye shine when light is directed into the animal's eyes at night. The choroid has a great deal of pigment, and is almost entirely made of blood vessels. The vascular tunic (or uvea) is made up of the choroid, the ciliary body, and the iris.The optic disk of the eye, however, does not contain any of these light-sensitive cells, as it is where the optic nerve leaves to the brain, so is a blind spot within the eye. The part of the retina covered by light-sensitive cells is therefore termed the pars-optica retinae, and the blind part of the eye is termed the pars-ceaca retinae. Since only two-thirds of the eye can receive light, the receptor cells do not need to cover the entire interior of the eye, and line only the area from pupil to the optic disk. These receptors are light-sensitive, and include cones, which are less light-sensitive, but allow the eye to see color and provide visual acuity, and rod cells, which are more light-sensitive, providing night vision, but only seeing light and dark differences. The nervous tunic (or retina) is made up of cells which are extensions of the brain, coming off the optic nerve.The wall of the eye is made up of three layers: the internal or nervous tunic, the vascular tunic, and the fibrous tunic. However, research has found the horse does not have a ramped retina, as was once thought. The eyeball of the horse is not perfectly spherical, but rather is flattened anterior to posterior. ![]() The equine eye includes the eyeball and the surrounding muscles and structures, termed the adnexa. Both the strengths and weaknesses of the horse's visual abilities should be taken into consideration when training the animal, as an understanding of the horse's eye can help to discover why the animal behaves the way it does in various situations. Its visual abilities are directly related to the animal's behavior for example, it is active during both day and night, and it is a prey animal. The equine eye is one of the largest of any land mammal.
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