The Normal Eye

Over the last year we have talked a lot about problems with the eye. What we would like to do now is take a step back and talk about the normal eye. We hope that this will help add understanding to our previous articles. The picture was created by our own retina specialist, Dr. Chen.

Cornea:eye

This is the clear front portion of the eye.  It excludes the white portion of the eye which is the sclera (to be discussed later). Simplistically, it has 3 basic layers (more in reality).  The first is the epithelium, the second is the stroma, and the third is the endothelium. The epithelium covers the front surface of the cornea and is kept smooth by the tears produced elsewhere.  Without the wet film over the epithelium, the surface is roughened and the vision decreased, and if it is severely roughened and/or abraded, nerve endings are exposed which results in pain.  Common disorders of the cornea are dry eye syndrome, foreign bodies, ulcers, trichiasis, edema, etc. The stroma makes up the major portion of the cornea, creating its curvature and participating in the focusing of the image to the back of the eye.  An irregular loss of or scarring of the stroma will result in distortion or hazing of the image projected to the back of the eye and thus blurred vision. The endothelium is a single cell layer on the back side of the stroma.  It is responsible for keeping the stroma and epithelium dry.  Loss of the endothelium cells permits any fluid entering the epithelium or stroma to result in swelling and then, loss of vision.  Loss of the endothelium cells are most commonly caused by trauma, multiple eye surgeries or hereditary.

Anterior chamber:

This is the space behind the cornea and in front of the iris, it is filled with a liquid called aqueous, which is produced by the ciliary body (to be discussed later).  The fluid remains clear, but in cases of inflammation,  cells and flare are observed.

Iris:

This is the colored part of the eye, which is seen through the cornea.  It acts like the shutter of a camera, adjusting the amount of light which enters the eye.  Closure of the pupil, or constriction of the pupil, occurs when in bright light and when focusing up close. Common disorders include trauma which damages the muscles around the pupil (the round opening in the center of the Iris) causing it to become irregular.  With inflammation the edge of the pupil may adhere to the surface of the lens preventing it from either opening up or closing down, depending where the adhesion occurs.  Severe trauma may also cause the iris to tear away from its peripheral location near the cornea.

Ciliary body:

This is the structure just behind and connected to the iris.  It has two functions.  The first is to produce aqueous and, the second is to adjust the lens, thus focusing the image on to the macula (to be discussed later). Aqueous is important to the eye because it keeps the pressure in the eye.  Without the aqueous the eye would collapse, just as a basketball would without air.  The continuous production of aqueous is necessary because there is a drainage system in the eye which allows for its outflow.  If there were not an outflow the pressure in the eye would elevate leading to glaucoma. Damage to the ciliary body would result in low pressure in the eye and effect vision.

Zonules:

These are fine fibers that originate from the ciliary body and connect to the periphery of the lens.  As the muscles in the ciliary body contract, the zonules pull on the lens resulting in its flattening thus decreasing its refractive power, and if the muscles relax, there is not tension of the lens through the zonules, and the lens thickens, thus increasing the refractive power of the lens.

Lens:

This is the primary means by which an image is focused on the back of the eye, and more specifically on the macula.  As mention above its ability change its focusing power is due to the ciliary body through the zonules.  The lens is also the object of discussion when dealing with cataracts. Nuclear cataracts are the most common type.  They are due to aging the center of the lens initially becomes less pliable, resulting in a shift toward myopia (near sightedness) and presbyopia (inability to focus up near).  With further aging there is greater hardening of the central portion of the lens, which also becomes discolored, progressing from light yellow to dark yellow, then turning to brown, and in unusually advanced cataracts, black. Cortical cataracts involve the part of the lens peripheral to the nucleus of the lens.  It can develop clefts, vacuoles and/or opacities, causing disturbances in the uniformity of the lens and thus decreased vision.  Rarely the cortex can swell to such an extent that it pushes so tightly on the back of the iris that it prevents the natural flow of aqueous out of the eye resulting in a form of glaucoma. Posterior subcapsular cataract involves an opacification on the back surface of the lens.  It can vary in density, but even if small it can significantly affect vision when the pupil constricts which occurs in bright lighting and reading.   Mel Chen, MD Vitreo-Retinal Surgeon Sarasota Retinal Institute  Sarasota, Florida