Anatomy, physiology, and embryology of the visual system
The anatomy, physiology and embryology domain tests whether you can rebuild the eye from memory and use that structure to explain clinical signs. Examiners rarely ask isolated facts; they ask which layer is spared, which derivative failed, or which fluid pathway is blocked, so learn the anatomy as functional layers and the embryology as named migrations.
Layers and their numbers
Cornea: epithelium, Bowman layer, stroma, Descemet membrane, endothelium; central thickness about 0.52 to 0.55 mm, thicker peripherally. The endothelium is a pump, not only a barrier; human endothelial cells barely divide, so cell loss is permanent and decompensation follows density loss. Aqueous is secreted by the ciliary epithelium at roughly 2 to 2.5 microlitres per minute; outflow is about 85 percent trabecular and pressure-dependent and 15 percent uveoscleral and pressure-independent. The vitreous base straddles the ora serrata and is the strongest vitreoretinal adhesion, which is why tears occur there. The optic nerve carries about 1.2 million axons and is vulnerable at the orbital apex, where the annulus of Zinn gives origin to four recti.
Physiology worth quoting
Phototransduction hyperpolarises the photoreceptor; rhodopsin regeneration underlies dark adaptation. Foveal cones are avascular and depend on the choriocapillaris; the foveola has no inner retinal layers. The tear film is lipid, aqueous and mucin, and the mucin layer comes from goblet cells.
Embryology that explains disease
The optic vesicle evaginates around day 22 to 25, invaginates to form the cup, and the embryonic fissure closes by week 7. Failure gives a typical inferonasal coloboma of iris, ciliary body, choroid and disc. Lens and corneal epithelium come from surface ectoderm; corneal stroma and endothelium, sclera, iris stroma and ciliary muscle are neural crest; extraocular muscles and vascular endothelium are mesoderm.
Worked example
A newborn has a keyhole pupil inferonasally and a corresponding chorioretinal defect. Reasoning: the defect respects the inferonasal meridian, exactly the line of the embryonic fissure, so closure failed in week 7. The retina itself formed, so this is a coloboma, not retinal dysplasia. Look for midline systemic defects, not for a retinal dystrophy.
Traps
Calling the lens neural ectoderm; attributing corneal endothelium to mesoderm; forgetting the vitreous base when explaining where a retinal tear occurs.