The sclera is the tough, opaque white portion of the fibrous layer of the eyeball. It is the largest part of the fibrous layer, forming its posterior five-sixths and extending from the cornea (Read more!) to the optic disc of the retina (Read more!).

The thickness of the sclera varies, decreasing from 0.04 - 0.05 inches (1 - 1.3 mm) at the posterior pole to 0.02 inches (0.5 mm) at the equator. Then it becomes progressively thicker anteriorly, increasing to 0.03 inches (0.8 mm) at the limbus. It is the thinnest behind the insertion sites of the extraocular muscles (around 0.01 inches (0.3 mm)).

The sclera is further subdivided into three sublayers:

The sclera also serves as an attachment site for the extraocular muscles. Overall, its outer surface is smooth, except at the sites where the tendons of the muscles attach. Additionally, the fibers of the optic nerve (CN II) (Read more!) penetrate it at the posterior pole of the eyeball via the posterior scleral foramen. Here, the outer surface of the sclera fuses with the dural sheath of the optic nerve (CN II). In contrast, the inner surface forms the lamina cribrosa, which is a sieve-like area with many small perforations that allow the fibers of the optic nerve to pass through.

The sclera contains three sets of openings that allow the nerves and blood vessels to enter and exit the eyeball. The anterior apertures are found near the limbus at the attachment sites of the rectus muscles. They transmit the anterior ciliary vessels. The middle apertures are four to five in number and are found slightly behind the equator. They transmit the vortex veins. And finally, the posterior apertures are situated around the optic disc and the posterior scleral foramen, transmitting the short and long posterior ciliary nerves and vessels.

The sclera acts as a barrier to other structures of the eyeball and serves as the main protective layer for the structures lying beneath it. Also, its rigid structure helps the eyeball to maintain its shape and keep all other structures in place.

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Sclera (part 1)