Key Concepts

The wall of the eye has three layers (tunics): the fibrous tunic (sclera and cornea), the vascular tunic (choroid, ciliary body, and iris), and the neural tunic (the retina). Light passes through the cornea, aqueous humor, pupil, lens, and vitreous humor before it reaches the photoreceptors.

The cornea provides about two-thirds of the eye’s focusing power. The lens fine-tunes focus: when you look at something near, the ciliary muscle contracts, the zonular fibers (also called the suspensory ligament) slacken, and the lens rounds up (accommodation). The iris controls pupil size through the parasympathetic and sympathetic nervous systems.

Nearsightedness (myopia), farsightedness (hyperopia), and presbyopia are all problems of where light comes to a focus relative to the retina. The optic disc has no photoreceptors, which creates the blind spot.