The human eyeSense organs are receptors that receive stimuli and inform the body of changes in the environment.

The structure of the human eye

Each eyeball lies in a hollow in the skull called the orbit and is attached to the skull by rectus muscles which controls eye movement.

Cornea

Dome-shaped transparent layer continuous with the sclera or the white part of the eye.

Specialised form of the conjunctiva.

Refracts or bends light rays into the eye.

Conjunctiva

A thin transparent membrane covering the sclera in front.

Secretes mucus in order to keep the front of the eyeball moist.

Iris

The amount of light entering the eye is controlled by the two sets of involuntary muscles in the iris, the circular muscles and the radial muscles.

Pupil

Allows light to enter the eye.

Eyelids

Protects the cornea from mechanical damage.

Can be close partially, preventing excessive light from entering the eye and damaging the light-sensitive tissues inside. This is known as squinting.

Blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the cornea.

Eyelashes

Shields the eye from dust particles.

Chapter 14 - The human eye 1

Tear glands

Secretes tears which wash away dust particles, keep the cornea moist for atmospheric oxygen to dissolve which diffuses into the cornea, and lubricates the conjunctiva reducing friction when the eyelids move.

Internal structure of the eye

Sclera

The ‘white of the eye’ which protects the eye from mechanical damage.

Choroid

The middle layer of the eyeball.

Pigmented black to prevent internal reflection of light.

Contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products.

Ciliary body

Contains ciliary muscles which control the curvature or thickness of the lens.

Lens

Transparent, circular, and biconcave structure.

Elastic and changes its shape or thickness in order to refract light onto the retina.

Suspensory ligament

Attaches the edge of the lens to the ciliary body.

Chapter 14 - The human eye 2

Aqueous chamber

Space between the lens and the cornea.

Filled with a transparent, watery fluid known as aqueous humour which keeps the front of the eyeball firm and helps to refract light into the retina.

Vitreous chamber

Space behind the lens.

Filled with vitreous humour which is transparent and jelly-like. Vitreous humour keeps the eyeball firm and helps to refract light into the retina.

Retina

The light-sensitive layer on which images are formed.

Contains photoreceptors which are light-sensitive. They consists of rods and cones.

Cones enable us to see colours in bright light. Each cones contains a different pigment which absorbs light of different wavelengths, working together to allow us to see a variety of colours. They do not work well in dim light.

Rods enable us to see in black and white in dim light. They contain a pigment called visual purple. When the eye is exposed to bright light, all the visual purple is bleached and must be re-formed for a person to see in the dark.

Photoreceptors are connected to the nerve-endings from the optic nerve.

Fovea (yellow spot)

Small yellow depression in the retina which is situated directly behind the lens.

Images are normally focused at the yellow spot.

Contains the greatest concentration of cones, but no rods. Hence, the yellow spot enables a person to have detailed colour vision in bright light.

Optic nerve

Transmits nerve impulses to the brain when the photoreceptors in the retina are stimulated.

Blind spot

The region where the optic nerve leaves the eye.

Does not contain rods nor cones, therefore it is not sensitive to light.

The size of the pupil is controlled by two sets of involuntary muscles (circular and radial muscles) in the iris.

Chapter 14 - The human eye 3

Controlling the amount of light entering the eye

The size of the pupil determines how much light enters the eye.

In bright light, the circular muscles of the iris contract and the radial muscles relax. The pupil becomes smaller or constricts, reducing the amount of light entering the eye.

In dim light, the circular muscles of the iris relax and the radial muscles contract. The pupil enlarges or dilates, increasing the amount of light entering the eye.

The ciliary and radial muscles are antagonistic muscles because when one set contracts, the other set relaxes.

Sight

Process• Light rays are refracted through the cornea and the aqueous humour onto the lens• Lens causes further refraction and the rays converge to a focus on the retina• Image on retina stimulates either the rods or cones, depending on light intensity• Image formed on retina is - Inverted- Laterally inverted- Smaller in size than the actual object

Focus

Distant object1) Ciliary muscles relax, pulling on the suspensory ligaments2) Suspensory ligaments become taut, pulling on the edge of the lens3) Lens becomes thinner and less convex, increasing its focal length4) Light rays from the distant object are sharply focused on the retina5) Photoreceptors are stimulated6) Nerve impulses produced are transmitted by the optic nerve to the brain which

interprets the impulses and the person sees the distant object

Near object

Ur,--

Foeusing art a flear on:iect

When a person is looking at a near object, for

example, when reading a book, diverging light

rays reflecting off the near object are

refracted through the cornea and

the aqueous humour into the pupil.

The following changes occur in the

eye when focusing on a near object:,

'

raysfromnearobject

@ ci l iary muscles contract

fi suspensory ligaments

- slacken

@ lens,becomestnrcKer

focus onretina

focal length oflens deireases

Vertical section of the eye

Figure 14.9 Focusing on a near object

Enlarged portion of eye

{front view)

What is the near point of

your eye?

The nearer an object is to

your eye, the more your

cil iary muscles wil l contract

and the thicker your lens

will be. However, there is

a point where the object

is so close to the eye that

the cil iary muscles must

contract fully in order for

you to see the object clearly.

At this point the lens is most

convex.This ooint is called

the near point of the eye.

lf the object is moved even

nearer to the eye, the image

formed on the retina will be

blurred as the lens cannot

adjust further. Move this

page towards your eye.At

what distance from your eye

does the text blur? Estimate

the near point of your eye.

@ Ciliaty muscles contract, relaxing their pull on the

suspensory ligaments.

@ S,trpensory ligaments slacken, relaxing their pull on the lens.

@ fne lens, being elastic, becomes thicker and more convex,

decreasing its focal length.

@ Ught rays from the near object are sharply focused on

the retina.

@ Photoreceptors are stimulated.

@ Netrre impulses produced are transmitted by the optic nerve

to the brain. The brain interprers the impulses and the

person sees the near object.

Focusing or accommodation is necessary so that objects at different

distances can be seen clearly. In focusing, the thickness or curvature

of the lens is adjusted.

To focus on a distant object, the cil iary muscles relax, the suspensory

Iigaments become taut, and the lens becomes thinner and less

convex.

To focus on a near object, the cil iary muscles contract, the

suspensory l igaments slacken, and the lens becomes thicker and

more convex.

Chapter 14 - The human eye 4