When we are born our eyes are only 1.6 to 1.7 centimeters in diameter. Over the first three years of life, the eyes grow rapidly, reaching their full size (just shy of one inch, or 2.4 cm) by the age of 13. The visible part of the eyeball makes up 1/6 of the eye’s total surface area, with the rest hidden behind the eyelids. Although it is small, the eye is a complex organ. To enable clear vision, all structures within the eye must function properly in order to capture light, focus it, and relay messages back to the brain to create a visual image.
How the Eye Works
The eye is a complicated machine with many parts. If we compare the body to a car, then the engine is the heart, the gas tank is the stomach, and the eye would represent all the windows and gauges. Imagine driving with all the windows fogged so that you are unable to see. Imagine that the gauges do not work and you do not know how fast you are going or how much gas is left in the tank. You would have to drive by feeling only. Without the eye you would be in the same predicament. The eye allows you not only to view objects, but to see depth, color, size, and detail. The eye works by refracting and focusing light onto the retina. When light strikes the retina, millions of rhodopsin-containing rods, which are responsible for night vision, convert the light into electrical impulses, which are sent to the brain. The brain then translates what it receives from the optic nerves so that we can understand what we see. The retina also contains millions of cones that contain iodopsin and are used for bright light vision and color perception. There are approximately 17 times more rods than cones—about 120 million rods and 7 million cones—in the retina of each eye.
Parts of the Eye
The many intricate parts of the eye allow light refraction, maintain the shape of the eye, convert light into electrical impulses, and much more. Here is a look at the various parts of the eye:
The cornea is the dome-shaped outer covering of the eye. It is like the windows of the car we discussed earlier. It protects your eye and allows you to see around you. The cornea is where light is focused. It comprises many layers, including the outer layer, the epithelium. The epithelium is often removed or cut during surgical procedures that reshape the cornea to focus light better. Unlike other organs in the human body, there are no blood vessels in the cornea, since blood vessels block light from entering the eye. Instead, the cornea receives its oxygen and nutrients from tears, from the atmosphere, and from the aqueous humor.
The Sclera is the white outer part of the eye that you can see. It provides protection and structure for the inner parts of the eye.
The Conjunctiva and Lacrimal Glands
The conjunctiva is a mucus layer that keeps the eye moist. It covers the sclera and the inner surfaces of the eyelids. Infections in this area are commonly known as “Pink Eye.” Lacrimal glands, which produce tears, are found on the outer part of each eye.
The Vitreous Humor and Aqueous Humor
Vitreous humor makes up approximately 80 percent of the volume of the eyeball. It is a gel-like substance in the back part of the eye that provides the shape of the eyeball. The vitreous humor is located between the lens and the retina, in an area called the vitreous cavity. Besides helping to maintain the shape of the eyeball, the vitreous cavity also provides a clear pathway for light going through the eye to the retina. The Aqueous Humor is the watery region in the front of the eyeball. It is separated into two regions, the anterior chamber in front of the iris, and the posterior chamber behind it. The canal of Schlemm drains water in this region. Blockage of this canal leads to glaucoma and other complications. The main function of the aqueous humor is to carry nutrients to the cornea and the lens and to remove waste products from inside the front of the eye via the canal of Schlemm.
The Iris and Pupil
The Pupil is the black hole in the center of the colored iris. It contracts when exposed to bright light and expands in darkness to allow more light into the eye. The iris is the colored part of the eye. This coloring is due to pigment cells in the tissue. People with blue eyes have less pigment in their iris than those who have brown eyes. The iris contains the sphincter pupillae, the muscle used to narrow the pupil, and the dilator pupillae, the muscle used to widen it. The iris controls how much light enters the eye by blocking extraneous light from entering the pupil.
The lens is a clear structure behind the pupil that does just what a regular lens does. The main purpose of the lens is to focus light by changing shape. The ciliary body is a muscle group attached to the lens that help the lens change its shape to better focus light on the retina. As we age, our lenses naturally deteriorate, sometimes resulting in cataracts.
The retina is the innermost layer of sensitive tissue that transmits light to the brain. The retina consists of several types of cells, including a layer of rods and cones, which transform light into chemical and electrical energy that is transmitted to the optic nerves. The center of the retina contains the macula. The macula is a highly sensitive part of the retina that is responsible for our detail vision. The center of the macula, which has a major role in detail perception, is called the fovea. When there is damage to the macula, we are unable to see fine details.
The Macula and Fovea
The macula is the center portion of the retina. Its main function is to provide clear, distinct central vision. The fovea is the center portion of the macula that provides the sharpest vision. The fovea only contains cones. Damage to the macula or fovea often results in a decline in one’s central vision.
The Optic Nerve
Also known as Cranial Nerve 2, the optic nerve is what carries messages from the eye to the brain. It consists of over one million axons, which carry visual information to different parts of the brain.
Located between the retinal pigment epithelium (see below) and the back wall of the eye, the choroid carries nutrients to the retina and the retinal pigment epithelium. The choroid is made up of melanin, which absorbs any extraneous light that may interfere with the image the eye is sending to the brain.
The Retinal Pigment Epithelium
The retinal pigment epithelium can be found between the retina and the choroid. The retinal pigment epithelium:
- Protects the retina from excess incoming light.
- Supplies omega 3 fatty acids for building photoreceptive membranes
- Supplies glucose for energy.
- Helps transport water from the retina to the choroid
- Maintains the pH balance of the retina
- Helps remove dead segments of photoreceptor cells.
- Secretes substances to help build and sustain the choroid and retina.
The Eye Muscles The eyes have four groups of muscles:
- The extra-ocular muscles that control eye movement. Each eye has six of these muscles, which control each eye’s movement, allowing both eyes to see the same image simultaneously.
- The muscles of the iris. These dilate and constrict the pupil of the eye, controlling how much light enters it.
- The eyelid muscles that control opening and closing of the lids.
- The ciliary muscles. These control lens focusing within the eye.
The Orbit The orbit is the pocket of tissue each eyeball sits in. Seven separate facial bones create the walls around the orbit. Besides the eyeball, several muscles, nerves, blood vessels, fat, and the lacrimal drainage system create the complex structure. The optic nerve rests at the back of the orbit. The Eyelids The eyelids’ main function is to protect the eyes by blinking. Blinking prevents debris from getting into the eye. The average blink rate is 10 blinks per minute. Men and women blink at about the same rate unless the woman is taking oral contraceptives; she’ll then blink at about 14 blinks per minute. When a person is concentrating on reading or working on a computer, they’ll blink about three or four times a minute. This is the major reason that eyes dry out and become fatigued when reading. The Lacrimal Drainage System As mentioned above, the lacrimal glands, which are part of the drainage system, also produce tears. The lacrimal drainage system functions by distributing those tears over the surface of the eye and removing excess tears. The puncta consist of small holes that allow tears to drain from the eyes into the nose. If you were to split your eyelids into thirds vertically, you would see that the innermost third of both the upper and lower lids contains the puncta. The lacrimal drainage system also contains the nasolacrimal sac and the nasolacrimal duct. The sac is a pouch located under the skin between the eye and the nose. Its main function is to collect tears leaving the eye and ensure that they continue on their path out of the eye and into the nose. The duct is a tube that transports the tears from the eye to the sac to the nose. The Tear Film Also part of the lacrimal drainage system, tears are made of three components: water, lipid, and mucus. Once they are produced from the lacrimal gland, they bathe the surface of the eye. Tears provide moisture and nourishment for the cornea and remove surface debris. Once they have performed their duties, they enter the puncta and travel through the nasolacrimal sac and duct, making their way into the nose and down the throat. As you can see, the eye is small but very complex. So take care of your eyes. Visit your eye care professional regularly, or if changes occur in your vision. Did you know … the eye is capable of seeing a candle flame from more than thirty miles away? Did you know … messages are sent from the eye to the brain via the optic nerve at a speed of 423 miles per hour?
- J. Weizer, MD; J. Stein, MD, MS “Reader’s Digest Guide to Eye Care” (Quantum Publishing Ltd, 2009) 10-20
- J. Anshel, MD “Smart Medicine for Your Eyes” (SquareOne Publishers, 2011) 7-9
- P. Abrahams “A Complete Gude to How the Body Works – The Atlas of The Human Body” (Amber Books, 2009) 40-45
- H. F. Ullmann “Atlas of Anatomy” (Ullmann Publishing, 2009) 168-177