Anatomy of the Outer Layers of the Globe

The sclera, along with the cornea anteriorly, forms a tough protective shell for the eye. On the outer surface of the sclera are three distinct layers, from inner to outer, the episclera, Tenon’s capsule (fascia bulbi) and conjunctiva (highlighted in red in the image).

The sclera is avascular and is supplied by the choroid inside the eye and also from a deep vascular plexus lying in the episclera. It is this plexus which becomes engorged, dilated and therefore easily visible, as part of the inflammatory process accompanying scleritis.

The episclera also contains a superficial vascular plexus, which is distinct from the conjunctival vessel network, and which becomes inflamed in episcleritis.

Learning bite

An understanding of the vascular supply of the different layers of the eye is important in differentiating clinically between episcleritis and scleritis.