Special SensesSpecial Senses

The EyeThe Eye

Overview of the EyeOverview of the Eye

Eye acts like a camera– Lens adjusts to bring object into focus– Pupil constricts to allow less light to

enter in bright setting or dilates to allow more light to enter in darker setting

– Through bending of light rays, images reaches retina

Sensitive nerve cell layer of eye Image is transmitted to brain for

interpretation

Front view of the eyeFront view of the eye Sclera = white

portion of eye– tough, maintains

shape of eyeball– serves as protective

covering for the eye Iris = colored

portion of the eye Pupil= opening in

center of eye that controls amount of light entering eye

Conjunctiva = thin mucous membrane that lines the eye and inner part of eyelid

Lacrimal gland = produces tears– located at upper outer

edge of each eye Lacrimal Duct =

Tears drain thru duct– located at inner edge

of eye

Cornea = transparent layer covering colored part of eye that holds in fluid

Aqueous humor = maintains pressure to keep outer cornea inflated & helps fight infection

Lens = colorless biconvex structure that focuses image clearly on retina

Aqueous Humor

■ Vitreous humor = fluid that prevents eye from collapsing forward

Vitreous humor

RetinaRetina Sensitive nerve cell layer

– changes energy of light rays into electrical nerve impulses

– transmits nerve impulses via optic nerve to brain for interpretation

Nerve cells of retina– Rods are responsible for

vision in dim light and for peripheral vision

– Cones responsible for colors and vision in bright light

ConesCones The retina contains

approximately 6 million cones. – contained in the

macula, the portion of the retina responsible for central vision.

– They are most densely packed within the fovea, the very center portion of the macula.

– Cones function best in bright light and allow us to appreciate color.

Fovea

RodsRods

There are approximately 125 million rods. – They are spread

throughout the peripheral retina and function best in dim lighting.

– The rods are responsible for peripheral and night vision.

How the eye How the eye focusesfocuses

Lenses and cornea focuses image precisely on the retina

Cornea of eye is overly curved or eyeball is too long. This combination

brings images of distant objects to a focus point in

front of the retina. cornea of eye is not curved enough or eyeball is too short. This combination causes the focus point of the eye to be located behind the retina.

Cornea of eye has a football shape and does not bring

light to focus at a single point. Instead, it focuses

images over a range of points producing a blurred image

Normal Vision

Near-sighted Vision

Far-Sighted Vision

Astigmatism

Choroid

Retina

Blood vessels

Optic nerve

Fovea

Vitreous humor

Sclera

Ligaments

Iris

Pupil

Cornea

Aqueous humor

Lens

Muscle

Section 35-4

Figure 35-14 The Eye

Pathway for Vision Pathway for Vision ReceptionReception

Light Cornea Iris and pupil Lens Retina to photoreceptors

– Rods– Cones (macula and fovea)

Optic Nerve Brain