Vision

Canthi and Lacrimal Structures

  • Canthus: The angle where the upper and lower eyelids meet.
    • Singular: Canthus
    • Plural: Canthi
  • Lacrimal Caruncle: Reddish elevation at the medial canthus; it has a function in eye moisture.

Accessory Structures of the Eye

  • Tarsal Plates: Connective tissue providing structure to the eyelids; associated with tarsal glands.
  • Tarsal Glands: Sebaceous glands that secrete sebum, a light oil that prevents eyelid sticking.

Tear Production and Drainage

  • Lacrimal Gland: Located lateral to the eye; secretes tears onto the conjunctiva, which are then funneled into the lacrimal canaliculi and subsequently into the nasolacrimal duct; this can cause a runny nose.

Extrinsic Eye Muscles

  • Six muscles control eye movement, attached to the outer surface of the eye:
    1. Superior Rectus
    2. Inferior Rectus
    3. Lateral Rectus
    4. Medial Rectus
    5. Superior Oblique
    6. Inferior Oblique
  • Trochlea: U-shaped structure through which the tendon of the superior oblique muscle passes.
  • Muscles are supplied by cranial nerves; notable is LR6 (lateral rectus) innervated by cranial nerve VI, while other rectus muscles are innervated by cranial nerve III.

Eye Anatomy

Anterior Segment

  • Cornea: Transparent front part of the eye, refraction begins here.
  • Sclera: White opaque tissue surrounding the eye, continuing from the cornea.
  • Pupil: Adjusts light entry, surrounded by the iris (colored part).
  • Aqueous Humor: Fluid filling anterior segment; helps maintain intraocular pressure.

Posterior Segment

  • Lens: Adjusts shape to focus light; thickens for near vision and elongates for distance sight, facilitated by suspensory ligaments connected to the ciliary body.
  • Vitreous Humor: Jelly-like substance in posterior segment, helps maintain eye shape and pressure.

Retina and Photoreception

  • Retina: Nervous tissue at the back of the eye containing photoreceptors (rods and cones).
    • Rods: Provide vision in dim light (night vision).
    • Cones: Enable color vision and function in bright light.
  • Fovea Centralis: Area with the highest concentration of cones, responsible for sharp vision.
  • Blind Spot: Area of optic disc where no photoreceptors are present.

Vision and Neural Pathway

  • Light is refracted and focused upside down on the retina, corrected by the brain at the visual primary cortex after traveling via the optic chiasma (crossing of optic nerves).

Myopia (Nearsightedness)

  • Eyeball appears too long, images are focused in front of the retina.
  • Correction: Concave lens is used to move the focal point back to the retina.

Accommodation and Adjustment

  • Accommodation: Change in lens shape allows focusing on near and far objects.
    • Ciliary Body contracts, affecting tension on the suspensory ligaments and lens shape.

Summary of Cranial Nerve Functions

  • Various extrinsic muscles are innervated by specific cranial nerves, influencing the direction of eye movement.
  • Ciliary Body and Iris: Two smooth muscle structures controlling pupil size based on light regulation, with sympathetic and parasympathetic responses affecting contraction and dilation respectively.

Emergency and Health Precautions

  • Reminder: Any changes in vision should be addressed immediately to prevent further damage or complications.