At least eighty to ninety percent of the information we receive from the world comes through the eye, making it the most valuable sensitive organ we have. Yet most people know very little about the function of the eye or about the disorders that commonly affect it.

When light first hits the eye it passes through the cornea. The cornea is the tough, transparent window at the front of the eye that focuses the objects we observe by bending the light onto the retina. If the surface of the cornea is irregular, light cannot pass through it clearly, and vision is therefore obscured. Epithelium, the top layer of the cornea, is the fastest regenerating tissue in the body, and is able to regenerate itself in about 24 hours, without scarring.

The iris is the colored portion of the eye that regulates how much light can enter the eye through the pupil. The iris surrounds the pupil. The dilator muscles of the iris can either open the pupil (dilation) or close the pupil using a different muscle. Dilation of the pupil during an eye exam allows eye doctors to (more) thoroughly evaluate the eye’s lens, vitreous, retina and optic nerve.

The anterior chamber lies between the cornea and the iris. It is filled with fluid called aqueous humor which is continually being drained and reformed by a structure at the base of the iris. If this draining system fails then the aqueous humor becomes blocked inside the anterior chamber. The pressure that can develop can damage the optic nerve which may result in glaucoma. Glaucoma occurs when the point at which the optic nerve enters the eye. The optic disk becomes enlarged and the nerves around the area become damaged. Glaucoma first damages peripheral vision which is why it sometimes goes unnoticed by patients for a long time.

Behind the iris is the lens which further refracts the light entering the eye. The ciliary muscle controls the shape of the lens, which enables the lens to bend incoming light depending on the distance from the object being focused on.

Behind the lens is the posterior segment which contains the retina. The retina consists of nine layers that contain rods and cones. The cones are contained within the smallest part of the retina and the eye called the macula. The macula is responsible for fine, central, and color vision. Macular degeneration is the progressive deterioration of the macula that results in the loss of these types of vision. The rods, contained in the retina, are responsible for peripheral and night vision.

Common Problems with Vision and Treatment Options

Nearsightedness (Myopia) is a condition where a person has good sight at ranges that are close up but lacks clarity in their distance vision. The problem here occurs because the incoming light is focused before it reaches the macula. This usually happens because the eyeball is too long or the cornea is too steep. The minus lens that corrects myopia works because it spreads the incoming light rays apart so that the focal point of the image reaches the macula.

Farsightedness (Hyperopia) describes a condition where a person has good vision when looking into the distance but lacks clarity of vision at close up ranges. When the eyeball is too short or the dome of the cornea is too flat, then the light from the image is focused on a point past the macula. A plus powered lens corrects farsightedness because it forces the light of the image forward so that it hits the macula.

Astigmatism occurs in eyes where the cornea is not a perfect spherical shape. Instead, the cornea is more curved in one direction than in another and therefore an image will be stretched out rather than focused to a point. Astigmatism is corrected using eyeglasses that are positioned with a cylinder that coincides with the cornea’s irregular curvature.

As we age our vision gets worse due to many factors. The main cause of age related near vision loss is when the lens, normally soft, flexible and capable of a large degree of focusing, becomes hard and the muscles that move the lens become less toned. This type of vision loss, due to age, is called Presbyopia.

Refractive errors in vision occur when the eye does not “bend” the light that enters it properly. Refraction problems lead to impeded vision which may be corrected with glasses, contact lenses and corrective surgery.