The sclera (from the Greek skleros, meaning hard), also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the eye containing collagen and elastic fiber.
The sclera has received its name from its extreme density and hardness; it is a firm, unyielding membrane, serving to maintain the form of the bulb.
It is much thicker behind than in front; the thickness of its posterior part is 1 mm.
Its external surface is of white color, and is in contact with the inner surface of the fascia of the bulb; it is quite smooth, except at the points where the Recti and Obliqui are inserted into it; its anterior part is covered by the conjunctival membrane.
Its inner surface is brown in color and marked by grooves, in which the ciliary nerves and vessels are lodged; it is separated from the outer surface of the choroid by an extensive lymph space (spatium perichorioideale) which is traversed by an exceedingly fine cellular tissue, the lamina suprachorioidea.
Behind it is pierced by the optic nerve, and is continuous through the fibrous sheath of this nerve with the dura mater.
Where the optic nerve passes through the sclera, the latter forms a thin cribriform lamina, the lamina cribrosa scleræ; the minute orifices in this lamina serve for the transmission of the nervous filaments, and the fibrous septa dividing them from one another are continuous with the membranous processes which separate the bundles of nerve fibers. One of these openings, larger than the rest, occupies the center of the lamina; it transmits the central artery and vein of the retina.
Around the entrance of the optic nerve are numerous small apertures for the transmission of the ciliary vessels and nerves, and about midway between this entrance and the sclerocorneal junction are four or five large apertures for the transmission of veins (venæ vorticosæ).
In front, the sclera is directly continuous with the cornea, the line of union being termed the sclero-corneal junction.
In the inner part of the sclera close to this junction is a circular canal, the sinus venosus scleræ (canal of Schlemm). In a meridional section of this region this sinus presents the appearance of a cleft, the outer wall of which consists of the firm tissue of the sclera, while its inner wall is formed by a triangular mass of trabecular tissue; the apex of the mass is directed forward and is continuous with the posterior elastic lamina of the cornea. The sinus is lined by endothelium and communicates externally with the anterior ciliary veins.