The document discusses the anatomy and physiology of the limbus and sclera. The limbus is the transitional zone between the cornea and sclera, characterized externally by the anterior limbal border, mid limbal line, and posterior limbal border. The sclera forms the opaque outer layer of the eye and varies in thickness, being thickest posteriorly. Microscopically it consists of episcleral tissue, sclera proper with dense collagen bundles, and inner lamina fusca. The arrangement and composition of collagen fibers gives the sclera its opacity compared to the cornea. Physiologically, the sclera maintains ocular shape and volume through its hydration state and ability