sclera
Meaning of sclera: The white of the eye is a shell of dense collagen, and its color is one of the easiest clues to read.
Definition of sclera
The sclera is the tough, fibrous white outer coat that surrounds most of the eyeball, meeting the clear cornea at the front. It is built from dense layers of collagen, which give the eye its shape, protect the delicate structures inside, and provide a firm anchor for the muscles that swivel the eyeball in its socket.
Why is the sclera white when the cornea is clear?
Both are made largely of the same protein, collagen, and the difference lies entirely in the arrangement. In the cornea the fibers are thin, uniform and stacked in precise lattices with regular spacing, so light passes through almost untouched. In the sclera the fibers vary in thickness and run in every direction, scattering light rather than transmitting it, which an observer reads as white. The sclera is also thicker and, unlike the cornea, carries its own blood supply. The two coats meet at a ring called the limbus, where transparent tissue gives way to opaque.
What sits on top of it?
A thin, transparent membrane called the conjunctiva covers only the exposed front portion of the sclera before folding back to line the inside of the eyelids. It does not wrap the whole globe. Between the two lies a loose, vessel rich layer, the episclera. That layering explains a distinction which looks trivial and is not: inflammation of the surface vessels is uncomfortable and largely harmless, while inflammation of the sclera itself brings a deep, boring ache, often disturbs sleep, and is frequently tied to a whole body inflammatory disease such as rheumatoid arthritis. The six muscles that turn the eye anchor into the sclera, and the optic nerve leaves through a sieve like opening at the back.
What does a change in its color mean?
| Color | What lies behind it |
|---|---|
| Yellow | Bilirubin building up in the blood and binding to the elastic fibers of the sclera, the visible sign of jaundice |
| Red, spread across the surface | Widening of the small vessels of the overlying conjunctiva, as in conjunctivitis or simple irritation |
| Red, a solid patch | A small bleed beneath the conjunctiva, painless and reabsorbed over a couple of weeks |
| Blue or gray | An unusually thin sclera letting the dark layer beneath show through, seen in some inherited collagen disorders and normally in newborns |
The sclera needs no equipment to inspect and sits in plain view, which is why its color earns attention out of all proportion to the size of the tissue involved.
Also known as
Used in a sentence
Examination of the sclera revealed a mild yellow discoloration consistent with elevated bilirubin.
Good to know
Yellowing of the sclera reflects a buildup of bilirubin in the blood, which is a different finding from the redness of inflamed vessels in the conjunctiva covering its front surface.
Word origin
Where “sclera” comes from
Formed from Greek skleros, meaning hard, reflecting the tissue's firm, fibrous consistency.