The equine eye is made up of a number of distinct parts (see illustration below).
The front surface, visible from the outside, is the cornea which is a clear window which allows light into the interior of the eye.
Behind the cornea is the iris which dilates and contracts according to the lighting conditions. The equine iris has a modification at the upper edge -- corpora negra (dark pigmentation) acts as a visor and filters light.
Between the cornea and the iris is a space filled with fluid called the anterior chamber.
The space between the iris and the lens is called the posterior chamber
The posterior chamber is filled with the same type of fluid as the anterior chamber, and in fact the pupil, which is the central opening in the iris, allows the fluid in the two areas to mingle.
Although it can be hard to see from outside, if you look closely and in the right light, you can see that the pupil is horizontal when it is contracted and wide and round when it is dilated.
The lens, which is held in place behind the iris, has a special muscular arrangement which allows it to relax or tense, becoming thicker or thinner as it does so. This enables the eye to focus near and far objects.
Behind the lens, the main cavity of the eyeball is filled with a clear gelatinous substance called the vitreous body.
Lining the rear of the eyeball is the retina which comprises millions of light receptors which collect information and transmit it to the brain.
The exterior of the eye comprises the conjunctiva, which is divided into the palpebral and bulbar portions, and the eyelids. The upper lid has lashes.
Unlike humans, horses also have a third eyelid, or nictitating membrane, which originates from the inside corner and closes horizontally across the eye. The third eyelid functions as protection for the cornea.