Optic Nerve

The cornea is divisible into five distinctive layers: the epithelium, Bowman's layer, the stroma, Descemet's membrane and the endothelium. The non-keratinising stratified squamous epithelium consists in the centre of the cornea of five layers, increasing to nine or ten layers at the periphery. Bowman's layer is believed to represent a modified layer of the stroma. It is composed of small collagen fibrils surrounded by a mucoprotein ground substance. The stroma is avascular and consists of col-lagenous lamellae arranged in an almost parallel manner interspersed with flattened fibroblasts (keratocytes). Descemet's membrane is a faintly eosinophilic staining, PAS-positive, acellular structure, formed by corneal en-dothelial cells. The underlying endothelium is a single layer of polygonal cells.

The axons from the retinal nerve fibre layer converge at the optic disc. The axons in the nerve fibre layer of the optic disc form a bulge as they pass through the lamina cribrosa, a sieve-like plate of connective tissue formed by fibroblasts, ingrowing from the posterior part of the adjacent sclera. This bulge is larger on the nasal side. The tissue anterior to the lamina derives blood supply from the posterior ciliary arteries, and the tissue posterior to the lamina has a meningeal blood supply derived from branches of the ophthalmic and central retinal artery.

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