November 15, 2006
Its beauty and intricate patterns have been the basis of art and poetry. Countless ballads and sonnets have praised it. It bestows individuality and character to the face that frames it. As much as it is all this, the iris is a complex tissue, and is critical to the physiology and function of the eye.
The center of this highly muscular tissue forms the pupil, through which light enters and focuses inside the eye. There are two main groups of muscles in the iris. Radial muscles fan out from its center and act to open the pupil. A circular muscle at the inner fringe of the pupil serves to close the pupil. Different intensities of light cause the pupil to constrict in bright light and dilate in dimness. The pigment of the iris, besides doing all I mentioned in my opening paragraph, absorbs and blocks unfocused and unwanted light from getting inside the eye. People without iris pigment, as seen in albinos, generally have very poor vision, in addition to extreme light sensitivity. The role of pigment, along with the ability of the pupil to constrict and dilate, allows us to see with proper contrast.
What can happen to the iris? If there is damage to the brain, (stroke, concussion, etc.), one or both of the pupils will dilate and will not react to light. This is an important indication that immediate medical care is required. Keep in mind that it is not too unusual for pupils to normally be unequal in size; however, you should never ignore a sudden change in pupil size. In many inflammatory conditions of the body (arthritis, lupus, colitis), the iris can inflame as well. This condition, called iritis, can upset the critical function of the iris. Pain, extreme redness, sudden reduction of vision, and acute glaucoma (more on this in a moment) can rapidly develop if untreated. The key here is rapid onset, so if these symptoms occur, prompt treatment is essential.
The iris has other vital roles in vision as well. The rear portion of the iris includes a tissue called the ciliary body. Its muscles are connected to tiny ligaments which control shape of the crystalline lens. The crystalline lens change its shape to focus to different distances.
The ciliary body also produces a clear liquid called aqueous humor. This fluid, filtered from the blood, circulates from the back of the iris, through the pupil, and exits the eye at the junction of the iris and the tough outer tissue called the sclera (the "white" of the eye). Aqueous humor nourishes the inside of the eye and maintains the internal pressure of the eye.
A delicate balance between the production and the drainage of aqueous humor is critical to ocular health. When production is less than drainage, the eye can lose the critical and precise shape it requires for effective vision. When production is greater than the drainage, the ocular pressure can gradually rise to damaging levels. This results in the well-known disease, glaucoma. Glaucoma is one of the most common causes of blindness in the world today, affecting close to 70 million people worldwide. Keep in mind that most types of glaucoma are treatable; glaucoma testing is therefore one the most important reasons to visit your eye doctor.