Anatomy of the Eye: A Quick Overview

Vision is widely considered to be the most important of our five senses. Knowing about the basic structures of the eye helps people understand where an eye problem may be located and what is normal or not normal.

Basic knowledge in this area can also help decide if a patient needs to be seen by an eyecare practitioner and whether the problem may be need to be seen at once as an emergency, or how urgent it may be. An ocular emergency indicates a situation that must be seen at once, even if it is in the middle of the night, because it may cause loss of sight if left untreated, while the term "urgent" still indicates something important that could become more serious if left untreated, but can usually wait for an appointment the following day.

Going from front to back, some of the main structures of the eye include the cornea and conjunctiva, the iris and the pupils, the crystalline lens, the vitreous body and the retina and its circulatory system.

The Cornea and Conjunctiva

The first surface of the eye is the cornea, which is a clear, curved structure that allows light to pass through and into the eye. It is surrounded by the white of the eye, or sclera, which is covered with transparent skin called the conjunctiva, which contains tiny blood vessels called capillaries. These structures are lubricated and protected by the tear film.

Contact lenses float on the tear film over the cornea.

When a foreign object gets into the eye, the first defense is increased tearing, which may rinse the offending particle out of the eye before it can damage the surface. It has been shown that the tears have some antibacterial effect as well. In case of a foreign body in the eye, avoid rubbing it, as this may cause it to scratch the corneal surface. Instead, allow the tears to rinse the eye. If necessary, rinse the eye with other fluids, such as lubricating eye drops, contact lens rinsing solution or, if nothing else is available, water.

A foreign body that stays in the eye longer than an hour should be checked and removed by an eyecare practitioner. Most of these can wait until the morning, as keeping the eye still and not blinking is the best therapy, so sleep is actually good, because the object will not tend to move around very much. If it is still present in the morning, see your eye doctor as soon as possible.

The longer a foreign body stays in the eye, the more likely it will become embedded in the tissues, either the conjunctiva or the cornea itself. The more embedded a foreign object is, the more tissue will have grown over it, and the more difficult it will be to remove. Whenever the corneal surface is scratched, there is a possibility of infection, which can be quite serious if not treated right away.

There is a fairly common condition known as subconjunctival hemorrhage, which is caused by the rupture of one of the tiny blood vessels in this clear skin covering the white of the eye. It will be bright red and will usually be painless, although it may cause significant swelling of the tissue. There can be various causes for the blood vessel breakage, which may include excessive rubbing, scratching the tissue while inserting or removing a contact lens, or even a severe coughing attack.

If the blood vessel break has no clear cause, and especially if it happens more than once within a relatively short time, there could be serious underlying physical causes such as high blood pressure, so in the case of repeated episodes, a person should see their primary care physician for a checkup.

The conjunctiva and cornea are subject to infections and allergic reactions, resulting in "pinkeye," which is a generic, catch-all term for any type of redness, swelling or itching of these tissues. An infection may be carried into the eye on a foreign body, or can be caused by the usual microbes living on the skin of the lids and eyelashes. Redness, swelling, itching or burning, light sensitivity and any unusual discharge from the eyes, either watery or thicker, indicates a problem that must be checked by an eyecare practitioner, as either an emergency or urgent basis, depending on the severity of the symptoms and the length of time they have been present.

Because common microbes are everywhere, it is not recommended for eyeliner to be placed within the line of the eyelashes. Mascara and eyeliner should never be used by another person, and they should be replaced every six months or so, because the tube of a mascara brush is a great place for them to live and grow. Although this might sound obvious, all makeup should always be removed completely each evening before sleep; there is no sense in letting such a great growth medium stay overnight so close to the eyes.

A watery, itchy eye with a moderate amount of redness, which has already been present for a week or so, is no longer an emergency, but the eyecare practitioner will want to check on it as soon as a visit can be arranged. An infection causing severe redness, thick, colored discharge and pain is a true ocular emergency and should be seen immediately, as even a delay of a few hours may be crucial.

The Iris and the Pupils

The colored part of the eye is called the iris, which adapts to the level of light present to contract and make the pupils smaller, or allow the pupil to get larger in low light.

The amount of pigment present in the iris determines its color; less pigment means a lighter-colored iris or blue eyes, while a lot of pigment makes the eyes appear brown. People with blue eyes are generally more light-sensitive than those with dark brown eyes.

The pupils of the eyes are not actually structures, but little circular windows that appear black because there is no light coming out of the eye. Pupils should be equal in size, round in shape, and expand or contract equally and quickly to changes in light level.

Pupils that are unequal in size or that do not react to light indicate the need for an urgent appointment with an eyecare practitioner, especially in the presence of head injury. Concussion often results in pupils that are not equal, which is an indication of injury to the brain.

The iris is a heavily vascular tissue, which makes it susceptible to inflammation. This is a serious condition that causes pain and light sensitivity, and requires emergency treatment. The inflamed iris may leak fluids and cause it to stick to the front of the crystalline lens, which can cause the pressure within the eye to increase dramatically. Patients with iritis should be seen by their eyecare practitioner at once, because this is a sight-threatening emergency. It is treated with drops to dilate the pupil and steroid medications to decrease the inflammation.

The Crystalline Lens

The lens of the eye is located just behind the iris. It is shaped like a rounded disc and its function is to change the focus of the eye from distant objects to those that are close up, and back again.

The focusing itself is done by muscles of the uvea, which includes the iris and ciliary body. When these muscles contract, the focus moves closer to the eye. The shape of the lens changes, which changes the focus.

The lens is usually completely clear and transparent, but it does thicken as we age, making it more difficult for the muscles to change its shape. This is a condition known as presbyopia which is completely normal, but gradually causes reading to become more and more difficult and causes most people to need reading glasses later in life, usually at about age forty.

Cataracts can also form in the crystalline lens and cause vision difficulties. By definition, any decrease in the clarity of the crystalline lens is a cataract; by that definition, we probably all have some form of them. They become clinically significant only when they begin to interfere with daily life and activities. There is no cure for cataracts, but the lens can be removed by an eye surgeon and replaced with an artificial implant. The specific lens implant is chosen after carefully measuring the eye, so many patients become less dependent on prescription eyewear after they have cataract surgery.

The eyes should be protected by UV-blocking sunglasses whenever a person is outdoors, particularly if he or she is outside a lot, or participates in winter sports at high altitudes, such as skiing or snowboarding; this is also recommended if the person spends time on the water. By limiting the amount of UV rays absorbed by the eye, both cataracts and macular degeneration may be prevented or at least delayed.

The Vitreous Body

The vitreous body fills the eyeball behind the lens. Also called the vitreous humour, this is not actually liquid, but a rather thick gelatin-like substance. Its main purpose is to hold the retina in place against the back of the eye.

Sometimes the vitreous body detaches from one of its anchoring points, particularly around the head of the optic nerve. This is not usually dangerous, although it may cause the patient to see arc-shaped or ring-shaped floaters, which are small bits of tissue detached with the vitreous. Usually, these floaters will gradually disappear. There is no treatment for a detached vitreous, but patients should have their eyes examined by an eyecare practitioner if they notice any spots in the vision, or floaters, especially if they are new or numerous.

The Retina

The retina is made up of millions of tiny light-sensing nerve cells, which transmit information back into the brain via the optic nerves. Vision actually takes place in the brain, when the information coming from the retina is received and processed by the visual cortex.

The retina is a thin sheet of nerve tissue stretched across the back of the eye and attached to underlying tissues. When it becomes unattached to the back of the eye, it is known as a detached retina. Symptoms of a detached retina include vision distortion or floaters or may even appear as a curtain falling over the vision. This is a sight-threatening emergency.

There are various causes for detached retina, including a history of vitreous detachment and impact to the eye, but many times there is no precipitating event.

The retina may be re-attached by a surgeon using a laser, or by the use of various other means to stabilize it and prevent further detachment. Time is of the essence because the nerve cells cannot survive for very long when they are floating free without nourishment. Even a matter of hours or minutes can be significant to the amount of vision loss or recovery.

The retina has its own circulatory system, composed of small arteries and veins that bring blood and nutrients into the eye through the optic nerve. In fact, the only place in the entire body where the circulation can actually be seen without cutting into the tissues is inside the eye. Hypertension, diabetes, atherosclerosis and arterial sclerosis are all systemic (meaning that they effect the entire body, not just the eye) conditions that can be detected by an eyecare practitioner looking at the retina and its circulatory system. Changes seen to the arteries and veins in the retina are also present in the whole body.

Some Further Considerations

When there has been an impact to the facial or eye area, resulting in the bruising known as a black eye, an urgent visit to an eyecare practitioner is recommended. Many people think that a black eye isn’t serious, and thankfully, it usually isn’t, but the cone-shaped spaces within the skull where the eyeballs sit are filled up with the muscles that are responsible for eye movement. In the event of an impact, the eye may be pushed back into its orbit and can cause some breakage to occur in the relatively fragile bones separating the orbits from the spaces in the skull known as sinuses. Sometimes, the muscle tissues may become caught within the broken bones and cause eye movement problems that result in permanent double vision. A physician might want x-rays or CAT-scans to determine if any bones have been broken.

Children should have their first eye examination by an eyecare specialist before the age of three and yearly thereafter, until they are about the age of twenty, or have completed their education. (This is because serious students study a lot, and may need eyewear updates more frequently.)

Healthy adults from the age of twenty to about age forty should see an eyecare practitioner every two years, or if they notice unusual symptoms not otherwise explainable.

Any person who has experienced any head injury or impact to the facial area should be seen as soon as possible by either their primary care physician or an eyecare specialist.

Anyone who has a history of healthcare issues such as diabetes or high blood pressure should be seen at least yearly for a complete eye examination by an eyecare practitioner, or more often if changes are seen such an exam.

People over the age of forty should be seen by their eyecare practitioner every year or every two years at the least, depending on general health history and family history. After the age of 55 years, people should be seen on a yearly basis.

Vision can change rapidly, often without causing specific symptoms, especially if changes occur in only one eye. People should take precautions to protect this most important of all the senses.