21st February 2024

Layers of the Eyeball – Vascular Layer

The vascular layer (uveal tract) is the middle layer of the eye that includes the choroid, ciliary body, and iris. It plays several essential roles in supporting and nourishing the eye.

The Choroid

Positioned between the sclera and retina, the choroid is highly vascularised and rich in blood vessels. Its functions include providing nutrients and oxygen to the retina, absorbing excess light via melanocytes and thermoregulation.

Ciliary Body

The ciliary body, positioned just behind the iris, is a crucial structure for adjusting the eye’s focus. The ciliary muscle contracts and relaxes regularly, which alters the shape of the lens, allowing the eye to focus on objects at different distances. The muscle has smooth muscle fibres which attach to the lens of the eye by the ciliary processes. The ciliary body produces aqueous humour, which nourishes the cornea and lens while maintaining intraocular pressure. 

Iris

The iris is the coloured part of the eye, and it regulates the amount of light entering the eye through its central opening, the pupil. Its muscles control the pupil’s size in response to changes in light intensity. The iris is a thin, circular diaphragm suspended between the cornea and the lens. It divides the space between these structures into two chambers: the anterior chamber (in front of the iris) and the posterior chamber (behind the iris).

The iris has two types of smooth muscle fibres: dilator and sphincter pupillae muscles. Dilator pupillae, when contracted, enlarges the pupil, which allows more light to enter. Sphincter pupillae, when contracted, constrict the pupil, which prevents more light from entering.