Anatomy of the eye

The eye is a complex organ composed of many parts. Good vision depends on the way in which those parts work together.

Anatomie van het oog Sclérotique Rétine Fovéa Tache jaune (ou macula) Corps vitré Cornée Iris Cristallin

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The cornea

One-sixth of the outer layer of the eye bulges forward as the cornea, the transparent dome that serves as the outer window of the eye. The cornea is the primary structure focusing light entering the eye. The transparency of the cornea is due to the fact that it contains hardly any cells and no blood vessels. On the other hand, the cornea contains the highest concentration of nerve fibers of any body structure, making it extremely sensitive to pain.


The sclera

Along its circumference, the cornea is continuous with the sclera: the white, opaque portion of the eye. The sclera makes up the back five-sixths of the eye’s outer layer. It provides protection and serves as an attachment for the extra-ocular muscles, which move the eye.


The iris

The iris, visible through the clear cornea as the colored disc inside of the eye, is a thin diaphragm composed mostly of connective tissue and smooth muscle fibers. It is situated between the cornea and the crystalline lens. The color(s), texture, and patterns of each person’s iris are as unique as a fingerprint.


The lens

Lens is a transparent structure situated behind the iris and it is enclosed in a thin transparent capsule. It helps to refract incoming light and focus it onto the retina. A cataract is when the lens becomes cloudy, and a cataract operation involves the replacement of the cloudy lens with a man-made plastic lens.


The vitreous humor

The vitreous humor is a clear gel which occupies the posterior compartment of the eye, located between the crystalline lens and the retina and occupying about 80% of the volume of the eyeball. Light initially entering the eye through the cornea, pupil, and lens, is transmitted through the vitreous to the retina.

Posterior vitreous detachment (PVD)

With age, the vitreous humor changes from a gel to a liquid. As it does so, the vitreous mass gradually shrinks and collapses, separating and falling away from the retina. This is called a “posterior vitreous detachment” (PVD) and is a normal occurrence between ages 40 and 70.

Commonly, a person having experienced a PVD will report seeing flashing lights and/or floaters in his or her field of vision. The flashes of light occur when the vitreous tugs on the sensory layer of the retina, as the vitreous is detaching. The floaters — which are cells or debris released when the vitreous detaches — can appear as little dots, circles, lines, cobwebs, clouds, or a puff of smoke.

Floaters can be apparent especially when looking at a bright background, as the light entering the eye casts shadows of the floaters onto the retina. Sometimes a large, single floater actually can obstruct print that is being read. The observance of flashes and floaters can last two or more weeks. Episodes lasting even as long as six months can occur.


The retina

The retina is the innermost layer of the eye and is comparable to the film inside of a camera. It is composed of nerve tissue which senses the light entering the eye.

This complex system of nerves sends impulses through the optic nerve back to the brain, which translates these messages into images that we see. In fact, we “see” with our brains; our eyes merely collect the information to do so.

The image is collected by light sensitive cells known as rods and cones. The human eye contains about 125 million rods, which are necessary for seeing in dim light. Cones on the other hand function best in bright light - there are between 6 and 7 million in the eye , most are concentrated in the macula and they are essential for receiving a sharp accurate image; cones can also distinguish colours.


The macula

Macula is a yellow spot on the retina at the back of the eye which surrounds the fovea. This is the area with the greatest concentration of cone cells, and when the eye is directed toward an object, the part of the image that is focused on the fovea is the image most accurately registered by the brain.


The fovea

Fovea forms a small indentation at the centre of the macula and is described as the area with the greatest concentration of cone cells.