Sclera

The sclera makes up 5% of the dense fibrous membrane of the eye and performs a protective and skeletal function, i.e. it determines and provides the shape of the eye. It is opaque, has a shiny white, tendon-like appearance.

The sclera consists of dense collagen tissue and elastic fibers, especially numerous in the places where the eye muscles are attached. The sclera is poor in cellular elements, but it does contain pigment cells, which are grouped mainly around the vessels and nerves passing through the sclera and are sometimes visible on the outer surface as dark spots. The sclera lacks its own epithelial and endothelial coverings.

On the outside, the superficial layers of the sclera are loose, they form a thin layer of episclera, which merges with the even looser subconjunctival tissue of the eyeball. In front, the sclera passes into the cornea, and behind, its superficial layers merge with the hard shell of the optic nerve.

The thickness of the sclera in different places varies within the range of 0.4-1.2 mm. The thickness of the sclera is insignificant in the area of the equator of the eye (up to 0.4 mm) and in front of the attachment of the rectus muscles of the eye. In the place of attachment of the eye muscles and especially in the circumference of the optic nerve, where its hard shell is woven into the sclera, the thickness of the sclera reaches 1.2 mm.

The sclera is poor in vessels and nerves. It receives blood from the anterior and posterior ciliary vessels, which form the episcleral network, giving off branches to the sclera; sensory nerves go to the sclera from the long and short ciliary nerves. Numerous arteries, veins and nerves pass through the sclera (near the optic nerve, in the equator region, near the cornea) to supply and innervate the cornea and the vascular tract of the eye. The sclera contains less water than the cornea, 10% protein and mucopolysaccharides.

The scleral stroma consists of collagen bundles of varying sizes and shapes that are not oriented as orderly as in the cornea.

The inner layer of the sclera (lamina fusca) passes into the suprachoroidal and supraciliary layers of the uveal tract.

Anteriorly, the episclera consists of dense vascularized connective tissue that lies between the superficial scleral stroma and Tenon's capsule.

The anterior surface of the sclera is covered by three vascular layers.

1. The conjunctival vessels are the most superficial layer; the arteries are tortuous, the veins are straight.
2. The vessels in Tenon's capsule have a straight course with a radial configuration. In episcleritis, the greatest stagnation of blood occurs in this vascular plexus. When palpated, it shifts above the surface of the sclera. Tenon's capsule and episclera are infiltrated with inflammatory cells, and the sclera itself does not swell. Instillation of phenylephrine causes pallor of the conjunctiva and partly of Tenon's capsule, allowing the underlying sclera to be seen.
3. The deep vascular plexus is located in the superficial layers of the sclera and is associated with maximum congestion in scleritis. Some injection of the superficial vessels is inevitable, but it is insignificant. Instillation of phenylephrine has no effect on the dilated vessels of this plexus. To localize the level of maximum injection, examination in daylight is necessary. The stroma of the sclera is largely avascular.

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