Basic Ocular Anatomy and Physiology
What the exam tests
Basic Ocular Anatomy and Physiology asks you to link each structure to its function and to the clinical sign that follows when it fails. Expect questions on corneal layers, the aqueous pathway, retinal photoreceptors, extraocular muscles and the lacrimal drainage system.
Cornea and chambers
The cornea has five layers: epithelium, Bowman's layer, stroma, Descemet's membrane and endothelium. The endothelium does not regenerate; it actively pumps fluid out of the stroma. Endothelial loss therefore causes stromal oedema and corneal haze. The anterior chamber lies between cornea and iris; the posterior chamber lies between iris and lens. Aqueous is secreted by the ciliary epithelium at about 2–3 µL/min, passes through the pupil and drains through the trabecular meshwork into Schlemm's canal. Resistance here raises intraocular pressure.
Retina and muscles
The retina has about 120 million rods and 6 million cones. Rods contain rhodopsin and serve dim-light vision; cones cluster at the fovea and serve daylight and colour vision. The optic nerve carries about 1.2 million axons. Four rectus and two oblique muscles move the eye; the superior oblique is supplied by cranial nerve IV and the lateral rectus by VI.
Worked example
A patient has corneal haze after a chemical injury. Reasoning: the endothelium has been damaged, so it cannot remove stromal fluid; the stroma hydrates and loses transparency. The correct conclusion is endothelial dysfunction, not a simple epithelial defect.
Traps
Do not confuse Bowman's layer with Descemet's membrane, describe the lens as having a blood supply, or say aqueous is produced in the anterior chamber. Remember that the fovea is cone-only and avascular.