What is an Eye’s Iris?

Diagram showing the iris and pupil of an eye

Iris Defined

The iris is the part of the eye that gives it color, such as in blue, green, hazel, or brown eyes. It is composed of pigment, connective tissue, and muscles (which control the size of the pupil – the black hole in the center of the iris – of the eye). It is also a part of the “uveal tract” of the eye, which includes the tissues of the eye called the iris, ciliary body, and choroid.

The term “iris” is derived from a Greek term for the word “rainbow”. The Greek goddess of rainbows was named Iris. And so, the color portion of the eye is called the “iris”.

The root words “irid-” and “iri-” are often used when medically defining conditions that are associated with the iris. As in “Iritis,” which is inflammation of the iris.

Optometry and Ophthalmology are both practices that scientifically evaluate the iris along with ALL of the structures that constitute the eye. The iris can direct attention to certain diseases of the body and brain, and therefore is an important structure to evaluate within an eye exam.

Iridology, however, is a non-scientific interpretation of the structures and shapes of the iris and pupil that is sometimes utilized in homeopathic care. Be aware, iridology is not science based and may rightly be considered in line with palmistry (the study of lines on a person’s lines of their hand, “palm reading”) and phrenology (the study of a person’s bumps on their head) in determining health issues. Again, it is unrecognized scientifically and should only be used for its entertainment value and not as a diagnostic tool for health. Though people who practice iridology may have good intentions, and altruistically want to help others, the teachings of iridology are misleading. Also, you are likely to find an iridology exam followed by the selling of vitamins or other treatment suggestions.

The Iris’s Role in Eyesight

The iris of the eye forms and controls the pupil. The pupil is the hole through which light enters the eye and is often compared to an aperture on a camera. When using the camera as an analogy, the iris would be comparable to the diaphragm of the camera. A diaphragm uses overlapping blades to increase or decrease the size of the aperture. Instead of overlapping blades, the iris uses muscles. 

The two main muscles that form the iris are the circular iris/pupillary sphincter muscles which immediately encircle the pupil, and the radial dilator muscles, which radiate out like spokes of the sun from the sphincter muscle to the outer edge of the iris. In low lighting conditions, the pupil gets larger (dilates) and allows more light in the eye because the dilator muscles constrict. In brighter light conditions, the pupil gets smaller (constricts) and allows less light into the eye because the sphincter muscles constrict. Cranial nerve 3 (the oculomotor nerve) signals control the message to the iris to control pupil size. 

How do Contact Lenses Work with the Iris?

There are some contact lenses that are used to alter the apparent color of the iris by forming an opaque or translucent color design over the iris. They are often referred to as cosmetic lenses, but they can also correct refractive error of the eye so that the wearer can both look good and see good.

Check out colors contacts → 

Since light enters the eye through the pupil, the only part of a contact lens that requires the corrective power is the center area directly over the pupil, called the Optic Zone of the contact lens. Since the iris can change the pupil size, the Optic Zone is usually made to be larger than a large pupil. Cosmetic lenses generally do not overlap the optic zone.

Common Vision Conditions that Involve the Iris

The brain controls the iris. Both irides (plural form of iris) react in unison to change pupil sizes together, not separately. Therefore, if you shine light in one pupil, it will get smaller, and so will the other pupil that did not have a light shined into it. If the pupils do not react together, that is a bad sign and a doctor should evaluate immediately. Actually, anything that affects the brain can affect the pupil size. Here things that can make the pupil bigger (dilate) or smaller (constrict)…

Bigger Pupils (dilated):

  • Idiopathic (normal for that individual)
  • Low light
  • Myopia (nearsighted)
    • Youth
    • Distant viewing
  • Head injury
  • Brain problem
  • Eye disease
  • Systemic conditions
  • Drugs

Smaller Pupils (constricted):

  • Idiopathic (normal for that individual)
  • Bright light
  • Hyperopia (farsighted)
    • Older age
    • Near viewing
  • Head injury
  • Brain problem
  • Eye disease
  • Systemic conditions
  • Drugs

When the iris inflames, it is referred to as iritis. Iritis creates eye pain and tenderness with light sensitivity and is often recognized as creating a red ring of vessels that surround the iris. Iritis is often referred to as uveitis (remember: the iris is part of the uveal tract), especially when the ciliary body is also involved.

Tips for Maintaining Eye Health of the Iris

Since the iris which is controlled by the brain, anything that you can do to maintain good neuronal health can be beneficial to iris health. For example (but not limited to): cardiovascular health, proper nutrition, and mental stability. 

Iritis is a condition that is often associated with inflammatory diseases such as arthritis. To best avoid iritis is to do what is necessary to control the specific inflammatory disease.

Learn about more parts of the eye! →

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