The Sclerotic and Cornea

The sclerotic and cornea form the external tunic of the eyeball; they are essentially fibrous in eye structure, the sclerotic being opaque, and forming the posterior five sixths of the globe; the cornea, which forms the remaining sixth, being transparent.

The sclerotic has received its name from its extreme density and hardness; it is a firm, unyielding, fibrous membrane, serving to maintain the form of the globe. It is much thicker behind than front. Its external surface is of a white color, and is in contact with the inner surface of the capsule of Tenon; it is quite smooth, except at the points where the Rocti and Obliqui muscles are in the whitness and brilliancy of the front of the eyeball. Its inner surface is stained of a brown color, marked by grooves, in which are lodged the ciliary nerves and vessels; this is loosely connected by an exceedingly fine cellular tissue (lamina fusca) with the outer surface of the choroids, an extensive lymph-space (perichoroidal) intervening between the sclerotic and choroids. Behind it is pierced by the optic nerve, and is continuous with its fibrous sheath, which is derived from the dura mater. At the point where the optic nerve passes through the sclerotic, this tunic forms a thin cribriform lamina (the lamina cribrosa); the minute orifices in this region 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-fibres. One of these openings larger than the rest, occupies the center of the lamella; it transmits the arteria centralis retinae to the interior of the eyeball. Around the eribriform lamella are numerous small apertures for the margin of the cornea and the entrance of the optic nerve are four or five large apertures, for the transmission with that of the cornea by direct continuity of tissue, but the opaque sclerotic slightly overlaps the outer surface of the transparent cornea.