The sclera is composed of collagen and a few elastic fibers embedded in a matrix of proteoglycans. It terminates at the histologists’ limbus. The sclera does not have uniform thickness; it is thinnest immediately behind the muscle insertion, whereas it is thicker just posterior to the limbus.
The sclera is normally permeable to the passage of molecules in both directions. Up to 40% of the aqueous leaves the eye via uveoscleral outflow, making the sclera an important path of fluid movement. Scleral permeability allows drugs to be delivered to the eye by means of injection into the sub-Tenon space. The sclera is a hydrophilic tissue and is therefore only variably permeable to hydrophobic or amphiphilic substances or medications. This characteristic is an important consideration for periocular injection of pharmacologic agents.
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Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.