Out of all the domestic species of animals, horses have the lowest concentration of nerve cells which relay information from within a centrally located field. In most other animals, the optical nerve cells are concentrated within the visual streak. Horses' optical nerves extend downward towards the nose and temporally to the sides. Within the temporal arm lies a field of optic nerve cells which is denser and provides greater acuity. This offers the horse the abilities of "binocular" vision ... that of having the ability to focus on one thing with both eyes at the same time just as human eyes are capable.
Each eye has a full optical vision of about 215 degrees. The panoramic view allows the horse to see just about everything around him with just slight movement of the head. The "blind spots" of the horse are directly in front of his nose and directly in back of his body unless he turns his head and neck. The horse's "monocular" vision allows the horse to view two seperate views at the same time; each view with each seperate eye. Horses' eyes do not adjust readily to light changes.
The pupil in the eye is rectangular shaped in a horizontal position. This pupil cannot constrict well, even when a light is shone directly into the eye. The eyeball, itself, contains black orbs on iris called Corpora nigra or granula iridica. It is believed that these black nodules help to decrease the amount of light which enters the eye. It is not believed that horse can distinguish colors but has yet to be founded and proven. Located in the inner corner of each eye is a "third eyelid" called the "nictitating membrane". This third eyelid is what moves across the eye, cleaning
