THE EYE The eye is the receptor organ that is highly specialized to trap light and very sensitive to changes in the immediate surroundings. It is located in a bony socket in the cranium called the orbit and has a number of muscles attached to it so it can be moved. Eyelashes and Eyelids protect the eye from foreign particles. Tear glands produce tears which possess the enzyme lysozyme which destroys microorganisms.

The Eye

Functions of parts of the Eye Part of the EyeFunction ScleroidLight proof fibrous tissue ChoroidPrevents internal reflection, supplies cells with oxygen and food, removes waste RetinaLight sensitive layer, contains Rods and Cones Fovea(yellow spot)Contains cones which work in bright light and gives us our sense of color. Optic nerveCarries impulses to the brain Vitreous humourJelly -like material which maintains the shape of the eye Ciliary muscleInvolved in focusing Suspensory ligamentsInvolved in focusing IrisControl the amount of light entering the eye PupilHole in the iris which allows the light through CorneaTransparent front part of the eye where the greatest amount of refraction (bending) of light occurs LensUsed to focus objects onto the retina Aqueous humourMaintains the shape of the eye ConjunctivaProtects the Cornea

Control of light entering the Eye The Iris consist of involuntary muscles arranged in two layers : radial and circular. The muscles work in antagonistic pairs and control the size of the pupil in response to the amount of light entering the eye. DIM LIGHTBRIGHT LIGHT Circular muscle relaxes Circular muscle contracted Radial muscle contracted Pupil dilates (large) Radial muscle relaxes Pupil constricted (small)

IMAGE FORMATION Light is made up of small particles called photons that travel in straight lines called rays. When light rays strike an object and rebound off them and enter our eyes we see the object. When light rays enter the eye they are bent (refracted) by the cornea and the lens; this leads to the rays being focused to form an image on the Retina. Nervous impulses are sent to the cerebrum of the brain. During focusing of the rays the image is turned upside down then back to front on the retina and the cerebrum of the brain then interprets the image, turns them the right side up and the right way around.

RETINA : Rods and Cones Light sensitive part of the eye that contain light sensitive cells called rods and cones. Rods function in dark light. They contain a light sensitive pigment called Rhodopsin which is broken down(bleached) by even low intensity light resulting in an impulse being sent to the brain. Dark adaptation is the time taken for the eye to adjust from a transition from light to dark and during this time bleached rhodopsin is resynthesised. Cones works only in bright light. The pigment Iodopsin in the cones is not broken down in dim light. Found mainly in the fovea and allow us to see bright light. There are three types and each are sensitive to only blue, red and green light. The brain interprets the impulses as different colors depending on the type and ratio of the cones stimulated.

Diagram of Rods and Cones in the eye

ACCOMMODATION The Cornea bends most of the light however it is the lens that can change its shape. The bending of the light by the lens to focus on near and distant objects is called Accommodation. The ciliary muscle adjust by contracting or relaxing the suspensory ligaments that hold the lens, hence changing its shape. EYE COMPONENTSUnaccommodated eye Accomodated eye Ciliary musclesRelaxedContracted Suspensory ligamentsTaut (tighten)Slacken Lens shapePulled flatBulges (fattens) Distance of focused object farnear

Accommodation in the eye (near & far)

Defects of the eye Size of the eyeball, shape of the lens and ability of the lens to change shape determines the ability of the lens to focus images on the retina. Any changes to these factors leads to images not forming properly in the eye. The main eye defects is Long sightedness, Short sightedness, Astigmatism, Cataract, Glaucoma and old sight (presbyopia).

Long sightedness Images from distant objects are focused on the retina but near objects are focused behind the retina so they appear out of focus. It is corrected using a converging (convex) lens in a spectacles of a contact lenses. The lens refracts the light sufficient to focus the diverging light rays from a near object

Short sightedness People with this defect are able to see near objects but find it difficult to see distant objects because the light rays reflecting from it are focused in front of the retina. This defect is corrected by using a diverging (concave) lens in spectacles or contacts.

Astigmatism Cornea or lens has an uneven curvature leading to one side of the lens focusing too much light while the other side does not focus enough light. Most of the image is seen out of focus. Special lenses made of glass or hard plastic must be used in glasses or as contact lenses to counteract this defect.

Cataract and Glaucoma Cataract – the lens become opaque or milky with age. It can be treated by surgically removing the tainted lens and replacing it with an artificial lens or a transplant. Glaucoma- the aqueous humour is formed at a faster rate than it’s reabsorbed thus increasing the pressure inside the eyeball. It requires immediate treatment either by use of drugs or by operation to drain out the excess fluid. If nothing is done the person will lose sight in the effected eye.

Old sight (Presbyopia) Over the age of 40 the elasticity of the lens is gradually lost leading to the lens becoming more or less fixed in the eye and only able to focus on distant objects. Spectacles that have a single converging lens or bi- focal lenses for reading.