Structure of the Eye

1. Outer Layer — Corneoscleral Coat (Fibrous Layer)

Sclera

• Forms the posterior five-sixths of the eyeball — the "white of the eye"
• Composed of dense fibrous connective tissue; provides mechanical protection and attachment points for extraocular muscles
• In children, slightly blue-tinged due to thinness; in elderly, yellowish due to lipofuscin accumulation
• Penetrated posteriorly by the optic nerve

Cornea

• Covers the anterior one-sixth of the eye; transparent and avascular
• 0.5 mm thick at center, ~1 mm at periphery
• The most powerful focusing (refracting) element of the eye
• Consists of five layers:
  1. Corneal epithelium — nonkeratinized stratified squamous epithelium
  2. Bowman membrane — acellular anterior basement membrane
  3. Corneal stroma — thickest layer; regularly arranged collagen lamellae and keratocytes
  4. Descemet membrane — acellular posterior basement membrane
  5. Corneal endothelium — single layer of flat cells; maintains corneal transparency via active fluid pumping

Limbus

• The corneoscleral junction; contains the trabecular meshwork (spaces of Fontana) and the canal of Schlemm (scleral venous sinus), which drain aqueous humor

2. Middle Layer — Vascular Coat (Uvea)

Choroid

• Lines the posterior portion of the eye between sclera and retina
• Richly vascular (venous plexuses, capillary layers); nourishes the outer retina
• Dense melanin pigment absorbs stray light to minimize internal glare
• Firmly attached to the retinal pigment epithelium

Ciliary Body

• A ring-like thickening just posterior to the corneoscleral junction
• Contains the ciliary muscle (smooth muscle): contraction changes lens shape → accommodation for near vision
• Ciliary processes produce aqueous humor, which fills the posterior and anterior chambers
• Zonular fibers (suspensory ligaments) run from ciliary processes to the lens equator

Iris

• A pigmented muscular diaphragm with a central aperture — the pupil
• Contains two smooth muscles:
  • Sphincter pupillae — constricts pupil (parasympathetic)
  • Dilator pupillae — dilates pupil (sympathetic)
• Iris pigmentation determines eye color; its stroma arises from mesoderm, while both epithelial layers are derived from neural ectoderm

3. Inner Layer — Retina

Retinal Pigment Epithelium (RPE)

• Single layer of pigmented cells; outer layer of the retina
• Supports and maintains photoreceptors; phagocytoses shed outer segments

Neural Retina (Photosensitive Part)

• Lines the posterior two-thirds of the eye (terminates anteriorly at the ora serrata)
• Contains five principal cell types arranged in layers:
  1. Photoreceptors (rods and cones) — detect light
  2. Bipolar cells — relay signals from photoreceptors
  3. Ganglion cells — axons form the optic nerve
  4. Horizontal cells — lateral processing in outer plexiform layer
  5. Amacrine cells — lateral processing in inner plexiform layer

• Fovea centralis (within the macula) — highest visual acuity; densely packed cones, no overlying neurons
• Optic disk (optic disc) — where ganglion cell axons exit; no photoreceptors → "blind spot"
• Macula lutea — surrounds the fovea; contains high cone density

Non-Photosensitive Retina

• Anterior to the ora serrata; a simple two-layered epithelium lining the ciliary body and posterior iris surface

Optical Media (Transparent Structures)

Lens

• Biconvex, avascular, transparent structure
• Enclosed in an elastic capsule; composed of lens fibers derived from epithelial cells
• Shape changed by ciliary muscle via zonular fibers for accommodation

Aqueous Humor

• Produced by ciliary processes → flows from posterior chamber → through pupil → anterior chamber → drained via trabecular meshwork into canal of Schlemm
• Blockage of this pathway raises intraocular pressure → glaucoma

Vitreous Body

• Transparent gel (mostly water + collagen + hyaluronic acid)
• Occupies the space between lens and retina; maintains the globe's spherical form

Accessory Structures

• Conjunctiva — thin mucous membrane lining the inner eyelids and folding over the anterior sclera (not cornea)
• Extraocular muscles (6 muscles) — superior/inferior rectus, medial/lateral rectus, superior/inferior oblique — inserted into the sclera for eye movement
• Eyelids and lacrimal apparatus — provide mechanical protection and corneal lubrication