Q1 LONG ANSWER 10M ★ High-yield

Describe the anatomy of the eyeball with the structure of the cornea and the angle of the anterior chamber.

Ophthalmology · 3rd Professional

Dimensions — the adult eyeball has an anteroposterior (axial) diameter of 24 mm, a horizontal diameter of 23 mm and a vertical of 23.5 mm; it weighs 7 g and has a volume of 6.5 mL. The axial length is the single most important dimension clinically, since a variation of 1 mm alters the refraction by 3 dioptres

Dimensions — horizontally 11 to 12 mm and vertically 10 to 11 mm, so it is elliptical from the front and circular from behind; thickness 0.52 mm centrally and 0.67 mm peripherally
Refractive power +43 to +45 dioptres, which is nearly two-thirds of the total refractive power of the eye (58 to 60 D) — the cornea, not the lens, is the principal refracting surface

Layer	Features and clinical significance
1. Epithelium	5 to 6 layers of stratified squamous cells; regenerates completely from the limbal stem cells in 48 hours, leaving NO scar. Loss of limbal stem cells (chemical burns, Stevens-Johnson syndrome) causes conjunctivalisation of the cornea
2. BOWMAN’S membrane	An acellular condensation of stroma. It does not regenerate, so any injury breaching it heals with a permanent scar — which is why a corneal abrasion heals clear and a deeper ulcer does not
3. Stroma (substantia propria)	90% of the thickness; regularly arranged lamellae of collagen fibrils of uniform diameter and spacing.
4. Descemet’S membrane	The basement membrane of the endothelium; strong, elastic and regenerates; it resists infection, so an ulcer may perforate through it as a descemetocele — a clear bulge that is a surgical emergency
5. Endothelium	A single layer of hexagonal cells, 3,000 per square millimetre at birth, falling with age. It does not regenerate — cell loss is compensated by spreading. Below 500 cells per square millimetre the cornea decompensates and becomes oedematous. Its Na-K ATPase pump maintains deturgescence
(6.) DUA’S layer	A pre-Descemet layer described in 2013; of interest in lamellar keratoplasty

The four factors maintaining corneal transparency — avascularity; the regular lamellar arrangement of uniform collagen fibrils spaced less than a wavelength of light apart; relative dehydration (deturgescence), maintained actively by the endothelial pump and by the epithelial barrier; and non-myelinated nerve fibres

Sclera — opaque because its collagen fibrils are irregular in diameter and arrangement, the exact converse of the cornea. Thickest posteriorly (1 mm) and thinnest (0.3 mm) just behind the insertions of the recti, which is where it ruptures in blunt trauma and where a staphyloma forms
Limbus — the 1.5 to 2 mm transition zone between cornea and sclera. It contains the limbal stem cells in the palisades of Vogt, and is the surgical landmark for cataract and glaucoma incisions
Iris — contains the sphincter pupillae (circular, parasympathetic via the third nerve) and the dilator pupillae (radial, sympathetic). The pupil is normally 3 to 4 mm

Retina — ten layers. Rods number 120 million, are peripheral, and subserve scotopic (dim light) vision using rhodopsin; cones number 6 to 7 million, are concentrated at the macula, and subserve photopic vision, colour and acuity
The macula is 5.5 mm across; the fovea is 1.5 mm and the foveola 0.35 mm, containing only cones, with the inner layers displaced aside so that light reaches them directly. The foveal avascular zone has NO retinal vessels and depends entirely on the choroid — which is why a central retinal artery occlusion spares a small central island (the cherry-red spot with a patent cilioretinal artery)
Optic disc — 1.5 mm across, with no photoreceptors, giving the physiological blind spot 15 degrees temporal to fixation

Structure	Features
Crystalline lens	Biconvex, 9 to 10 mm across and 4 mm thick, of +16 to +20 D. It has NO blood supply and NO nerve supply, and is nourished entirely by the aqueous. It grows throughout life, laying down new fibres externally so that the nucleus is compressed and hardens — the basis of both presbyopia and nuclear cataract
Lens capsule	The thickest basement membrane in the body; thinnest at the posterior pole.
Zonules of ZINN	Suspend the lens from the ciliary body. Weak or absent in MARFAN syndrome (dislocation upward and temporally) and homocystinuria (downward and nasally) — a distinction worth remembering
Vitreous	4 mL, 99% water with collagen and hyaluronan; attached most firmly at the vitreous base astride the ora serrata, and also at the disc, macula and vessels. It liquefies with age (syneresis).
Extraocular muscles	Four recti and two obliques. All are supplied by the third nerve except the superior oblique (fourth, trochlear) and the lateral rectus (sixth, abducent) — remembered as LR6 SO4, all the rest 3
Insertions — the spiral of tillaux	The recti insert at increasing distances from the limbus: medial 5.5 mm, inferior 6.5, lateral 6.9 and superior 7.7 — a surgical landmark in squint operations

1 mm of axial length is 3 dioptres; this single relationship explains axial myopia, axial hypermetropia and why biometry before cataract surgery must be accurate to a fraction of a millimetre
Inject into the pars plana, 3.5 to 4 mm behind the limbus; anterior to it lies the ciliary body and posterior to it the retina, and the safe window is narrow
Count the endothelium before intraocular surgery in a compromised cornea; below 500 cells per square millimetre the cornea will decompensate, and the cells do not regenerate

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