01 / Location & anatomical relationships
Behind the clear cornea and in front of the lens.
02 / Structure & organization
The iris contains pigmented tissue and smooth muscles controlling the pupil. Circular sphincter fibers constrict the opening, while radial dilator fibers enlarge it. Autonomic pathways regulate these muscles in response to light and other influences.
03 / Function
It changes the size of the pupil to control how much light enters.
04 / Mechanism
Tiny muscles change the diameter of the pupil in the middle. A smaller pupil admits less light in bright conditions; a wider one admits more when light is dim. Pigment contributes to the iris's color.
Part of a bigger picture
The iris belongs in our brain & senses collection. Explore the nearby parts to see how this area of the body fits together.
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Connect structure with function
The iris is tissue; the pupil is its opening. Pupil size affects light entry, but focusing depends mainly on the cornea and lens.
Check your understanding
Locate the iris in relation to its neighbors. Explain how the structural features described above support its function, then distinguish it from a nearby structure with a different role.
Keep learning
Continue with OpenStax Anatomy and Physiology for system-level study. This atlas is educational and has not undergone independent medical peer review.