Equine vision
The equine eye is the largest of any land mammal. Its visual abilities are directly related to the animal's behavior and the fact that the horse is a flight animal. 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.
The anatomy of the equine eye
The equine eye includes the eyeball and the surrounding muscles and structures around the eyeball, termed the adnexa.
The eyeball
The eyeball of the horse is not perfectly spherical, but rather is flattened anterior to posterior. However, research has found the horse does not have a ramped retina, as was once thought.
The wall of the eye is made up of three layers: the internal or nervous tunic, the vascular tunic, and the fibrous tunic.
The nervous tunic (or retina) is made up of cells which are extensions of the brain, coming off the optic nerve. 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. 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. 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. 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.
