142. Functional Anatomy of Eye – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• The eye is a specialized sensory organ that converts light into neural signals. Its optical system focuses light on the retina, while the neural system converts it into electrical signals that reach the visual cortex through the optic nerve.
• The eyeball is approximately 24 millimetres in diameter and has three coats:
  • Sclera – tough, opaque outer coat that protects the eye and provides attachment for extraocular muscles.
  • Choroid – vascular middle coat that supplies the outer retina and absorbs scattered light.
  • Retina – inner neural coat containing rods, cones and associated neurons.
• The cornea forms the anterior one-sixth of the fibrous coat and provides nearly two-thirds of the total refractive power. It is avascular and receives oxygen mainly from the tear film. Its five layers from anterior to posterior are: epithelium, Bowman’s layer, stroma, Descemet’s membrane and endothelium.
• The lacrimal gland produces tears that lubricate, nourish and protect the cornea. Tears contain antimicrobial substances such as lysozyme and immunoglobulin A and drain into the nasal cavity through the nasolacrimal duct.
• The ciliary body produces aqueous humor and controls accommodation. Contraction of the ciliary muscle relaxes zonular fibers, allowing the elastic lens to become more convex for near vision. Relaxation of the muscle flattens the lens for distance vision.
• The iris controls the amount of light entering the eye. Parasympathetic contraction of the sphincter pupillae causes miosis, whereas sympathetic contraction of the dilator pupillae causes mydriasis.
• Aqueous humor is formed by the ciliary epithelium, enters the posterior chamber, passes through the pupil into the anterior chamber, and drains mainly through the trabecular meshwork → canal of Schlemm → episcleral veins. Normal intraocular pressure is about 10–21 millimetres of mercury.
• Increased resistance to aqueous drainage causes glaucoma, which produces optic nerve damage and progressive visual field loss. Open-angle glaucoma develops from increased resistance in the trabecular pathway, whereas angle-closure glaucoma results from obstruction of the drainage angle by the peripheral iris.
• The lens is transparent, biconvex and avascular. It focuses light on the retina and changes its curvature during accommodation. Loss of lens elasticity with age causes presbyopia, while loss of transparency produces cataract.
• Vitreous humor is a transparent gel containing water, collagen and hyaluronic acid. It supports the retina, maintains the shape of the eyeball and absorbs mechanical shocks.
• The retina contains rods and cones. Its ten layers, from outer to inner, are: retinal pigment epithelium, photoreceptor layer, external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fibre layer and internal limiting membrane.
• The fovea centralis provides the highest visual acuity because it contains densely packed cones, minimal neuronal convergence and displaced inner retinal layers. The optic disc contains no photoreceptors and therefore produces the physiological blind spot.
• The fundus contains the optic disc, macula, fovea, retina and retinal vessels. Fundus examination can reveal ocular and systemic diseases such as diabetes, hypertension and atherosclerosis.