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Eye anatomy

Vision is very important for just about everything you do. Such as reading this page, seeing what you have for dinner, seeing the one you love etc. We can hardly imagine what it would be like if we did not have the ability to see the world. Even the loss of colour is a major disadvantage. Imagine if the world was a black and white film.

The eye is a marvel of biological optics and about 24 mm. in diameter. The eye is filled with a jelly like fluid, vitreous humor which is under slight pressure giving the eyeball its firmness. It is surrounded by the white sclera, except for the clear cornea in front of the eye. Inside to cornea we find the iris which give the eye its colour. Behind the iris we find the cilliary body which produce a constant flow of acqueous humor. This fluid is flowing slowly and provides nutrients for the lens and the cornea.

The lens is about 10 m.m. in diameter and is suspended from the cilliary body by a lot of small threads called zonules. Around the cilliary body there is a muscle which contracts and relaxes allowing the lens to make fine adjustments to visual acuity. The lens is responsible for approx. 10% of your focusing ability. The cells in the lens of the eye do not regenerate they are retained throughout life. Each year a thin layer grows around the lens so the thickness of your lens doubles from age 20 to age 80.

The cornea is responsible for about 80% of the eyes focusing power, which is derived mainly from the outer interface between air and the cornea. Depending on the opening of the iris, we use an area of about 4 to 8 m.m. only at the very center of the cornea. The cornea is about half a millimeter thick and consists of several layers of cell structures.

Laser surgery is performed on this very thin tissue and as you can imagine must be very precise in order not to cause damage. One of the major problems with this kind of surgery is that the iris opens wider than the 6 m.m. area of surgery, or worse if the surgery is done off center of the cornea. Due to the absence of blood vessels the cornea also take several months to heal.

Microsurgery is performed by making radial incisions in what is known as the intermediate support zone or the ring that surrounds the optical zone at the very center of the cornea. This type of surgery is not performed very much anymore since the results are unpredictable.

Any kind of surgery in the eyes ought to be done only when all other options have been exhausted. Any changes made to the cornea are permanent and virtually impossible to correct afterwards. For more information about what can happen after laser surgery check out www.surgicaleyes.org

At the back of the eye we find the retina. This area is populated with light sensitive cells. At the very center, directly behind the lens there is an area of about 1 x 1.5 m.m. (about the size of a lover case "o" on this page) called the fovea, where we have absolute sharp vision and can distinguish colour. In that area there are three types of cone cells, sensitive to red, blue and green light enabling you to see colours. In the fovea the cone cells are packed very tight and each cell has a direct nerve fiber connection to the brain.

In the areas around the fovea, called the macula, there is a progressive transition into what we call peripheral vision, which consists of more and more rod cells. The rod cells are very sensitive to low light but cannot distinguish colour. On the eye chart peripheral vision would be about 20/200 and foveal vision is 20/20. The foveal cone cells are thus about ten times sharper than the rod cells found in the periphery.

Nerve fibers lead from the retina to the brain via the optic nerve. There is a blind spot where the nerves collect into the optic nerve. The brain automatically fills in the blind spot so we do not notice this.

Around the eye we find six muscles, which we use to move the eyes in different directions. There are four recti muscles, one on top, superior rectus muscle, one on each side of the eye, lateral rectus muscle and the medial rectus muscle, below the eye is the inferior rectus muscle. There are another two muscles forming a belt around the eyeball. The lower one is called the inferior oblique muscle and the upper one is called the superior oblique muscle.

Vision is a very fine balance of interaction between the 12 muscles found around the eyes and to a lesser degree the cilliary muscle around the lens. Any imbalance or tension in any of the muscles will affect your vision. The eye muscles are capable of quickly adjusting their relationship of tension and relaxation so youcan focus your eyes on a near object, an instant later you can look out the window and see a bird high in the sky.

Vision training involves regaining flexibility in these muscles so natural clear vision can be restored.

The visual field is divided into two parts. If you are looking straight ahead the inner half of the image, closer to the nose, will be transferred to the opposite brain hemisphere. The outer half goes to the same side of the brain. The slight difference in the images enables the brain to process the input into what we perceive as a three dimensional image with depth perception. Most of our vision takes place in the brains visual cortex located at the back of your head. In fact about two thirds of your brain is involved in seeing. There are areas of the brain that are processing shape, another is in charge of shadows, and yet another is processing colour impressions. In addition to that we probably also have thoughts and feelings about what is going on so it is easy to imagine how active the brain is.

For example your peripheral vision is extremely sensitive to movement. If you detect a slight movement out of the corner of your eye you will immediately swing your head and eyes to bring this movement into the center focus so you can get a clear look at what's going on. This was very important when we were hunters and needed to protect ourselves from dangerous animals. Today it's more about avoiding dangerous motorists on the highway.

 

 

 

 

 

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