Layers and Internal Structure of the Eyeball

The Three Tunics

1. Fibrous Tunic (External Layer)

Sclera

• The posterior 5/6 of the fibrous layer
• Opaque, whitish, ~0.5 mm thick (thickens to ~1 mm posteriorly)
• Composed of dense connective tissue — flat bundles of type I collagen oriented parallel to the surface but intersecting in multiple directions
• Microvasculature is mainly in the outer surface (episclera)
• Tendons of the six extraocular muscles insert into the anterior sclera
• Posteriorly joins with the epineurium of the optic nerve
• Inner surface: suprachoroidal lamina — less collagen, more fibroblasts, elastic fibers, and melanocytes

Cornea

• The anterior 1/6 of the eye; completely transparent and avascular
• Has five distinct layers (from anterior to posterior):

2. Vascular Tunic / Uvea (Middle Layer)

Choroid

• Located in the posterior 2/3 of the eye, between sclera and retina
• Consists of well-vascularized connective tissue with large flattened melanocytes that block excess light penetrating the sclera
• Two main sublayers:
  • Choroidocapillary lamina (inner): rich microvasculature that nourishes the outer retinal layers
  • Bruch's membrane (basal complex): a layer of elastin sandwiched between two sheets of collagen, continuous with the basement membranes of the capillaries and retinal pigment epithelium

Ciliary Body

• Anterior expansion of the uvea that encircles the lens; lies posterior to the limbus
• Major components:
  • Ciliary muscle: smooth muscle in three complementary orientations; contraction/relaxation changes lens curvature (accommodation)
  • Ciliary processes: finger-like folds covered by a bilaminar cuboidal epithelium (inner nonpigmented + outer pigmented); they secrete aqueous humor into the posterior chamber
  • Ciliary zonule (zonular fibers): microfibrils connecting the ciliary body to the lens capsule — tension regulated by ciliary muscle to change lens shape
• When ciliary muscles relax → zonule taut → lens flattens (distant vision)
• When ciliary muscles contract → zonule loosens → lens rounds up (near vision / accommodation)

Iris

• Opaque, pigmented extension of the uvea anterior to the lens
• Surrounds the central aperture: the pupil
• Contains two layers of smooth muscle:
  • Sphincter pupillae: circular muscle — constricts pupil (parasympathetic)
  • Dilator pupillae: radial muscle — dilates pupil (sympathetic)
• Separates the anterior chamber (in front) from the posterior chamber (behind)

3. Retina (Inner Sensory Layer)

Pigmented Layer (RPE)

• Single layer of cuboidal epithelial cells containing melanin
• Rests on Bruch's membrane of the choroid
• Functions: absorbs stray light, recycles photopigments, provides vitamin A to photoreceptors

Neural Layer — 9 Distinct Sublayers (outer → inner)

Special Regions of the Retina

Internal Contents (Non-layered)

Lens

• Transparent, biconvex, avascular, elastic structure suspended by the ciliary zonule
• Three components:
  • Lens capsule: very thick (~10–20 µm) basement membrane of type IV collagen surrounding the entire lens
  • Lens epithelium: single layer of cuboidal cells on the anterior surface only; proliferates at the equator to form new lens fibers
  • Lens fibers: densely packed, anucleate, elongated cells filled with crystallins — the basis of transparency
• Loses elasticity with age → presbyopia (typically after 4th decade)

Aqueous Humor

• Clear fluid secreted by the ciliary process epithelium
• Fills the anterior chamber (between cornea and iris) and posterior chamber (between iris and lens)
• Flows through the pupil from posterior to anterior chamber, then drains via the scleral venous sinus (canal of Schlemm) at the iridocorneal angle

Vitreous Body

• Large gelatinous mass of transparent connective tissue filling the posterior (vitreous) chamber behind the lens
• Occupies ~80% of the eyeball's volume
• Contains the hyaloid canal — a remnant of the fetal hyaloid artery

Summary Table