Chapter 24: Eye: Structure & Histology

The outer fibrous layer, or corneoscleral coat, includes the transparent cornea—the primary refractive element composed of five distinct layers, including the highly regular corneal stroma necessary for clarity—and the opaque, dense fibrous connective tissue of the sclera, which provides attachment for the six extrinsic muscles controlling eye movement. The middle vascular layer, known as the uvea, consists of the posterior, nutrient-supplying choroid; the anterior, ring-like ciliary body, which generates aqueous humor and houses the ciliary muscle responsible for changing lens shape (accommodation); and the iris, a contractile diaphragm that adjusts pupil size using the sphincter pupillae (constriction) and dilator pupillae (dilation) muscles. The aqueous humor, which fills the anterior and posterior chambers and nourishes the avascular cornea and lens, drains through the trabecular meshwork near the iridocorneal angle into the scleral venous sinus (canal of Schlemm); interference with this outflow mechanism is the underlying cause of glaucoma. The innermost layer is the retina, comprising the outer retinal pigment epithelium (RPE), which phagocytoses discarded photoreceptor discs and maintains the blood-retina barrier, and the nine-layered neural retina. Light is detected by specialized photoreceptor cells: the highly sensitive rods, utilized for low-light black-and-white vision via the pigment rhodopsin, and the cones, responsible for color and detailed vision via iodopsin, concentrated most heavily at the fovea centralis. Vision involves light inducing a conformational change in retinal (a Vitamin A derivative) that hyperpolarizes the photoreceptor cell, decreasing neurotransmitter (glutamate) release to the bipolar cells, which then signal the ganglion cells whose axons form the optic nerve. Supporting structures include the lens, a transparent, biconvex structure suspended by zonular fibers which loses elasticity with age (presbyopia) or transparency (cataract), and accessory structures like the eyelids and the protective, goblet cell-rich conjunctiva (which contains CALT and lines the conjunctival sac) and the lacrimal gland, which secretes tears.