Library · Anatomy
Iris
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The iris is the colored part of the eye. Anatomically it is a circular, pigmented diaphragm, meaning a sheet with a hole in the middle and with pigment that gives it its color, placed right in front of the crystalline lens, the eye's internal lens.
That hole in the middle has a name: it is the pupil, and it is not a separate piece but the free edge of the iris itself. When the iris changes the diameter of the pupil, it changes how much light reaches the retina, the tissue at the back of the eye that receives the image. It is a camera diaphragm: the same opening widens and narrows with the light.
What closes that opening is the iris sphincter muscle, also called the pupillary sphincter, which rings the pupil. It contracts in two situations: in bright light and during accommodation, the adjustment the eye makes when the gaze shifts from far to near.
In brief
The iris is the circular, pigmented diaphragm that sits in front of the lens.
Its free central edge forms the pupil, and by changing the diameter of that opening the iris controls how much light reaches the retina.
It is a camera diaphragm: the same opening widens and narrows with the light.
The iris sphincter muscle surrounds the pupil and constricts it in bright light or during accommodation.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingAnatomy, Head and Neck: Eye Iris Sphincter Muscle, StatPearlsPublished
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