Twenty-four structures, each a door into OphthoSpace

The clear, avascular window at the front of the eye, about half a millimetre thick, which provides most of the eye's focusing power. Five layers: epithelium, Bowman's layer, stroma, Descemet's membrane, endothelium.

Abrasions, ulcers and dendrites live here; fluorescein and cobalt blue light show them.

The aqueous-filled space between the cornea and the iris. Aqueous is made by the ciliary body, passes through the pupil, and drains at the angle through the trabecular meshwork.

Cells and flare mean uveitis; a hypopyon or hyphaema settles here; a shallow chamber warns of angle closure.

The coloured diaphragm with the pupil at its centre. The sphincter pupillae (parasympathetic, CN III) constricts it; the dilator pupillae (sympathetic) widens it.

Posterior synechiae stick it to the lens in uveitis; its root forms the angle that closes in acute angle-closure glaucoma.

The ring of tissue behind the iris root: the ciliary muscle, which drives accommodation, and the ciliary processes, which secrete aqueous humour into the posterior chamber.

It makes the aqueous that intraocular pressure depends on, and it is where intermediate uveitis and the pain of ciliary spasm arise.

A transparent biconvex body of about 10 mm diameter behind the iris, held by the zonules. It changes shape to focus near objects (accommodation) and stiffens with age (presbyopia).

Cataract is the lens losing its clarity; refractive error is the lens and cornea missing the retina.

The suspensory ligament of the lens: fine fibres running from the ciliary processes to the lens capsule. Ciliary muscle contraction relaxes them and lets the lens round up for near focus.

Weak zonules (trauma, pseudoexfoliation, Marfan syndrome) let the lens subluxate.

The clear gel filling the four fifths of the eye behind the lens, attached most firmly at the vitreous base near the ora serrata and around the optic disc.

As it liquefies with age it can pull away from the retina: flashes, floaters, and sometimes a retinal tear.

The light-sensitive neural layer lining the back of the eye, ten layers deep, from the photoreceptors against the pigment epithelium to the nerve fibres that gather at the disc.

Diabetic and hypertensive retinopathy, vein and artery occlusions, and detachment are all read on its surface.

The central five to six millimetres of retina, with the fovea at its centre: a pit of tightly packed cones and no vessels that gives fine, colour vision. It lies about four millimetres temporal to the disc.

Macular oedema, degeneration and holes take central vision; OCT shows them in cross section.

The vascular, pigmented layer between retina and sclera. Its choriocapillaris feeds the outer retina, and with the ciliary body and iris it makes up the uvea.

Choroidal neovascularisation is the wet form of macular degeneration; posterior uveitis and melanoma arise here.

The tough white coat of the eye, continuous with the cornea at the limbus, pierced posteriorly by the optic nerve and its vessels. The extraocular muscles insert onto it.

Scleritis is a deep, boring pain that wakes the patient; episcleritis is superficial and blanches with phenylephrine.

Where a million retinal nerve fibres turn to leave the eye, with the central retinal artery and vein at its centre. About 1.5 mm across, with a pale central cup and a pink rim.

Cupping means glaucoma; swelling means papilloedema or optic neuritis; pallor means atrophy.

The second cranial nerve: about 50 mm from the disc to the chiasm, myelinated behind the lamina cribrosa and wrapped in the meninges, so it carries cerebrospinal fluid and pressure with it.

Optic neuritis, ischaemic optic neuropathy and papilloedema each mark it in their own way; a relative afferent pupillary defect is its signature.

Elevates the eye, most purely in abduction; also intorts and adducts. Oculomotor nerve (CN III).

Tested with the eye turned out and up.

Depresses the eye, most purely in abduction; also extorts and adducts. Oculomotor nerve (CN III).

Entrapped in orbital floor fractures, which limits upgaze.

Adducts the eye. Oculomotor nerve (CN III).

Paired with the opposite lateral rectus in horizontal gaze through the medial longitudinal fasciculus.

Abducts the eye. Abducens nerve (CN VI).

A sixth nerve palsy leaves the eye unable to abduct, with horizontal diplopia worse looking to that side.

Runs through the trochlea and inserts behind the equator: it intorts, depresses (most in adduction) and abducts. Trochlear nerve (CN IV).

A fourth nerve palsy gives vertical diplopia worst looking down and in, with a head tilt away from the palsy.

Arises from the orbital floor near the lacrimal fossa and inserts behind the equator: it extorts, elevates (most in adduction) and abducts. Oculomotor nerve (CN III).

Overacts in a fourth nerve palsy, so the affected eye shoots up in adduction.

The cartilaginous pulley on the superomedial orbital rim that turns the superior oblique tendon back and out.

It is why the superior oblique's action is opposite to where its belly sits.

In the superotemporal orbit, it secretes the aqueous layer of the tear film onto the eye through ducts in the upper fornix.

Dry eye, dacryoadenitis and lymphoma involve it; it sits under the temporal upper lid.

Puncta, canaliculi, the lacrimal sac and the nasolacrimal duct carry tears to the nose beneath the inferior turbinate.

Obstruction means a watery eye; infection of the sac is dacryocystitis.

The dense fibrous plate that gives the upper lid its shape and holds the meibomian glands.

A chalazion is a blocked meibomian gland inside it; the lid everts over its upper edge.

The smaller plate of the lower lid.

Laxity of the lower lid turns it in (entropion) or out (ectropion).