Senses: Vision Ch. 17-2. Accessory Structures of Eye Eyelids Eyelashes Eyebrows Lacrimal apparatus...

Senses: Vision

Ch. 17-2

Accessory Structures of Eye

• Eyelids• Eyelashes• Eyebrows• Lacrimal apparatus• Extrinsic eye muscles

Accessory Structures of Eye

• Eyelids – palpebrae– Functions:

• Shade eyes during sleep• Protect eyes from light• Protect eyes from foreign objects• Spread lubrication over eyeballs

– upper eyelid is more moveable – levator palpebrae superioris muscle

– Where eyelids meet is called commissure – lateral and medial

– Lacrimal caruncle – reddish bump in medial commissure that secretes oil and sweat – causes “sleep” in your eyes

Infections

• Sty – infection in oil gland of eyelash follicle• Chalazion – infection in tarsal glands of eyelid

– Tarsal glands secrete fluid onto eye for lubrication

Accessory Structures of Eye

• Lacrimal apparatus– Produces and drains

lacrimal fluid or tears• Eye muscles

Anatomy of the Eyeball

• 2.5 cm in diameter• 3 anatomical layers

– Fibrous tunic– Vascular tunic– Retina

Tunic Layers

• Outer layer – fibrous– Cornea – transparent, focuses light rays– Sclera – white of the eye– Optic nerve – transmits info to brain

• Middle layer – vascular– Choroid coat – contains blood vessels– Ciliary body – holds the lens in place– Lens – focusing– Aqueous humor – fluid surrounding lens– Pupil – opening for light to enter

Tunic Layers

• Inner layer– Retina – visual receptor

cells– Vitreous humor – fluid

supports internal structures

Eye

Structures of Eyeball

Retina

• Made of cells that are light receptors – photoreceptors

• Rods and cones– Rods – black and white– Cones - color

• Color blindness– Lack of cones

ColorblindnessA genetic trait that affects boys more than girls. The location of the gene is on the X chromosome

Image Formation

• Seeing is like taking a picture• The object must be focused on a “film” –

retina• The correct amount of light must be present –

pupil

Light bends - refraction

Refraction

• When light bends as it moves between two mediums – air and water

• Images on the retina are upside-down and have right-to-left reversal

Image Formation Process

• Light hits the cornea and is bent• Light leaves the cornea and is bent again• Light enters the lens where it is focused on the

retina• So why don’t we see everything upside-down?

– Very early on the brain “learns” how to coordinate the images and make them correct

Accommodation

• Most of the focusing is done by the cornea

• 25% must be done by the lens• Our lens is convex on both

sides in order to produce clear images

• The lens will increase its curvature in order to focus all images exactly

Animations

• http://www.bbc.co.uk/science/humanbody/body/factfiles/sight/sight_animation.shtml

• http://www.biologymad.com/resources/eye.swf

Abnormalities and Changes

• Presbyopia – lens loses elasticity with age– Around age 40 the lens can’t focus near images and people

need glasses• Normal eye – emmetropic – can reflect images perfectly

of objects 20 ft away• Myopia - near-sightedness

– Eyeball is too long for the focusing power of the lens or lens is thicker than normal

• Hyperopia – far-sightedness– Eyeball is too short for the focusing power of the lens or lens

is thinner than normal

Abnormalities