2.2 Cornea, Sclera, Anterior Chamber, Angle, Aqueous Dynamics & Crystalline Lens

Cornea

The cornea supplies roughly 43 dioptres of the eye's approximately 60 D of refractive power — about two thirds of the total. It measures about 11.5 to 12 mm horizontally and 10.5 to 11 mm vertically, is about 540 to 550 micrometres thick centrally and 650 to 700 micrometres peripherally.

Five layers, front to back:

1. Epithelium — five to seven cell layers, regenerates in 7 to 10 days from limbal stem cells; the site of abrasions and recurrent erosions.
2. Bowman's layer — acellular condensed stroma; does not regenerate, so injuries through it scar.
3. Stroma — about 90% of thickness; regularly spaced collagen lamellae produce transparency. Loss of that regular spacing (oedema) causes haze.
4. Descemet's membrane — the endothelial basement membrane; thickens with age and can be stripped (DMEK) or detached.
5. Endothelium — a single layer of hexagonal cells pumping fluid out of the stroma via Na⁺/K⁺-ATPase. Cell density falls from about 4,000 cells/mm² at birth to 2,000 to 2,500 in adults and does not regenerate; surviving cells enlarge (polymegethism) and lose hexagonality (pleomorphism).

Below roughly 500 cells/mm² the pump fails and stromal oedema with bullous keratopathy follows. That number is why specular microscopy is performed before cataract surgery in Fuchs dystrophy.

The cornea is avascular, nourished by the tear film (oxygen), the aqueous humour and the limbal vessels. It is one of the most densely innervated tissues in the body, supplied by the long ciliary nerves from V1, which is the basis of Cochet-Bonnet aesthesiometry.

Sclera and limbus

The sclera is opaque because its collagen fibres are irregular in diameter and spacing. It is thinnest behind the rectus insertions (about 0.3 mm) — the reason globe rupture from blunt trauma often occurs there — and thickest posteriorly near the optic nerve (about 1 mm). The limbus is the 1.5 to 2 mm transition zone housing limbal stem cells, the trabecular meshwork and Schlemm's canal, and it is the surgical landmark for essentially every anterior segment incision.

Anterior chamber and angle

The anterior chamber holds about 250 microlitres and is roughly 3.0 mm deep centrally in a phakic adult eye, shallowing with age as the lens grows. The angle structures, seen on gonioscopy from posterior to anterior, are:

1. Ciliary body band
2. Scleral spur
3. Trabecular meshwork (posterior pigmented, anterior non-pigmented)
4. Schwalbe's line — the termination of Descemet's membrane

The scleral spur is the key landmark: it is the white line just posterior to the pigmented meshwork and its visibility defines an open angle in most grading schemes.

Aqueous dynamics

Production. The non-pigmented ciliary epithelium of the pars plicata secretes aqueous at about 2 to 3 microlitres per minute, mostly by active transport using carbonic anhydrase, with smaller contributions from ultrafiltration and diffusion. Production falls during sleep, which is why nocturnal intraocular pressure behaves differently from daytime pressure.

Circulation. Posterior chamber → through the pupil → anterior chamber → angle.

Outflow.

The arithmetic is worth internalising: at 2.5 microlitres per minute into a 250-microlitre chamber, the entire anterior chamber volume is replaced in roughly 100 minutes. That is why a topical drop reaches therapeutic aqueous levels quickly and why anterior chamber washout after surgery is fast.

Crystalline lens

The lens is avascular, has no innervation, and is suspended by zonules of Zinn from the ciliary body. Its parts are the capsule (the thickest basement membrane in the body, thinnest posteriorly — the reason posterior capsule rupture is the classic phacoemulsification complication), the anterior epithelium (which lays down new fibres at the equator lifelong), the cortex and the nucleus.

Because fibres are added continuously and none are shed, the lens grows throughout life: from about 3.5 mm thick at birth to 5 mm or more by the eighth decade. Two consequences dominate clinical work: presbyopia, as the hardening nucleus resists the accommodative change in shape, and progressive shallowing of the anterior chamber, raising angle-closure risk in the hyperopic elderly.

During accommodation, parasympathetic fibres from cranial nerve III contract the ciliary muscle, the ciliary ring narrows, zonular tension decreases, and the elastic capsule moulds the lens into a steeper, more powerful shape. The common misconception is that zonules pull the lens round; they do the opposite — relaxation of zonular tension is what allows accommodation.