Structures of the Eye

Structures of the External Eye

  • Only about 1/6 of the eyeball is visible from the anterior, external surface.
  • Observable parts from the external, anterior perspective:
    1. Cornea: Thin, transparent dome projecting forward; avascular; serves as the "window" into the eye.
    2. Sclera: Tough, fibrous, white outer covering protecting internal structures; commonly referred to as the "white" of the eye; corneal limbus is the junction of the sclera and cornea.
    3. Conjunctiva: Thin, translucent membrane lining the eyelid underside and covering the sclera; secretes mucus for lubrication.
    4. Iris: Pigmented ring of tissue surrounding the pupil (internal structure visible through the cornea); attracts wavelengths of light toward the pupil.
    5. Pupil: Opening in the iris through which light travels to the inner eye (lens & retina); also visible through the cornea.
  • Other external, anterior eye structures (peripheral organs) assist in light collection:
    1. Lacrimal Glands: Located on the inner, lateral surface of each superior eyelid; secrete tears (containing water and lysozyme) to moisten and protect the eye; the moisture helps channel light toward the cornea.
    2. Tears: As you blink, tears smear across the ocular surface and drain into two canaliculi (tear ducts) at the medial corner of the eye: the superior and inferior canaliculi.
    3. Lacrimal Canaliculi: Collect spent tears into the lacrimal sacs, then drain into the nasolacrimal ducts, and finally into the nasal passage.
    4. Eyelids (Palpebrae): Protective flaps containing eyelashes; sebaceous tarsal (Meibomian) glands inside the border release oily secretion for lubrication and to prevent sticking; ciliary glands are sweat glands between eyelashes.
    5. Canthi: Eyelids meet at the lateral and medial canthus; the space between eyelids is the palpebral fissure.
    6. Plica Semilunaris: Vertical fold of conjunctiva in the medial canthus; vestigial nictitating membrane.
    7. Lacrimal Caruncle: Small reddish body at the medial angle, containing modified sebaceous and sweat glands that produce thick secretions contributing to gritty deposits.

Lacrimal System

External Pathologies of the Eye

  • Conjunctivitis (pink eye)
  • Sty
  • Cataracts

Positioning of the Eye in the Eye Socket

Extraocular (Extrinsic) Skeletal Muscles

  • Located on the anterior of the eye
  • Located on the lateral side of the eye
  • Eye movements occur when extraocular muscles contract.
  • Strabismus (Cross-eyes): Occurs when one extraocular muscle contracts with too much force, or its counterpart in the other eye does not contract equally; the brain does not perceive a unified image.
  • Convergent Strabismus: Both right and left medial rectus muscles contract at the same time.
  • Divergent Strabismus: Left lateral rectus contracting more than right lateral rectus.

Internal Structures of the Eye

  • The eyeball is composed of three tunics (layers):
    • Fibrous tunic (outermost): sclera & cornea.
    • Vascular tunic (middle): iris, ciliary body & choroid.
    • Retinal tunic (innermost): neural & pigmented layer.
  • The eye is divided into two large cavities: anterior and posterior, containing aqueous and vitreous humor, respectively.
  • The anterior cavity is further divided into:
    • Anterior chamber: Between the cornea and the iris/pupil.
    • Posterior chamber: Behind the iris/pupil and extending to the lens.
  • Aqueous humor circulates from the posterior chamber, through the pupil, to the anterior chamber and drains through the canal of Schlemm.
  • Glaucoma: Blockage of the canal of Schlemm prevents proper drainage of aqueous humor, increasing fluid pressure and interfering with lens focusing ability.
  • Fibrous Tunic:
    • Composed of dense fibrous connective tissue (collagen and elastic fibers).
    • Thickest at the posterior (sclera), thinnest at the anterior (cornea).
    • Functions:
      1. Physical support and protection.
      2. Attachment site for extraocular muscles.
      3. Maintains eye shape for proper focusing.
  • Vascular Tunic:
    • Contains the iris, ciliary body, and choroid layer.
    • Iris: Contains smooth pupillary muscles behind the pigmented anterior surface that dilate or constrict the pupil.
      • Circular muscles (innermost): cause constriction (smaller).
      • Radial muscles (outermost): cause dilation (bigger).
    • Ciliary Body:
      • Contains smooth muscle cells for lens accommodation (focusing).
      • Epithelial cells secrete aqueous humor.
      • Suspensory ligaments (ciliary zonule) hold the lens in place.
    • Choroid Layer:
      • Underlies the sclera.
      • Extensive capillary network supplies oxygen and nutrients to retinal sensory cells.
      • Contains melanocytes for dark pigmentation to prevent light scattering.
    • Tapetum Lucidum: Not present in humans, this pigmented tissue in the choroid of nocturnal mammals reflects light (causing eye shine).
  • Sensory/Neural Tunic (Retina):
    1. Innermost surface has photoreceptors (rods and cones) that respond to light wavelengths.
    2. Outermost pigmented layer (facing the choroid) prevents light scattering and stores Vitamin A (converted into retinal).
    3. Neural axons from photoreceptors form the optic nerve (cranial nerve II).
      • Optic Disc: Blind spot where neural axons leave the retina (no photoreceptor dendrites or somae located here).
  • Photoreceptors:
    • Rods: Abundant along the retina margins; respond to dim light; permit black and grey vision ("night" vision); provide peripheral vision.
    • Cones: Respond to bright light; provide color vision; concentrated in the macula lutea, especially the fovea centralis.

Retinal Layers

The layers of the retina are as follows:

  1. Pigment Epithelium:
    • Pigment cells
    • The choroid lies beneath this layer
  2. Photoreceptor Layer:
    • Rod
    • Cone
  3. Outer Limiting Membrane
  4. Outer Nuclear Layer
  5. Outer plexiform layer
    • Horizontal cell
  6. Inner nuclear layer
    • Müller cell (supporting glial cell)
    • Bipolar cell
    • Amacrine cell
  7. Inner plexiform layer
  8. Ganglion cell layer
    • Ganglion cell
  9. Nerve fibre layer
    • Axons at surface of retina passing via optic nerve, chiasm and tract to lateral geniculate body
  10. Inner limiting membrane

Distribution of Rods & Cones on Surface of the Retina

  • Rods concentrate along the outer area of the retina surface
  • Cones concentrate in the inner Fovea area of the retina's surface

Eye Parts – Dissected Normal 20/20 Vision (Emmetropia)

  • Refraction: Bending of light as it travels from one medium to another (air to cornea, aqueous humor to lens, vitreous humor to retina).
  • The amount of refraction influences how well light focuses on the macula lutea.
  • The image inverts as it passes through the lens.
  • The lens provides enough refraction to project the image on a point of intersection on the retina, the focal point – focusing the image; this occurs in the area of the macula lutea.
  • Focal Distance: Distance between the center of the lens and its focal point on the retina.
    • Determined by:
      1. Object distance: Closer objects result in a greater focal distance.
      2. Lens shape: Rounder lenses cause more refraction and a shorter focal distance.
  • Accommodation: The lens changes shape to maintain the appropriate focal distance because the distance between the lens and macula lutea cannot change much.

Common Situations Involving Accommodation

  • Myopia (Nearsightedness): Eyeball is too elongated or lens is too rounded; the person can focus on near object only; corrected by concave (diverging) lens.
  • The rays of light from a near object are diverged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.
  • Hyperopia (Farsightedness): Eyeball is too shallow or the lens is too flat; the person can focus on distant objects only; corrected by convex (converging) lens.
  • The rays of light from a near object are converged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.
  • Presbyopia ('Old' Sightedness): Older individuals become farsighted as their lens loses elasticity.
  • Nyctalopia (Night Blindness): Trouble seeing in dimly lit conditions.
  • Astigmatism: Unusual curvature of the cornea or lens causes multiple refractions of light onto the retina, none of which are correctly focused by accommodation.

Accommodation Demonstration

  • Image demonstrates bulging of the lens when a muscle contracts
  • Image demonstrates when ligaments are pulled tight and the lens is pulled thin