One Miraculous Organ

Perhaps the well-known song expresses it best. “Let’s start at the very beginning, a very good place to start. When you read, you begin with A-B-C, when you sing you begin with Do-Re-Mi.” And when you attempt to understand something as entirely complex and profoundly miraculous as the eye and our generation’s miracle of low vision assistive technology, the place to start is with the organ itself, that body part that lets us see, read, watch, observe, witness, perceive, distinguish, notice, glimpse, view, examine, and behold. Let this article be your encyclopedia of anatomy, a reference point for sight. With it as a foundation, become better prepared to delve into the exciting world of low vision assistive technology and the diseases that it helps to conquer.

A transparent “front window,” the cornea is the thick, nearly circular structure covering the eye and providing nearly 2/3 of its ability to focus. Normally clear and with a shiny surface, the cornea has no blood vessels. Quite sensitive, there are more nerve endings in the cornea than anywhere else in the body. One half millimeter thick in an adult, it has five layers: the epithelium, Bowman’s Membrane, the stroma, Descemet’s Membrane and the endothelium:

The epithelium is a layer of cells covering the surface of the cornea. It’s nearly five to six cell layers thick and quickly regenerates when an injury occurs. If a wound penetrates too deeply into the cornea, an opaque scar is sometimes left that can result in a loss of clarity and luster.

Bowman’s Membrane lies just beneath the epithelium. Tough and difficult to penetrate, it protects the cornea.

The stroma is the thickest layer of the cornea and is comprised of tiny collagen fibers running parallel to each other. This special formation of collagen fibers gives the cornea its clarity.

Descemet’s Membrane lies between the stroma and the endothelium and serves as a protective barrier against infection and disease.

The endothelium is just one cell layer thick. It pumps water from the cornea, keeping it clear. If it is damaged or diseased, its cells will not regenerate.
The conjunctiva is a thin, transparent surface covering the outer surface of the eye. It begins at the outer edge of the cornea, covers the visible part of the eye and lines the inside of the eyelids. Nourished by tiny blood vessels invisible to the naked eye, it secretes oils and mucous that moisten and lubricate the eye.

The sclera is the white of the eye and is a tough, opaque surface serving as the eye’s protective outer coat. At the back of the eye, the optic nerve is attached to the sclera. In children, the sclera is thinner and more translucent and tends to take on a bluish cast, but by adulthood, it takes on more of a yellowish cast.

The iris is the colored membrane of the eye, located between the cornea and the lens. It controls light levels inside the eye and because of microscopic pigment cells called melanin, varies in color from pale blue to dark brown. Flat in perspective, it divides the front (or anterior) chamber of the eye from the back (or posterior) chamber, and its color, textures and patterns are as unique to each individual as a fingerprint.

Directly in the middle of the iris is the pupil, the round black hole that changes size with the help of tiny muscles that control the amount of light entering the eye. The sphincter muscle, around the very edge of the pupil, contracts in bright light to constrict the pupil. The dilator muscle runs radially throughout the pupil to widen the eye in dim lighting.

The ciliary body lies just behind the iris, and tiny fiber guy wires called zonules are attached to it. One of its functions is the production of aqueous humor, the clear liquid that fills the space between the cornea and the iris. Aqueous humor nourishes the cornea and the lens and gives the front of the eye its shape. The ciliary body also helps change the shape of the lens. When it contracts, the zonules relax, the lens thickens and close-up vision is made possible. When it relaxes, the zonules contract, the lens becomes thinner and distance vision is made possible.

Vitreous is a thick, transparent substance that fills the center of the eye. It is composed of water and is two-thirds of the eye’s volume, providing its form and shape. Firmly attached to certain areas of the retina, vitreous has the consistency of egg-white in children but thins as we age. As it thins, it can separate from the retina, and this causes the floaters with which older adults are familiar.

The choroids lie between the retina and the sclera and are composed of layers of blood vessels that nourish the back of the eye. They connect with the ciliary body towards the front of the eye and the edges of the optic nerve at the back of the eye.

The lens is transparent and convex on both sides. Located behind the iris, it focuses light rays entering through the pupil to form an image on the retina. The nucleus (or innermost part of the lens) is surrounded by soft material called the cortex. The lens is encased in a capsular-like bag and suspended within the eye by zonules (those tiny guy wires mentioned earlier). In young people, the lens changes shape to accommodate for near or far vision, but by older adulthood, it is often no longer capable of the same flexibility and vision correction becomes necessary.

The retina is a thin, multilayered membrane lining the back two-thirds of the eye. Composed of millions of visual cells, it is connected by the optic nerve to the brain. The retina captures light rays that enter the eye and sends electrical impulses to the brain that result in sight. Rods and cones located in the retina function as photoreceptors. The approximately six million cones permit us to appreciate color, and the approximately 125 million rods function best in dim light and are responsible for peripheral and night vision.

The macula is an area of the eye near the center of the retina where visual perception is most acute. It is responsible for the critical focusing vision so necessary for seeing fine detail. One hundred times more sensitive to detail than the peripheral retina, it’s often called the “bull’s eye center” of the retina. The fovea is the most center portion of the macula.

The optic nerve, composed of thousands of nerve fibers that connect the macula and retina to the brain, carries electrical impulses to the processing center of the brain and converts them to sight. The visual portion of the optic nerve is called the optic disc and is connected to the back of the eye near the macula.

The extra ocular muscles are six tiny muscles that surround the eye and control its movements. The four rectus muscles control the up, down, left and right movements of the eye, and the two oblique muscles control the inward and outward movements.

Tear film is produced by tiny glands that surround the eye. Comprised of three layers (oil, water and mucous), the lower mucous layer serves as an anchor for the tear film and helps it adhere to the eye. The middle layer is comprised of water. The upper oil layer seals the tear film and prevents evaporation. Tear film keeps the eye moist, provides a smooth surface for light to pass through the eye, nourishes the front of the eye and provides protection from injury and infection.

That’s it. Eyenatomy 101, a working knowledge of one of life’s greatest miracles. From the very beginning, when an object is in the line of sight, through the incredible process where light rays are reflected from the object to the cornea, the light rays are bent, refracted and focused by the cornea, lens, and vitreous, the rays come to a sharp focus on the retina, light rays are converted to electrical impulses and transmitted through the optic nerve to the brain where the image is perceived, it’s an astonishing journey of possibility. Truly an amazing grace.
 

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