Sight

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

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

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

Extrinsic Eye Muscles

Superior rectus• rotates eye up and medially

Inferior rectus• rotates eye down and medially

Medial rectus• rotates eye medially

Extrinsic Eye Muscles

Lateral rectus• rotates eye laterally

Superior oblique• rotates eye down and laterally

Inferior oblique• rotates eye up and laterally

Structure of the Eye

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

Outer Tunic

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

Sclera• posterior portion• opaque• protection

Middle TunicIris

• 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

Anterior Portion of Eye

• filled with aqueous humor

Lens

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

Ciliary Body

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

Accommodation• changing of lens shape to view objects

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

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

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

Posterior Cavity

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

Vision

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

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

Layers of the Eye

Light Refraction

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

Types of Lenses

Convex lenses cause light waves to converge

Concave lenses cause light waves to diverge

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

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

Rods and Cones

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

Rod Cells

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

Visual Nerve Pathway

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

Clinical ApplicationRefraction Disorders

• concave lens corrects nearsightedness (myopia)

• convex lens corrects farsightedness (hyperopia)