Chapter 15 – The Special Senses: Vision

Vision

Accessory Structures of the Eye

The accessory structures of the eye serve to protect, lubricate, and support the function of vision. These include the eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

• Eyebrows: Located above the supraorbital margin, they shade the eyes from sunlight, prevent perspiration from running down the forehead, and contribute to facial expression.

• Eyelids (Palpebrae): Positioned anteriorly on the face, inferior to the eyebrows, they protect the eyes and contain several important structures:

  • Palpebral fissure: The space between the upper and lower eyelids.

  • Lateral and medial commissures: The corners of the eyes.

  • Lacrimal caruncle: Located at the medial commissure, contains sebaceous and sweat glands, produces a white oily secretion.

  • Orbicularis oculi: Muscle that closes the eye.

  • Levator palpebrae superioris: Muscle that elevates the upper eyelid.

  • Tarsal plate: Connective tissue sheets supporting the eyelids.

  • Tarsal glands: Ducts producing oil to lubricate eyelids and prevent sticking.

  • Eyelashes: Richly innervated, trigger the blinking reflex.

• Conjunctiva: Transparent mucous membrane covering the cornea and lining the eyelids, produces lubricating mucus to prevent drying.

  • Palpebral conjunctiva: Lines the eyelids.

  • Bulbar conjunctiva: Covers the cornea.

  • Conjunctival sac: Space between conjunctivae, site for contact lenses.

  • Conjunctivitis: Inflammation of the conjunctiva.

• Lacrimal Apparatus: Produces and drains tears.

  • Lacrimal gland: Located above the lateral end of the orbit, releases tears.

  • Excretory ducts: Drain tears from gland to eye.

  • Lacrimal puncta: Small openings at the medial margin of each eyelid.

  • Lacrimal canaliculi: Drain tears from puncta to lacrimal sac.

  • Lacrimal sac: Drains tears into the nasolacrimal duct.

  • Nasolacrimal duct: Carries tears to the nasal cavity.

• Extrinsic Eye Muscles: Control eye movement, originate from the annular ring behind the orbit.

  • Superior rectus: Elevates and turns eye medially.

  • Inferior rectus: Depresses and turns eye medially.

  • Medial rectus: Moves eye medially.

  • Lateral rectus: Moves eye laterally.

  • Superior oblique: Depresses and turns eye laterally (passes through the trochlea).

  • Inferior oblique: Elevates and turns eye laterally.

  • Cranial nerves: LR6SO4R3 mnemonic (Lateral Rectus - CN VI, Superior Oblique - CN IV, Rest - CN III).

Structure of the Eye

The eye is a spherical organ about 1 inch in diameter, protected by the orbit and surrounded by fat. It consists of several layers and segments, each with specialized functions.

• Protection: The orbit (bone) and surrounding fat cushion the eye.

• Poles: Anterior and posterior poles refer to the front and back of the eye.

Layers of the Eye

• Fibrous Layer: Outermost, dense connective tissue.

  • Sclera: Posterior, majority of fibrous layer, tendon-like, protects and shapes the eyeball, anchors muscles, continuous with dura mater, penetrated by optic nerve.

  • Cornea: Anterior 1/6th, avascular, transparent, bulges anteriorly, allows light entry.

• Vascular Layer (Uvea): Middle coat, rich in blood vessels.

  • Choroid: Dark membrane, forms 5/6 of vascular layer, supplies blood.

  • Ciliary body: Thickened ring anteriorly, encircles lens.

    • Ciliary processes: Smooth muscle bundles, control lens shape.

    • Ciliary zonule: Suspensory ligaments, hold lens upright.

  • Iris: Colored part, anterior, between cornea and lens, continuous with ciliary body.

    • Pupil: Central opening, regulates light entry.

    • Sphincter pupillae: Circular muscle, contracts in bright light, constricts pupil.

    • Dilator pupillae: Radial muscle, contracts in dim light, dilates pupil.

• Inner Layer (Retina): Innermost, delicate, two-layered.

  • Pigmented layer: Single-cell thick, next to choroid, extends anteriorly, contains vitamin A, absorbs light.

  • Neural layer: Transparent, covers posterior margin of ciliary body.

    • Ora serrata: Sawtooth margin, transition to ciliary body.

    • Photoreceptors: Respond to light.

      • Rods: Numerous, dim light and peripheral vision, detect shades of gray.

      • Cones: Bright light, high acuity color vision (blue, green, red).

    • Bipolar cells: Receive signals from photoreceptors, pass to ganglion cells.

    • Ganglion cells: Generate action potentials.

    • Horizontal and amacrine cells: Neurons involved in processing.

    • Optic nerve: Transmits visual information to the brain.

    • Optic disc: Blind spot, lacks photoreceptors, exit point for optic nerve.

    • Macula lutea: Oval region lateral to blind spot.

    • Fovea centralis: Pit in macula lutea, high concentration of cones, high acuity vision.

    • Central artery and vein: Supply inner retina, enter via optic nerve.

Segments and Humors of the Eye

The eye contains two main segments, each filled with a specific fluid (humor) that maintains its shape and function.

• Posterior Segment: Located behind the lens and ciliary zonule, filled with vitreous humor.

  • Vitreous humor: Formed in embryo, lasts lifetime, transmits light, supports lens and retina, maintains intraocular pressure, counters muscle pull.

• Anterior Segment: Subdivided by the iris.

  • Posterior chamber: Between iris and lens.

  • Anterior chamber: Between cornea and iris.

  • Aqueous humor: Fills anterior segment, clear fluid similar to blood plasma.

  • Scleral venous sinus (Canal of Schlemm): At junction of sclera and cornea, drains aqueous humor. Blockage increases intraocular pressure, causing glaucoma.

Lens

The lens is a transparent, biconvex structure that changes shape to focus light precisely on the retina. It is avascular and held in place by ciliary zonules.

• Lens epithelium: Confined to anterior surface.

• Lens fibers: Make up the bulk of the lens, lack nucleus and most organelles.

• Crystalline: Transparent, perfectly folded protein, essential for lens clarity.

Summary Table: Layers and Functions of the Eye

Layer	Main Structures	Function
Fibrous	Sclera, Cornea	Protection, shape, light entry
Vascular (Uvea)	Choroid, Ciliary body, Iris	Blood supply, lens shape, light regulation
Inner (Retina)	Pigmented layer, Neural layer	Light absorption, photoreception, signal transmission

Example: Accommodation of the Lens

When focusing on near objects, the ciliary muscles contract, reducing tension on the ciliary zonule, allowing the lens to become more convex and increase its refractive power.

Key Terms

• Conjunctivitis: Inflammation of the conjunctiva, commonly known as "pink eye".

• Glaucoma: Increased intraocular pressure due to blocked drainage of aqueous humor, can lead to vision loss.

• Fovea centralis: Area of highest visual acuity due to dense concentration of cones.

Additional info:

• The retina's neural layer is responsible for converting light into electrical signals, which are then processed and transmitted to the brain via the optic nerve.

• Color vision is mediated by three types of cones, each sensitive to different wavelengths (red, green, blue).