Sight

1
Sight

• Visual Accessory Organs
• eyelids
• lacrimal apparatus
• extrinsic eye muscles

2
Eyelid

• palpebra
• composed of four layers
• skin
• muscle
• connective tissue
• conjunctiva
• orbicularis oculi - closes
• levator palperbrae superioris opens
• tarsal glands secrete oil onto eyelashes
• conjunctiva mucous membrane lines eyelid and
  covers portion of eyeball

3
Lacrimal Apparatus

• lacrimal gland
• lateral to eye
• secretes tears
• canaliculi
• collect tears
• lacrimal sac
• collects from canaliculi
• nasolacrimal duct
• collects from lacrimal sac
• empties tears into nasal cavity

4
Extrinsic Eye Muscles

• Superior rectus
• rotates eye up and medially

• Inferior rectus
• rotates eye down and medially

• Medial rectus
• rotates eye medially

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Extrinsic Eye Muscles

• Lateral rectus
• rotates eye laterally

• Superior oblique
• rotates eye down and laterally

• Inferior oblique
• rotates eye up and laterally

6
Structure of the Eye

• hollow
• spherical
• wall has 3 layers
• outer fibrous tunic
• middle vascular tunic
• inner nervous tunic

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Outer Tunic

• Cornea
• anterior portion
• transparent
• light transmission
• light refraction

• Sclera
• posterior portion
• opaque
• protection

8
Middle Tunic

• Iris
• anterior portion
• pigmented
• controls light intensity

• Ciliary body
• anterior portion
• pigmented
• holds lens
• moves lens for focusing

• Choroid coat
• provides blood supply
• pigments absorb extra light

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Anterior Portion of Eye

• filled with aqueous humor

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Lens

• transparent
• biconvex
• lies behind iris
• largely composed of lens fibers
• elastic
• held in place by suspensory ligaments of ciliary
  body

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Ciliary Body

• forms internal ring around front of eye
• ciliary processes radiating folds
• ciliary muscles contract and relax to move
  lens

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Accommodation

• changing of lens shape to view objects

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Iris

• composed of connective tissue and smooth muscle
• pupil is hole in iris
• dim light stimulates radial muscles and pupil
  dilates
• bright light stimulates circular muscles and
  pupil constricts

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Aqueous Humor

• fluid in anterior cavity of eye
• secreted by epithelium on inner surface of the
  ciliary body
• provides nutrients
• maintains shape of anterior portion of eye
• leaves cavity through canal of Schlemm

15
Inner Tunic

• retina
• contains visual receptors
• continuous with optic nerve
• ends just behind margin of the ciliary body
• composed of several layers
• macula lutea yellowish spot in retina
• fovea centralis center of macula lutea
  produces sharpest vision (you move your eye to
  put images here)
• Only cones (no rods)
• optic disc blind spot contains no visual
  receptors
• vitreous humor thick gel that holds retina
  flat against choroid coat

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Posterior Cavity

• contains vitreous humor thick gel that holds
  retina flat against choroid coat

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Vision

• Pass through
• Cornea
• Aqueous humor
• Lens
• Vitreous humor
• Retinal layers
• Photoreceptor cells

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Major Groups of Retinal Neurons

• receptor cells, bipolar cells, and ganglion
  cells - provide pathway for impulses triggered by
  photoreceptors to reach the optic nerve
• horizontal cells and amacrine cells modify
  impulses

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Layers of the Eye
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Light Refraction

• Refraction
• bending of light
• occurs when light waves pass at an oblique angle
  into mediums of different densities

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Types of Lenses
Convex lenses cause light waves to converge
Concave lenses cause light waves to diverge
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Focusing On Retina

• as light enters eye, it is refracted by
• convex surface of cornea
• convex surface of lens
• image focused on retina is upside down and
  reversed from left to right

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Visual Receptors

• Rods
• long, thin projections
• contain light sensitive pigment called
  rhodopsin
• hundred times more sensitive to light than cones
• provide vision in dim light
• produce colorless vision
• produce outlines of objects

• Cones
• short, blunt projections
• contain light sensitive pigments called
  erythrolabe, chlorolabe, and cyanolabe
• provide vision in bright light
• produce sharp images
• produce color vision

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Rods and Cones
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Visual Pigments

• Rhodopsin
• light-sensitive pigment in rods
• decomposes in presence of light
• triggers a complex series of reactions that
  initiate nerve impulses
• impulses travel along optic nerve

• Pigments on Cones
• each set contains different light-sensitive
  pigment
• each set is sensitive to different wavelengths
• color perceived depends on which sets of cones
  are stimulated
• erythrolabe responds to red
• chlorolabe responds to green
• cyanolabe responds to blue

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Rod Cells
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Stereoscopic Vision

• provides perception of distance and depth
• results from formation of two slightly different
  retinal images

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Visual Nerve Pathway
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Life-Span Changes

• Age related hearing loss due to
• damage of hair cells in organ of Corti
• degeneration of nerve pathways to the brain
• tinnitus

• Age-related visual problems include
• dry eyes
• floaters (crystals in vitreous humor)
• loss of elasticity of lens
• glaucoma
• cataracts
• macular degeneration

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Clinical Application
Refraction Disorders

• concave lens corrects nearsightedness

• convex lens corrects farsightedness