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The Eye: Structure

Macro and Microscopic. The Eye: Structure. Eyes. Protected by bony orbits of the skull Send information to brain directly via optic nerve. Development. From 2 outgrowths of the brain that form the optic nerves & the optic cup: The posterior lining of the eye contains photoreceptors

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The Eye: Structure

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  1. Macro and Microscopic The Eye: Structure

  2. Eyes • Protected by bony orbits of the skull • Send information to brain directly via optic nerve.

  3. Development • From 2 outgrowths of the brain that form the optic nerves & the optic cup: • The posterior lining of the eye contains photoreceptors • In a mature eye called the retina

  4. External features • Palpebrae: eyelids! • Thin flaps of skin • Controlled by orbicular muscles • Close when objects are placed near the eye: reflex arc • Eyelashes • Edges of eyelid • Protect from dirt

  5. Eye Musculature

  6. External Structures of the Eye 1. Iris 2. Lacrimalcaruncle 5. Lower lid 7. Pupil 8. Sclera 9. Upper lid

  7. 3 primary layers… • Sclera • Choroid • Retina

  8. Sclera • Tough, outermost, white layer • Surrounds and protects the eyeball. • Its front surface, the cornea, is transparent to let light enter the eye. • Lacks blood vessels • Gets nutrition through diffusion • Ideal for transplants

  9. Cornea

  10. Choroid • The choroid coat is the middle layer of the eyeball. • Consists of the colored portion of the eye known as the iris. • Iris has a hole in its center called the pupil. • Light enters through the pupil and the size of the pupil is regulated by the iris. • Constricting = parasympathetic muscles • Dilating = Sympathetic muscles

  11. Why an iris? • Too much light “bleaches” the photopigment, rhodopsin to opsin • Reduces ability to see • Opsin must be “reconverted” to rhodopsin • This is related to “night vision” issues with sudden darkness…

  12. Iris and Choroid

  13. Lens • Transparent body • Lies directly behind the pupil • Held in place by ciliary muscles (run in circular, longitudinal, radial orientation; change lens shape) • Focuses light rays of imageson the retina • Images inverted (both L to R and Up to Down) • Visual cortex reorients these

  14. Lens Function • Increases amount of energy reaching photoreceptive cells • What happens when source moves closer? • All light isn’t focused on retina • Makes image “fuzzy” because adjacent cells stimulated • An “accommodating” lens clears image up by refocusing light

  15. To focus image… • Close image = ROUND lens (decreases radius of curvature) • Far image = FLAT lens (increases radius of curvature) • To round the lens, contract muscles in ciliary body: • Contracting a circular muscle reduces the aperture • This decreases the tension on the suspensory ligaments, allowing lens to “round up”

  16. Pupil, Lens, Ciliary muscle (body)

  17. Retina • Innermost layer of the eyeball. • Contains microscopic structures: • Rods • Low-light • Non-color vision • Cones • Bright-light • Color vision

  18. Vertebrate Eye • At center of retina, have fovea centralis • Concentration of cone cells • 1:1 cell/neuron ratio (gives good resolution) • Farther outward, mix of rods and cones, with just rods in peripheral vision • Mostly b & w, low-light, low-resolution peripheral vision • Eye directs fovea centralis at objects to maximize clarity • A “blind spot” occurs where the optic nerve/tract exits the eye

  19. Retina through Opthalmascope Optimal vision Blind spot

  20. Eye Fluids • Aqueous humor—in the anterior cavity in front of the lens • Provides nutrients to cornea, lens, other structures • Vitreous humor—in the posterior cavity behind the lens • Gelatinous • Holds retina to outer wall of choroid • May contain “floaters”, which must be removed surgically

  21. Anterior and Posterior Chambers

  22. Conjunctiva • Mucous membrane covering the front surface of the sclera and lining the eyelid • Produces tears • Barrier to microbes • Susceptible to trauma, infections, chemical irritation, and allergic reactions

  23. Conjunctiva: thin, transparent epithelium covers the surface • Tiny blood vessels

  24. Visual Pathway • Innermost layer of retina contains rods and cones • Impulse travels from the rods and cones through the bipolar and ganglionic layers of retina • Nerve impulse leaves the eye through the optic nerve; the point of exit is free of receptors and is therefore called a blind spot • Visual interpretation occurs in the visual cortex of the cerebrum

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