What is the structure of the sclera, episclera and Tenons capsule?

The Fibrous Tunic (tunica fibrosa oculi).—The sclera and cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.


Tenon's capsule is a tenuous tissue layer composed of dense collagen (a fascia of sorts)that lies between the episclera and substantia propria. It extends forward from the rectus muscle insertions becoming thinner as it moves anteriorly. It is thicker in children than adults. It forms a surgical plane that in histologic sections is quite ill-defined.

The Episclera is composed of loose vascularized layers of collagen that lie immediately beneath Tenon's capsule. The collagen fibrils in the episclera are larger than they are in the substantia propria and they are circumferentially arranged.

The Sclera.—The sclera has received its name from its extreme density and hardness; it serves to maintain the form of the eye. It is much thicker behind than in front; the thickness of its posterior part at the macula 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. The sclera thins to 0.3 mm just behind the rectus muscle insertions and this area is extremely vulnerable to traumatic rupture. In fact this is the most common site of a ruptured globe due to blunt trauma. At the equator the sclera measures 0.4-0.5 mm in thickness. The anterior sclera is covered by the conjunctiva.
The inner surface of the sclera is brown in color and marked by grooves, in which the ciliary nerves and vessels are lodged. Behind the sclera 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 the lamina cribrosa. Small holes in this lamina transmit nerve fibers of the optic nerve, and the fibrous septa dividing the lamina 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 the vortex veins . 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, (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.
The sclera is formed of white fibrous tissue intermixed with fine elastic fibers; flattened connective-tissue corpuscles, some of which are pigmented, are contained in cell spaces between the fibers. The fibers are aggregated into bundles, which are arranged chiefly in a longitudinal direction. Its vessels are not numerous, the capillaries being of small size, uniting at long and wide intervals. Its nerves are derived from the ciliary nerves.