Iris

👁️ What Is the Iris?

The iris is the anterior (front) portion of the uveal tract, lying between the cornea and the lens. It functions like a camera's aperture—regulating the amount of light entering the eye by adjusting the size of the pupil.

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📍 Anatomical Location:

• Anterior to: the lens and the anterior surface of the vitreous

• Posterior to: the cornea and aqueous humor of the anterior chamber

• Suspended in the aqueous humor, it divides the anterior and posterior chambers

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📚 Layers and Structures of the Iris:

1. Anterior Border Layer

• Thin layer of fibroblasts and melanocytes

• Responsible for variations in eye color (more melanin = darker eyes)

2. Stroma

• Loose connective tissue containing:

  • Blood vessels

  • Pigment cells (melanocytes)

  • Immune cells

  • Nerves

• Contains the sphincter pupillae muscle

3. Muscles of the Iris

• Sphincter pupillae (circular muscle)

  • Located in the stroma, near the pupillary margin

  • Parasympathetic innervation (via CN III/Edinger–Westphal nucleus)

  • Constricts the pupil (miosis)

• Dilator pupillae (radial muscle)

  • Lies just anterior to the pigmented epithelium

  • Sympathetic innervation

  • Dilates the pupil (mydriasis)

4. Posterior Pigmented Epithelium

• Two layers of pigmented cuboidal cells

• Non-light-transmitting (dark) → prevents light scatter

• Continuous with the ciliary epithelium

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🎯 Functions of the Iris:

• Regulates the amount of light entering the eye

• Maintains the blood-aqueous barrier

• Helps form the anterior and posterior chambers

• Plays a role in ocular immune defense

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🧾 Clinical Notes:

• Anisocoria: Unequal pupil sizes (can be physiologic or pathologic)

• Iritis / Anterior uveitis: Inflammation of the iris, often painful with photophobia

• Synechiae: Adhesion of iris to lens (posterior) or cornea (anterior) due to inflammation

• Coloboma: Congenital defect where part of the iris is missing (keyhole-shaped pupil)