The uvea is the middle layer of the eye, a vascular coat situated between the outer fibrous layer, which includes the sclera and cornea, and the inner light-sensitive retina. This structure, sometimes referred to as the vascular tunic, wraps around most of the eyeball. It supports surrounding tissues and contributes to various eye functions, playing an important role in overall eye health and vision.
The Iris
The iris is the visible colored part of the eye, a thin, circular structure in front of the lens. At its center lies the pupil, a small, black opening that regulates the amount of light entering the eye. Muscles within the iris, specifically the sphincter pupillae and dilator pupillae, work in opposition to control the pupil’s size. The sphincter muscle constricts the pupil in bright light, while the dilator muscle expands it in dimmer conditions, a process known as the pupillary reflex.
The iris’s color is determined by the amount of melanin, a natural pigment, present within its stroma. This melanin content is influenced by genetics. The iris attaches to the sclera and the ciliary body. The iris and the ciliary body together form the anterior uvea.
The Ciliary Body
The ciliary body is a ring-shaped structure located behind the iris. It connects the choroid to the iris. This structure contains the ciliary muscle and ciliary processes. The ciliary body performs two main functions.
One function is the production of aqueous humor, a transparent, water-like fluid similar to blood plasma. The ciliary epithelium secretes this fluid into the posterior chamber of the eye. Aqueous humor nourishes the eye’s front structures, such as the cornea and lens, which lack their own blood supply, and helps maintain intraocular pressure. The ciliary body also facilitates accommodation, the process by which the eye focuses on objects at varying distances. The ciliary muscles within the ciliary body contract or relax, changing the shape of the lens, allowing for clear vision at different focal lengths.
The Choroid
The choroid is a highly vascularized layer of the uvea, positioned between the sclera (the white outer layer of the eye) and the retina (the light-sensitive inner layer). It extends from the ciliary body to the optic nerve, surrounding the rear two-thirds of the eyeball. This thin tissue layer is densely packed with blood vessels, making it the primary supplier of oxygen and nutrients to the outer layers of the retina and the optic nerve, which require a substantial blood supply due to their high metabolic activity.
Beyond its role in nourishment, the choroid also contributes to light management within the eye. It contains melanin, which gives it a dark color. This dark color helps absorb excess light that enters the eye, preventing internal reflections that could otherwise scatter and distort the visual image. The choroid’s thickness varies, being approximately 0.2 mm at the back of the eye and narrowing to about 0.1 mm in the peripheral areas.