Anatomy of the Eye

Definition

Overview Diagram

Three Structural Layers (Tunics) of the Eye

1. Outer Layer (Fibrous Tunic)

Sclera

• The tough, white, opaque outer coat of the eye
• Composed of dense fibrous connective tissue
• Function: Provides structural rigidity and maintains the shape of the eyeball; protects internal structures; serves as attachment for the six extraocular muscles
• No light passes through it

Cornea

• The transparent, avascular, dome-shaped anterior continuation of the sclera
• Has a steeper radius of curvature than the sclera
• Function: The primary refracting surface of the eye (accounts for ~70% of total refractive power). Its avascular nature is maintained by nourishment from the aqueous humor behind it and tears in front
• The lateral margin of the cornea is continuous with the conjunctiva

2. Middle Layer (Uveal Tract / Vascular Tunic)

Choroid

• A vascular layer lining the inner surface of the sclera, covering the posterior two-thirds of the globe
• Contains large vessels, small vessels, and a network of capillaries (choriocapillaris)
• Function: Supplies oxygen and nutrients to the outer layers of the retina; its dark pigment absorbs stray light to prevent internal reflection

Ciliary Body

• A thickened ring of tissue located between the choroid and the iris
• Contains the ciliary muscle (smooth muscle) and produces aqueous humor
• Functions:
  • Ciliary muscle contraction/relaxation changes tension on the zonule fibers, altering lens shape for accommodation (focusing at different distances)
  • Actively secretes aqueous humor (via carbonic anhydrase and cytochrome oxidase systems at the ciliary epithelium) at ~2 µL/min
  • The iris dilator muscle is sympathetically innervated; the iris sphincter and ciliary muscle have parasympathetic innervation

Iris

• The pigmented, opaque, circular diaphragm in front of the lens
• Contains two sets of muscles:
  • Sphincter pupillae (circular fibers) - constricts the pupil (miosis); parasympathetic innervation
  • Dilator pupillae (radial fibers) - dilates the pupil (mydriasis); sympathetic innervation
• Function: Controls the amount of light entering the eye. Variations in pupil diameter can produce up to a 16-fold change in the amount of light reaching the retina

Pupil

• The central aperture/opening in the iris
• Appears dark because of the light-absorbing pigments in the retina
• Function: Regulates light entry

3. Inner Layer (Neural Tunic)

Retina

• A neurosensory membrane lining the posterior two-thirds of the choroid
• Composed of 10 histological layers that convert light impulses into neural impulses
• Contains two types of photoreceptors:
  • Rods - for dim light / peripheral vision / motion detection (~120 million)
  • Cones - for color vision and fine detail / high-acuity vision (~6 million; concentrated in the fovea)

Key retinal landmarks:

Internal Structures

Crystalline Lens

• A transparent, biconvex, elastic structure located posterior to the pupil and iris
• Held in place by the zonule fibers (suspensory ligaments) attached to the ciliary body
• Function: Fine-tunes refraction to focus images on the retina; changes shape (accommodation) to focus near and far objects
• When the ciliary muscle contracts, tension on zonule fibers decreases and the lens becomes more rounded (increases refractive power for near vision)

Aqueous Humor

• A clear, protein-free fluid filling the anterior chamber (between cornea and iris) and posterior chamber (narrow space between iris, zonule, and lens)
• Produced in the ciliary body, flows through the pupil into the anterior chamber, and drains via the trabecular meshwork into the Canal of Schlemm
• Function: Nourishes the avascular cornea and lens; maintains intraocular pressure (IOP). Obstruction of drainage raises IOP - a key risk factor for glaucoma

Vitreous Humor

• A clear, gelatinous material filling the large vitreous chamber (posterior cavity) between the lens and the retina
• Function: Keeps the eyeball spherical; maintains the retina pressed against the choroid; transmits light

Accessory / Protective Structures

Conjunctiva

• A clear mucous membrane covering the anterior sclera and lining the inner surface of the eyelids
• Function: Protects the eye; serves as a site for drug absorption (popular route for ophthalmic drug administration)

Eyelids

• Composed of four layers: conjunctiva, cartilaginous tarsal plate, muscle layer (orbicularis oculi + levator palpebrae superioris), and skin
• Function: Protect the eye from foreign objects; spreading of the tear film during blinking keeps the cornea moist

Lacrimal System

• Lacrimal gland - located in the superior temporal orbit; produces the tear film
• Lacrimal drainage system - composed of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct, draining into the nasal cavity
• Function: Maintain a moist anterior ocular surface; protect the cornea

Orbit

• A bony pyramidal cavity in the skull formed by seven bones: frontal, zygomatic, greater wing of sphenoid, maxilla, palatine, lacrimal, and ethmoid
• Function: Protects the globe and its associated structures from mechanical injury
• The optic foramen at the orbital apex transmits the optic nerve and ophthalmic artery

Extraocular Muscles (6 muscles)

Innervation Summary

• Optic nerve (CN II) - carries visual impulses from the retina to the brain
• Oculomotor nerve (CN III) - motor to 4 extraocular muscles + levator palpebrae; parasympathetic to iris sphincter and ciliary muscle (via ciliary ganglion)
• Trochlear nerve (CN IV) - motor to superior oblique muscle
• Abducens nerve (CN VI) - motor to lateral rectus muscle
• Trigeminal nerve (CN V1 - ophthalmic branch) - sensory to cornea, iris, ciliary body
• Sympathetic fibers (from carotid plexus, through superior orbital fissure) - dilator pupillae and lid retraction

Blood Supply

• Arterial supply: Branches of the ophthalmic artery (from the internal carotid artery), including the central retinal artery
• Venous drainage: Superior and inferior ophthalmic veins + central retinal vein, all draining into the cavernous sinus