The human eye is a complex sense organ that allows us to see. In order to process information, parts of the eye receive visual cues and other parts transmit that information to our brains. While it might sound simple, the process is quite complex and can be disrupted by a number of eye disorders.
Structure of the Eye: The Orbit
The orbit helps protect the eye from injury. The orbit is comprised of the following bones:
Portions of these seven bones converge and form a pyramid-shaped socket that points toward the back of the head. It is within this socket, which is typically called the eye socket, where the actual eyeball rests.
Surrounding the eye in the eye socket is a layer of fat. This layer cushions the eyeball and promotes smooth movement of the eye within the orbit.
The actual eyeball contains three layers:
- the outer layer, which is formed by the cornea and the sclera (see below)
- the middle layer, which holds the primary blood supply for the eye and contains the iris and the pupil (see below)
- the inner layer, which is comprised of the retina (see below).
The eyeball also contains three chambers of fluid:
- anterior chamber (between cornea and iris)
- posterior chamber (between the iris and lens)
- vitreous chamber (between the lens and the retina).
The first two chambers are filled with aqueous humor, a watery fluid that provides nourishment to the interior eye structures and keeps the eyeball inflated. The vitreous chamber is filled with a thicker fluid called the vitreous humor.
In addition to numerous eye blood vessels, the eye also features the optic nerve. The optic nerve runs from the back of the eyeball and through an opening in the orbit called the optic foramen. From this opening, the nerve connects to the brain to transmit visual information.
In addition to the optic nerve, the eye contains other nerves. However, most of these nerves carry non-visual information and convey messages about pain or help control motor activity in the eye.
The eyelids are thin folds of skin and muscle that cover the eyes. Eyelids help protect the eye from scratches, foreign objects and more and also work to lubricate the eyes.
While we are awake, eyelids carry secretions from the lacrimal (tear) glands across the eye when we blink. When we sleep, the eyelids close, helping to keep moisture on the surface of our eyes.
The eyelid actually has several layers, including:
- a fibrous layer to provide stability
- a layer of muscle that controls the opening and closing of the eyelid
- a layer of skin that contains glands and the eyelashes
- the conjunctiva, a mucous membrane that connects the eyeball to the eyelid and the eyeball to the orbit.
The lacrimal glands, or tear glands, help keep the eye moist. They are located under the upper eyelids and extend inward from the outer corners of the eye. Each gland can feature as many as 12 tear ducts. These ducts flow into a sac, which drops tears onto the eye via the puncta lacrimal, a small opening at the inner corner of the eyelid. The secretions of other glands within the eye help keep the tears from evaporating when they reach the eye’s surface.