Ocular Anatomy, Physiology, and Pathology
Anterior segment
The tear film (lipid from meibomian glands, aqueous from the lacrimal gland, mucin from goblet cells) is the eye's first refracting surface. The cornea has five layers: epithelium, Bowman membrane, stroma (about 90 percent of thickness), Descemet membrane, and endothelium. It supplies roughly two-thirds of the eye's total power (about 43 D). Endothelial cells do not regenerate; they pump fluid out of the stroma, so cell loss from age, surgery, or trauma produces corneal edema and haze. The ciliary epithelium produces aqueous humor, which passes through the pupil and drains through the trabecular meshwork into Schlemm canal, setting intraocular pressure and nourishing the avascular cornea and lens.
Posterior segment
The crystalline lens changes power by accommodation: ciliary muscle contraction relaxes the zonules and the lens rounds up. Rods serve dim light and peripheral vision; cones serve acuity and color and are densest at the fovea. The optic nerve head creates the physiologic blind spot, and the macula lies temporal to it.
Pathology candidates must recognize
Cataract (lens opacity), primary open-angle glaucoma (increased trabecular outflow resistance with gradual field loss), angle-closure glaucoma (iris occludes the angle, acute pain, halos, blurred vision), diabetic retinopathy (microaneurysms, hemorrhage, neovascularization), age-related macular degeneration (dry with drusen; wet with choroidal neovascularization), keratoconus (corneal thinning cone with irregular astigmatism), pterygium and pinguecula, dry eye, amblyopia (reduced acuity from abnormal visual development, no structural lesion), and retinal detachment.
Traps and rule of thumb
Questions confuse aqueous with vitreous production and open-angle with angle-closure mechanisms, and often treat Bowman membrane as regenerative. Sudden painless vision loss, new flashes or floaters, or a new field defect always warrants urgent referral.