THE EYE

 

The eye is a spherical structure situated in the orbit of the skull.

 

Label Handout     COMPONENTS OF THE EYE

 

Ciliary muscle, Suspensory Ligament, Lens, Iris, Pupil, Aqueous Humour, Cornea, Conjunctiva, Sclerotic Layer, Choroid Layer, Retina, Blood Vessels, Optic Nerve, Fovea, Blind Spot, Vitreous Humour.

 

FUNCTION OF COMPONENTS

 

EYELIDS                   Cover and protect eye, voluntary or involuntary movement.

 

Distribute fluid over eye surface.

 

TEAR GLANDS        Found in upper part of orbit.

 

Secrete tears to clean and moisten eye H20, NAC1, NaHCO3, and Lysozyme.

 

EYE MUSCLES         Each eye is held in place and moved by six muscles (extrinsic eye muscles).

Allow both eyes to look at the same spot.

 

SCLEROTIC LAYER           Thick layer of tough white fibres.

Protection of underlying structures.

Maintenance of shape of eye.

 

CORNEA                   Found at the front of the eye.

Transparent to allow passage of light.

Continuous with the sclerotic,

Causes refraction (bending) of light rays.

 

CHOROID LAYER   Layer containing blood vessels and dark coloured pigments.

Blood vessels -food and oxygen

waste disposal.

Pigments -                    Prevents light from being reflected inside the eye.

 

CILIARY BODY       Makes up the ciliary muscles.

Controls the shape of the lens during focusing.

 

SUSPENSORY

LIGAMENTS                        Connect ciliary muscle to lens.

Do not stretch or compress.

 

 

 

THE LENS                 Transparent tissue

Biconvex

Changes shape in response to ciliary movement.

Focus near and far objects on the retina.

 

Thin Lens       for distant objects

less refraction

 

Thick Lens     Ciliary muscles contracted

more refraction

focus for close objects

 

IRIS                            The coloured part of the eye.

In front of the lens.

Has a hole in it called the PUPIL.

Made up of radial and circular muscle.

Radial muscle contracts - pupil enlarged

Circular muscle contracts - pupil size reduced.

Regulation of amount of light entering the eye.

eg., Bright sunshine - pin point pupil.

 

RETINA                     Layer of light sensitive cells at the back of the eye.

Light is focused on the retina, by cornea - aqueous humour - lens ‑vitreous humour.

Light patterns are connected to nervous impulses which pass along the optic nerve to the visual cortex of the brain.

 

AQUEOUS

HUMOUR                  Found in front chamber of eye

Plasma like

Supplies lens and cornea with food and O2 and CO2 out.

 

VITREOUS

HUMOUR                  Give shape to eye

Maintenance of position of retina.

Focusing.

 

 

 

 

 

 

 

 

 

 

 

 

 

FOCUSING  (Accommodation)

 

 

ie., The changes in the eye to get an in focus image of the retina.

 

REFRACTION          In cornea, aqueous humour and vitreous humour is constant.

 

REFRACTION          By lens may change.

 

1.         Distant objects                       Lens becomes less convex

Light refracted less.

 

Achieved By                           Contraction of radial ciliary muscle

 

 

Low angle                                                                    These pull against the

of refraction                                                                  suspensory ligaments

Therefore lens is stretched.

 

 

2.         Near Objects              Lens becomes more convex (fatter), therefore more refraction to focus the diverging rays from a close object onto the retina.

 

Achieved By                           Contraction of circular ciliary muscles relaxation of radial circular muscles.

 

PARASYMPATHETIC

SYMPATHETIC

ATROPINE                            Reduction of tension on suspensory ligaments.

 

Lens allowed to become thickened.


 

THE RETINA

 

Label Handout           Direction of light rays

Nerve fibre to brain

Relay neurone

Cone, Rod

Pigment Cells

Choroid

 

 

Two forms of light sensitive cells.

 

Cone - for colour vision

Rod -  light and dark only

 

 

Rods only

Rods and cones

Blind spot - where optic nerve leaves the eye

Cones only (Fovea).

 

BRIGHT LIGHT       Most information carried to brain via Fovea.

Colour, lots of information.

 

DIM LIGHT              Rods only

No colour

Dim objects (eg., star) may therefore be seen more clearly if looked at to the side.

 

TEST FOR BLIND SPOT

 

 

+                                              .

 

Rods contain visual purple (Rhodopsin) which is bleached in bright light.

 

Vitamin A is needed for this pigment.

 

We do not usually see the blind spot because we have two eyes.

 

 

 

 

 

 

 

BINOCULAR

VISION                      1.         Distance judgements

2.         Three dimensional impression

3.         Larger field of vision

4.         Defect in one eye does not cause blindness.

 

 

EYE DEFECTS RAY DIAGRAM HANDOUT

 

LONG SIGHT           People who cannot focus on near objects.

 

SHORT SIGHT         People who can not focus on distant objects.

 

 

OPTIC NERVE                                 BRAIN                       SIGHT

 

 

 

 

 

OPTIC

CHIASMA