Your eye is wonderfully complex and intricately layered. Now, think of your eyelids as movable curtains on a stage. What we see of the eye is just the front surface that's visible between the opened lids; backstage in the eye is just as important and interesting.
The Orbit
The
Eyelids The Sclera and Cornea
The
Iris and Pupil
The Lens
The Retina
The Muscles
Nature provided the soft, vulnerable eye with excellent protection - a layered cradle, or socket, of bone called the orbit. The orbit has heavy bone on its outer edges, thinner bone on the inner (nasal) surfaces, and also a pillow of fat, which cushions the eye. This cavity also contains the muscles and nerves that allow your eyes and eyelids to move; blood vessels, which nourish and sustain eye tissue; and the lacrimal gland, which produces the tears that lubricate the eye. The lacrimal gland is also part of the eye's defense system: in response to the sting of chemicals or onions, or to such irritants as dust or pollen, it turns on the faucet to dilute and wash away anything that might harm the eye.
The eyelids are covered on their inner aspect with a sheet of thin, slippery membrane called conjunctiva, which folds back and connects to the front surface of the eye. Over this lining, and giving the eyelid some rigid support structure and strength is a tough, fibrous plate of connective tissue called the tarsus; then come layers of muscle and skin. A thinner layer of fibrous tissue, called the orbital septum, connects the tarsus to the periosteum, the outermost layer of bone covering the orbit. The orbital septum is a thin fence that keeps the previously mentioned layer of orbital fat confined inside the bony orbit.
At the edges of the eyelids are ducts for glands that secrete an oily substance, which helps keep tears from oozing out of the eyes and onto the skin. Here also are the bases of the eyelashes: delicate, efficient filters that protect our eyes from dust and myriad other foreign objects.
The 'white' of the eye visible between the lids is the front portion of the sclera, a thick, protective sheath that encircles the eye, with a porthole at its very front. At this porthole, sitting like a watch glass on its casing, is the cornea. The cornea is normally transparent; through it you can see the iris and pupil. The cornea has no blood vessels, while the conjunctiva and episclera (two tissue layers that cover the sclera) do.
The back surface of the sclera is connected to the tough outer covering of the optic nerve, the cable that links the eye with the brain. Sometimes the white sclera of the eye develops dark areas, but these dark areas are of no significance. They're caused by calcium deposits, which cause the sclera to lose its normal white color, allowing the dark pigment inside the eye to show through.
The cornea has five layers: the epithelial, or outer-lining, cells; Bowman's layer; the tough but transparent stroma, the bulk of the cornea; Descemet's membrane; and a single layer of endothelial, or inner-lining, cells. The cornea is more than a window; it is a converging lens. It takes light rays and bends and focuses them to the back of the inside of the eye, the retina.
The iris, visible through the cornea, is composed of connective tissue and muscle with a hole in the middle. The color of the iris is actually due to the amount of pigment in the iris connective tissue layer. Brown eyes have a lot of pigment, blue eyes very little.
The pupil is the hole in the iris, which allows light to reach the retina. The iris uses one muscle to constrict the pupil and another to actively dilate, or enlarge, it. The pupil is wider in a darkened room and narrower in full sunshine.
Around the iris, hidden by the sclera, is the ciliary body, the ring or tether that holds the lens in place, connected in back with the choroid, a bolstering, nourishing layer of blood vessels between the sclera and the retina. The ciliary body produces the aqueous, a watery solution that bathes the lens; manufactured behind the iris, it travels through the posterior chamber and the pupil and leaves the eye through a drain in the anterior chamber angle, where the iris meets the cornea.
The aqueous is secreted by one group of cells and eventually leaves the eye through a drain or meshwork created by other cells. This balance maintains what's called the intraocular pressure - and this is essential for maintaining the shape of the eye (and particularly for preserving the curvature of the cornea).
Behind the iris is the lens, fastened to the ciliary body by thin fibers called zonules. The lens is elastic: to focus, it stretches and snaps back into place. In the job of focusing light on the retina, the cornea does about three-quarters of the work and the lens about one-quarter; but the cornea has fixed focus power and the lens variable focus power. Accommodation takes place when a muscle in the ciliary body contracts, relaxing the tension on the zonules, thus allowing the lens to become less flattened and more spherical.
Inside the sclera is the vascular choroid; inside the choroid is the retina, the reason for being of everything else in the eye. The exquisitely complicated retina does far more than merely register an image like a bit of photographic film. Indeed, cells in the retina break down an image into countless elements - brightness, position, color, movement - then encode all these elements as electrical signals and transmit them to the brain.
There are layers of rods and cones receive the basic units of light, triggering a photochemical reaction in these cells. Then bipolar cells apparently receive, organize, and transmit this information to ganglion cells, which send these signals to the brain through a collection of nerve fibers called the optic nerve. The macula, the most significant part of the retina, is located next to the optic nerve and is responsible for our central vision. The macula is composed mostly of cones and is also an important part of color vision.
Most of the interior of the eye is filled with vitreous, a nearly transparent substance. Vitreous is supported by scaffolding, a meshwork of collagen fibers, and a gel of hyaluronic acid. The meshwork is thicker against the retina, at a site called the posterior vitreous membrane, attached at the optic nerve, the ciliary body, and various other places in the retina.
Part of the process of aiming the eyes to see an object is turning the body and moving the head. But the eyes can also move independently. To do so, there are six extraocular muscles fastened to the eye. The extraocular muscles are a medial and lateral rectus, mostly for horizontal movement, and superior and inferior rectus and oblique muscles, for vertical and torsional (twisting) movements. There is a sophisticated feedback system in the brain to keep the eyes focused together, to maintain binocular vision (that is, two eyes working together), and avoid double vision.
