Anatomy of the middle layer of the eyeball / uvea
The uveal tract consists of iris, ciliary body and choroid. Anatomically, they are continuous and so disease of one part may spread to the other. The uveal tract is the vascular layer of the eye.
I. IRIS
- It is a coloured, free, circular diaphragm with a central aperture— the pupil, measuring about 4 mm.
- It regulates the amount of light reaching the retina.
- When pupil is constricted, more of the posterior surface of the iris is in contact with the lens capsule. When pupil is fully dilated, the iris may not touch the lens.
- At the periphery, the iris is attached to the middle of anterior surface of the ciliary body.
- The iris divides the space between the cornea and lens into the anterior and posterior chambers of eye.
Structure
Grossly, anterior surface of the iris can be divided into two zones by a zigzag line called the collarette.
- Ciliary zone: There are series of radial streaks (due to underlying radial blood vessels) and crypts. Crypts are depressions where the endothelium layer is missing.
- Pupillary zone: It is situated in between the collarette and the pigmented pupillary frill. It is relatively smooth and flat.
Histologically, iris consists of three layers.
- Endothelium: It contains crypts or tissue spaces which communicate freely with the anterior chamber.
- Stroma: It consists of loosely arranged connective tissue, blood vessels, nerves and unstripped muscles namely,
- Sphincter pupillae: It is 1 mm, narrow circular band surrounding the pupil. It is supplied by the cervical parasympathetic nerves via third cranial nerve and causes constriction of the pupil.
- Dilator pupillae: These are radial fibres extending from ciliary body to the pupillary margin. It is supplied by cervical sympathetic nerves and causes dilatation of the pupil.
- Pigment epithelium: Two layers of pigment epithelium are situated on the posterior surface of iris.