Biology Today’s Lesson: The Eye

Ms. Pretty

Introductory Activity

Instructions: Close your right eye and stare at the cross below. Slowly move towards the image, staring at the cross all the time. Where it is about 20cm or 8in away, the dot will suddenly vanish.

What happened to the circle??

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The Eye: Review from Last Class

The EyeReview: Parts and Functions

Part Function

Aqueous Humor

Clear watery fluid found in the anterior chamber of the eye; maintains pressure and nourishes the cornea and lens

Vitreous Humor

Clear, jelly-like fluid found in the back portion of the eye; maintains shape of the eye and attaches to the retina

Cornea Transparent tissue covering the front of the eye; does not have blood vessels; does have nerves

Iris Circular band of muscles that controls the size of the pupil. The pigmentation of the iris gives color to the eye. Blue eyes have the least amount of pigment and brown have the most

Lens Transparent tissue that bends light passing through the eye; to focus light, the lens can change shape

Review: Parts and Functions(Cont’d)Part Function

Optic Nerve Bundle of over one million axons from ganglion cells that carry visual signals from the eye to the brain

Pupil Hole in the center of the eye where light passes through

Choroid Thin tissue layer containing blood vessels, sandwiched between the sclera and retina; because of the high melanocyte content, the choroid acts as a light absorbing layer

Retina Layer of tissue on the back portion of the eye that contains cells responsive to light (photoreceptors)

Sclera Tough, white outer covering of the eyeball; extraocular muscles attach here to move the eye

Today: Important Parts and FunctionsPart Function

Retina Layer of tissue on the back portion of the eye that contains cells responsive to light (photoreceptors)

Macula Small central area of the retina that provides vision for fine work and reading

Fovea Central part of the macula that provides sharpest vision; contains only cones

Optic Disc (‘Blind Spot’)

Small area of the retina where the optic nerve leaves the eye; any image falling here will not be seen; there are no rods or cones present here

Cones Photoreceptors responsive to color and in bright light conditions; used for fine detail

Rods Photoreceptors responsive in low light conditions; not useful for fine detail

Retina: PhotoreceptorsRods: Cones:

125 million rod cells

Photoreceptors responsive in LOW LIGHT conditions

NOT useful for fine detail

Best suited for NIGHT and PERIPHERAL vision

Absorbs all wavelengths of visible light but their inputs are perceived in GRAY TONES

Have HIGH sensitivity

6 million cone cells

Photoreceptors responsive to COLOUR and BRIGHT LIGHT conditions

Used for FINE DETAIL

Need bright light for activation

Have LOW sensitivity

Reading: Fun Cat Facts

• Please read the following handout

How did your eyes react?

Light Adaptation and Dark AdaptationLight Adaptation (time to adjust: seconds or minutes)

Dark Adaptation (time to adjust: up to hours)

Occurs when going from darkness into bright light (ex: movie matinee or waking in the morning)

Initially, we all see in white light because the sensitivity level of the retina is still set for dim light

Both Rods and Cones are stimulated strongly and therefore causes an influx of information

Compensations occur in this situation:

1. the sensitivity of the retina decreases dramatically

2. the retinal neurons adapt rapidly, switching from the Rod to the Cone System

Retinal sensitivity (rod function) is lost, but visual acuity is gained

Occurs when we go from a well-lit area to a dark area (ex. Walk indoors on a sunny day)

Initially we see blackness because our Cones stop functioning in low light our Rod pigments have been bleached out by the bright light

Compensations occur in this situation:

1. The sensitivity of the retina increases

2. The Rods adjust over time, thus switching from a Cone to a Rod system

Rod function is essential here as they are best for night and peripheral vision

Are You Colour Blind??

Are You Colour Blind?? (Cont’d)

Are you Colour Blind??Ishihara Test:

Normal Color Vision Red-Green Color Blind

Slide: 1 25 Slide 1: 25

Slide: 2 29 Slide 2: Spots

Slide: 3 45 Slide 3: Spots

Slide: 4 56 Slide 4: 56

Slide: 5 6 Slide 5: Spots

Slide: 6 8 Slide 6: Spots