The Outer Eye

When looking at the outside of the eye, several structures are readily available for viewing. The white part of the eye is a tough outer layer called the sclera. The sclera protects the inside of the eye and helps the eye keep its structure. The sclera is covered by a thin membrane called the conjunctiva. The conjunctiva has many blood vessels which may become dilated when irritated. Pink eye (conjunctivitis) is an infection of the conjunctiva. The colored part of the eye is called the iris. This portion of the eye is what controls the amount of light that is let into the inner eye. It is actually a circular muscle that contracts or relaxes depending on the amount of light it senses. The color of the iris is due to various pigmented cells incorporated into the iris. In the center of the iris is where we find the pupil. The pupil normally looks dark because of light absorbing pigments in the interior portions of the eye. The pupil can be large or small depending upon the amount of light that is needed by the posterior structures of the eye.

When we look at the outer structures of the eye from the side, we are able to see the various muscles that control the position of the eye as well as the optic nerve and the front of the eye. The transparent portion at the very front of the eye is called the cornea. The cornea is a thin layer of cells that allows light to pass through the front of the eye while protecting the iris and other structures beneath it. The portion of the eye beteen the cornea and the iris is called the anterior chamber. The anterior chamber is filled with a water-like substance that keeps the structures around it hydrated and lubricated. Another important structure we see when looking at the eye from this viewpoint is the optic nerve. The optic nerve is actually the connection from the eye to the brain. This the pathway that information gathered from the eye follows to deliver information to the brain.

The Eye in Cross-Section

When we look at a cross section of the eye, we can see the interior structures of the eye. The lens of the eye sits behind the iris. The lens focuses the light that comes through the cornea onto the back of the eye. Once the light has traveled through the lens, it travels through the transparent vitreous. The vitreous is a water-based viscous jelly-like substance that fills the middle of the eye. It is transparent and also helps to keep the structure of the eye. The retina is a thin layer of neural cells which sits on the back of the eye. It responsible for transmitting the information that it receives to the brain through the optic nerve.

The Vitreous Humor

The vitreous is composed of 99% water combined with salts, sugars, and a network of collagen fibers. The other 1% of the vitreous is composed of hyaluronic acid which increases the viscosity of the vitreous to 2 to 4 times that of pure water. The vitreous fills the inside of the eye but does not come in contact with the retina except in three places: the macula (the area of the retina responsible for central vision), the optic nerve disk (where the optic nerve attaches to the eye) and the anterior border of the retina (the border closest to the lens). There are no blood vessels in the vitreous and very few cells. Unlike the fluid in the anterior chamber which is continuously replenished, the vitreous does not turn over. As we age, the vitreous becomes more liquid and degenerates.

The Retina

The retina is a specialized layer of neural tissue that coats the back of the eye. It is an outgrowth of the brain and therefore can be classified as a member of the central nervous system. Although, the retina can easily be compared to the film inside a camera, that analogy is misleading because the retina is very complex. It has many blood vessels which supply nutrients and oxygen to the retinal cells.

The retina is composed of many specialized neural cells. The photoreceptor layer, which can be termed the sensory retina, is composed of rods (cells responsible for night vision) and cones (the cells responsible for color vision) which are excited by light and the first cells to recieve any visual signal. When excited, the photoreceptors pass their signals along to other cells, which can be termed the neural retina, which carry out the first steps in processing visual information and pass their signals on to the brain through the optic nerve.

A special part of the retina called the macula is responsible for our central vision. In this area, the retina thins to allow for more light to reach the photoreceptors. The center of the macula is called the fovea. This area has the greatest concentration of cones and is responsible for high acuity vision. There are no rods or blood vessels contained in the fovea. We automatically move our eye so that light focuses on the fovea when we want to look at something because this the the area of highest visual acuity. Therefore, the fovea is always responsible for our central vision when we are looking directly at something.

The Retinal Pigment Epithelium and the Choroid

The retina attaches to it's underlying tissue via the retinal pigment epithelium (RPE). The RPE is a highly specialized layer of pigmented cells which play a role in the maintence of visual function. The highest concentration of pigment in the RPE is found in the peripheral retina and the lowest concentration is in the macula. The RPE acts as an anchor for the retina as well as the blood-retinal barrier. It is involved in metabolism of certain compounds of the retina and the melanin (pigment) in the RPE works to help reduce scattering of light and blocks light absorbtion through the sclera. Attached to the opposite side of the RPE is a large vascular bed called the choroid. The circulation in the choroid has one of the highest flow rates in the entire body per tissue gram (higher than the coretx of the kidney). More than 70% of all the blood in the eye is found in the part of the choroid nearest the retina.

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