By “eye” we generally mean the “eyeball” which is the organ of sight. It has a complicated origin, the posterior and major portion is in fact part of the brain, whereas the anterior portion is ectodermal in origin.

The visible portion of the eye is the cornea, covered with the thin membrane of conjunctiva. The colored iris is in fact a circular muscle that can contract or expand to alter the size of its central hole, the pupil, and is also attached to the lens, suspended by its ligaments in a central position. The cavity between the lens and the cornea is filled with watery aqueous humor, the small space between the iris and the lens is the anterior chamber.

The lens is composed of collagen fibers, the same material that makes bone, hooves, tendons and glue, but organized in such a manner that light passes through them. It is not often recognized that the cornea does more to refract light than does the lens, but the shape of the lens can be altered to focus the light.

In the posterior five sevenths of the eye, the outer coat is the dense sclera, lined with the light interpreting cells of the retinal layer and the blood vessel bearing layer of choroid. The bulk of the eye behind the lens is filled with the gelatinous vitreous humor.

The eyeballs are moved by a set of small muscles inserted into the sclera. It rests on a pad of retro-orbital fat in the orbital cavity of the skull. At the back the optic nerve passes through a foramen in the skull to join the brain.