Eye — Horizontal Section
Right eyeball viewed from above · path of light and aqueous humour
A horizontal (transverse) section through the right eyeball seen from above: the cornea faces left, the temporal side is at the top and the nasal side — with the optic nerve leaving ≈3–4 mm nasal to the posterior pole — at the bottom. Proportions follow the adult eye (axial length ≈24 mm). The three tunics are shown as concentric layers: fibrous (sclera and cornea), vascular (choroid, ciliary body, iris) and neural (retina, with the yellow macula, the foveal pit and the optic disc).
Animated rays of light are refracted most strongly at the curved front of the cornea, then fine-focused by the lens to converge on the fovea. The inset magnifies the drainage (iridocorneal) angle: aqueous humour secreted by the ciliary processes flows through the posterior chamber, around the pupillary margin into the anterior chamber, and drains through the trabecular meshwork into the canal of Schlemm and the episcleral veins. Use the steps to follow both pathways.
Step-by-step
- Light 1 · Tear film and cornea
Light first meets the tear film and the steeply curved, transparent cornea. Because the change in refractive index is greatest at this air–tear interface, the cornea provides ≈43 D — about two-thirds of the eye's total focusing power — but its power is fixed.
- Light 2 · Anterior chamber and pupil
The converging rays cross the aqueous humour of the anterior chamber and pass through the pupil. The iris acts as an adjustable aperture (≈2–8 mm), controlled by the parasympathetic sphincter and sympathetic dilator pupillae.
- Light 3 · Lens and accommodation
The biconvex lens adds ≈15–20 D. For near vision the ciliary muscle contracts, the zonular fibres slacken and the elastic lens becomes rounder (accommodation); for distance the muscle relaxes and the taut zonule flattens the lens.
- Light 4 · Vitreous body
Light crosses the transparent vitreous gel (≈4 mL, ≈80% of the eye's volume), which keeps the globe's shape and holds the retina against the pigment epithelium.
- Light 5 · Focus on the fovea
In an emmetropic eye the rays converge on the fovea centralis — a cone-packed pit at the centre of the yellow macula where the inner retinal layers are displaced, giving the sharpest vision. Photoreceptors lie at the back of the retina, next to the pigment epithelium and choroid that nourish them.
- Light 6 · Optic disc and optic nerve
Ganglion-cell axons converge on the optic disc (the physiological blind spot, no photoreceptors) and leave through the lamina cribrosa of the sclera as the optic nerve, accompanied by the central retinal artery and vein.
- Aqueous 1 · Secretion by the ciliary body
The non-pigmented epithelium of the ciliary processes secretes aqueous humour (≈2–3 µL/min) into the posterior chamber, mainly by active transport involving Na⁺/K⁺-ATPase and carbonic anhydrase.
- Aqueous 2 · Through the pupil
Aqueous flows between the iris and lens, through the pupil into the anterior chamber, nourishing the avascular lens and cornea. If the iris is pressed against the lens (pupil block), pressure builds behind the iris and bows it forward.
- Aqueous 3 · Drainage at the angle
About 80–90% of aqueous leaves through the trabecular meshwork in the iridocorneal angle into the canal of Schlemm and on to the episcleral veins; the rest takes the uveoscleral route. Resistance here sets intraocular pressure (10–21 mmHg) — it is raised in open-angle glaucoma, and the angle is blocked in angle-closure glaucoma.