Iris (Eye)

The iris is the coloured part of the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Its pigmentation determines eye colour, which ranges from brown to blue, green, hazel, amber, and grey.

Microanatomy

The iris consists of several layers:

Anterior limiting layer: Outermost, thin layer.
Stroma: Contains blood vessels, melanocytes (melanin-producing cells), and connective tissue. The density and arrangement of these elements affect eye colour and the presence of features like the limbal ring.
Sphincter pupillae muscle: Constricts the pupil in response to bright light or accommodation.
Dilator pupillae muscle: Dilates the pupil in response to low light or emotional arousal.
Pigment epithelium: The back layer, rich in melanin, which prevents light from scattering inside the eye.

Function

The iris regulates the amount of light entering the eye by adjusting pupil size. This process is involuntary but can be influenced by light, emotional state, and certain drugs. Pupil size decreases with age. The iris also contributes to depth of field during accommodation.

Eye Colour Determination (Expanded)

Eye colour is determined by the amount and type of melanin in the iris stroma and pigment epithelium, as well as the scattering of light (Rayleigh and Tyndall effects). Blue and green eyes result from low melanin and structural colour, while brown eyes have high melanin content. See Eye colour.

Clinical Significance

Disorders involving the iris include:

Aniridia: Partial or complete absence of the iris (see Aniridia).
Heterochromia: Different colours in one or both irises (see Heterochromia).
Iritis/Uveitis: Inflammation of the iris, can cause pain and vision loss.
Pigment dispersion syndrome: Loss of pigment from the iris, can lead to glaucoma.
Kayser–Fleischer ring: Copper deposition at the corneal margin (see Kayser–Fleischer ring).

Evolutionary and Comparative Anatomy

The iris is present in all vertebrates with camera-type eyes. Its structure and pigmentation vary widely among species, often serving for camouflage, mate selection, or species recognition. The PAX6 gene is highly conserved and crucial for eye development across animal phyla.

Writing Tips (Expanded)

Use iris colour, texture, and changes (e.g., dilation, limbal ring prominence) to convey emotion, attraction, or health.
Describe the iris in close-up scenes to heighten intimacy or tension.
Consider using iris abnormalities (aniridia, heterochromia, pigment rings) for character distinction.

Structure & Function

Stroma: The front layer, containing blood vessels, melanocytes (melanin-producing cells), and connective tissue. The density and arrangement of these elements affect eye colour and the presence of features like the limbal ring.
Pigment epithelium: The back layer, rich in melanin, which prevents light from scattering inside the eye.
Sphincter and dilator muscles: Control pupil constriction and dilation, responding to light and emotional states (including sexual arousal).

Eye Colour Determination

Eye colour is determined by the amount and type of melanin in the iris stroma and pigment epithelium, as well as the scattering of light (Tyndall effect). Blue and green eyes result from low melanin and structural colour, while brown eyes have high melanin content.

Writing Tips

Use iris colour and changes (e.g., dilation, limbal ring prominence) to convey emotion, attraction, or health.
Describe the iris in close-up scenes to heighten intimacy or tension.