The choroid is the largest, posterior part of the vascular, pigmented middle layer of the eyeball. It appears dark brown in color and is located between the sclera (Read more!) externally and the retina (Read more!) internally.

Anteriorly, the choroid ends at the ora serrata (Read more!). Posteriorly, it is pierced by the optic nerve (CN II) (Read more!). The outer surface of the choroid is attached to the inner surface of the sclera, while the inner surface of the choroid is attached to the outer surface of the retina.

The choroid is continuous with the ciliary body (Read more!) anteriorly. It is the thickest at the posterior pole of the eyeball. There is a potential space between the sclera and choroid - the suprachoroidal or perichoroidal space. It is traversed by the long and short posterior ciliary arteries and nerves.

This layer is highly vascular, accounting for 85-90% of the total blood flow in the eyeball. Most of the blood vessels are branches of the short posterior ciliary arteries, which arise from the ophthalmic artery. Also, tributaries of the vortex veins are found in this layer.

Besides blood vessels, the choroid also contains connective tissue and melanocytes that produce melanin. Melanin helps absorb excess light, preventing light reflection and scattering within the eyeball and improving visual clarity.

As the choroid is highly vascular, it is responsible for the oxygen and nutrient supply to the outer layers of the retina. In addition, it contributes to thermoregulation within the eye and may indirectly influence intraocular fluid dynamics, helping modulate the intraocular pressure.

Unlock with Premium

Choroid