Organ of Vision - Eye Structure and Components

Organ of Vision

• Structure of Eyeball:

  • The eyeball is a spherical structure, approximately 2.5cm in diameter.
  • It has three distinct layers: outer tunic (fibrous), middle tunic (vascular), and inner tunic (nervous).
  • The spaces within the eyeball are filled with fluids, supporting the wall and internal parts, maintaining its shape.

• Visual Accessory Organs:

  • Eyelids (Palpebrae): Thin skin folds covering and protecting the eye.
    • Levator palpebrae superioris muscle retracts the eyelid.
    • Eyelashes along the edges provide additional protection from foreign objects.
  • Conjunctiva: Mucous membrane lining the inner surface of eyelids, folds back to cover the anterior surface of the eyeball excluding the cornea.
  • Lacrimal Apparatus:
    • Lacrimal gland and ducts
    • Conjunctival sac
    • Lacrimal puncta and canaliculi
    • Lacrimal sac
    • Nasolacrimal duct
  • Extrinsic Muscles: Responsible for eye movement.
    • Four recti muscles (superior, inferior, medial, lateral)
    • Two oblique muscles (superior, inferior)

• Eyeball Development:

  • Complex interaction between neighboring tissues during embryonic development forms the eye.
  • Neuroectoderm of the forebrain forms the sensory retina and accessory pigmented structures.
  • Surface ectoderm forms the lens and anterior corneal epithelium.
  • Neural crest cells contribute to the fibrous coats of the eye and tissues in the anterior segment.

Structure of Eyelids

• Skin: Thin and loose.
• Superficial fascia: Contains the palpebral part of orbicularis oculi muscle.
• Palpebral fascia: Forms orbital septum, thickens to form tarsal plates.
• Tarsi: Thin plates of condensed fibrous tissues at the edges of the lids.
• Tarsal glands: Meibomian glands embedded in the posterior surface of the tarsi.
• Conjunctiva: Lines the posterior surface of the tarsi.

Glands of Eyelids

• Zeis's glands: Large sebaceous glands at the lid margin.
• Moll's glands: Modified sweat glands closely associated with Zeis's glands; located at the lid margin.
• Meibomian glands: Sebaceous or tarsal glands.

Lacrimal Apparatus

• Lacrimal gland: Produces tears.
  • Orbital and palpebral lobes.
  • Excretory ducts.
• Lacrimal punctum: Opening of drainage ducts.
• Lacrimal canals: Collect tears.
• Lacrimal sac: Store and drain tears to the nasolacrimal duct.
• Nasolacrimal duct: Drains tears into the nasal cavity.

Orbit

• Bony cavity: Houses the eye, lacrimal gland, and extrinsic muscles.
• Periosteum: Lines the bony orbit.
• Fat: Supports and cushions the eye.
• Blood Vessels: Supply the eye and surrounding structures.
• Nerves: Control and support the eye

Structure of the Eye

• Outer Tunic (Fibrous Tunic):
  • Sclera: Forms the posterior 5/6 of the eyeball, composed of dense fibrous tissues.
  • Cornea: Transparent anterior portion, replacing the sclera.
• Middle Tunic (Vascular Tunic):
  • Choroid: pigmented layer separating the sclera from the retina; carries blood vessels.
  • Ciliary body: Thickened part of the middle tunic containing the ciliary muscle.
  • Iris: Colored portion of the eye containing muscles (sphincter and dilatator pupillae) controlling the size of the pupil.
• Inner Tunic (Nervous Tunic):

  • Retina: Innermost layer containing photoreceptors (rods and cones).

  • Macula lutea: Region with high concentration of cones.

  • Fovea centralis: Center of the macula containing only cones – sharpest vision.

  • Optic disc: Contains no photoreceptors (blind spot) where the optic nerve exits.

Lens

• Transparent, biconvex structure: Located between the anterior and posterior segments.
• Suspensory Ligaments: Maintain constant tension to the lens shape.
• Ciliary muscles: Change the shape of the lens to adjust focus (accommodation).

Aqueous Humor

• Clear fluid: Fills the anterior and posterior chambers.
• Secretion by ciliary processes: Flows through the pupil into the anterior chamber.
• Drainage through the canal of Schlemm: Maintains the shape of the eye's front and nourishes the cornea and lens.

Vitreous Body

• Clear, jelly-like substance: Fills the posterior segment of the eyeball.
• Hyaloid membrane: Encloses the vitreous body.
• Maintains eyeball shape: Anchors the retina in place.

Extrinsic Muscles

• Four recti muscles: Superior, Inferior, Medial, Lateral – responsible for moving the eye.
• Two oblique muscles: Superior and Inferior – responsible for moving the eye.

Blood Supply

• Ophthalmic artery: Primary source of blood to the eye.
• Branches of ophthalmic artery and other arteries provide blood supply to different parts of the eye.

Second Cranial Nerve (Optic Nerve)

• Axons of ganglion cells: Carry visual information from the retina.
• Optic Chiasm: Partial crossing of nerve fibers.
• Optic Tracts: Continue to transmit the information.

Lesions of Visual Pathway

• Total anopsia: Complete loss of vision in one eye.
• Bitemporal hemianopsia: Loss of temporal vision in both eyes.
• Contralateral homonymous hemianopsia: Losses of corresponding halves of the visual field.

Eyeball Development

• Inductive interactions: Neighboring tissues interact to develop the eye.
• Neuroectoderm, surface ectoderm, and mesenchyme: Contribute to the formation of the eye, including the sensory retina, lens, cornea, and other tissues.
• Optic vesicles: form the optic cup, nerve, optic chiasm, and optic tract, forming the components of the eye.

Questions

• Retina Layers: Ten layers
• Fovea Centralis Receptors: Cones
• Muscle attached to lens: Ciliary body
• Innermost tunic of the eye: Retina
• White portion of the eye: Sclera