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Worksheet 2: The Human Eye - structure and function.


THE HUMAN EYE

Human eye

We are able to see because light from an object can move through space and reach our eyes. Once light reaches our eyes, signals are sent to our brain, and our brain deciphers the information in order to detect the appearance, location and movement of the objects we are sighting at.

The whole process, as complex as it is, would not be possible if it were not for the presence of light. Without light, there would be no sight.

The human eye is the organ which gives us the sense of sight, allowing us to learn more about the surrounding world than any of the other five senses.

Diagram of human eye

The eyeball is set in a protective cone-shaped cavity in the skull called the orbit or socket and measures approximately one inch in diameter. The orbit is surrounded by layers of soft, fatty tissue which protect the eye and enable it to turn easily.

Structure of the Eye

In general the eyes of all animals resemble simple cameras in that the lens of the eye forms an inverted image of objects in front of it and projects it onto the sensitive retina, which corresponds to the film in a camera.

The front of the eye mainly contains an elaborate array of structures which are mainly concerned with the refraction (ie bending) of light rays and bringing them into focus on the retina. The structure most directly involved is the lens.

The eye changes light rays into electrical signals then sends them to the brain, which interprets these electrical signals as visual images via the optic nerve.

Structure of human eye and refion of retina

 

The Lens

Focusing the eye is accomplished by a flattening or thickening (rounding) of the lens. By altering its shape, the lens can focus on near and far objects.
The lens becomes flatter for distant objects and rounder for closer objects.

The transparent, double convex (outward curve on both sides) structure suspended between the aqueous and vitreous humours helps to focus light on the retina.

Suspensory Ligaments

These are elastic-like structures present in the eye that suspend the lens and pull it into shape for focusing distant objects onto the retina. This process is known as accommodation.

Ciliary Body

For nearer objects the lens is increasingly rounded by ciliary muscle contraction, which relaxes the suspensory ligament.
When the ciliary muscles are relaxed, the perimeter of the lens is pulled outwards giving it a flattened shape. The ciliary body projections are responsible for the secretion of the aqueous humour.

Diagram of visual accommodation

Aqueous Humour

As opposed to the vitreous humour, the aqueous humour (as its name suggests) is a clear watery fluid that occupies the front chamber of the eye and nourishes the lens and the cornea. It also helps to give the eye shape, and protection. The aqueous humour is secreted by the ciliary processes

The Vitreous Humour

The spherical shape of the eyeball is maintained by this jelly-like substance. It is the transparent, colorless mass of gel that lies behind lens and in front of retina.

Cornea

The front of the eye is protected by the thick transparent cornea, which like the lens, also refracts (bends) light rays.
The front of the cornea is covered by a thin layer of epithelium which is continuous further out with the conjunctiva

Sclera

The sclera is the tough, white, outer layer (coat) of the eyeball, and is continous with the cornea, it protects the entire eyeball.

Iris

The iris contains circular and radial muscles.
Different contractions of these 2 sets of muscles has the effect of varying the size of the pupil hole.

The iris is also what determines our eye color.
Eye colour depends on the amount of pigment on the back of the iris, which is constant, and the amount at the front, which is variable. The more melanin pigment found in the iris, the darker the eye colour will be.

Most new-born white babies have no pigment cells at the front, and light is reflected from the rear of the iris and scattered in such a way that it appears blue (a similar scattering of light makes blue skies). After a few months, the eventual colour of the eye emerges as cells bearing melanin build up on the front of the iris. Some people have more melanin than others. A large amount gives brown or black eyes, less gives green and the least amount, blue or grey. Albinos have pink eyes because their irises have no pigment on either the front or back and therefore the 'pinkness' occurs due to reflection of the choroid capillaries behind the retina .

By the age of six, most people's eye colour is fixed, and in most of us, it fades with old age, along with our hair.

Melanin's function in the iris is to help absorb excess light that might otherwise overwhelm our vision. Albinos who lack melanin, for instance, often have eyesight problems associated with an oversensitivity to light.

Pupil

Is the opening in the iris which permits entry of light into eye. Through the opening and closing of the iris' circular muscles excessive light is prevented from falling on the retina and damaging the light-sensitive photoreceptors.

Diagram of iris movements in response to light

The Retina

The retina is nourished by a layer rich in blood vessels called the choroid which is found immediately beneath it and protected by a thick connective tissue coat called the sclera. Heavy pigmentation in the choroid layer and in the epithelium on its inner side, shields the retina and prevents light being reflected within the eye.

All photoreceptors (light sensitive) cells are concentrated in the retina, which lines most of the interior of the eye ball.

Regions of the Retina

The Photoreceptors

Diagram of rods and cones
Diagram of retinal photoreceptors - the rods and cones

Photoreceptors are the unique cell types that are the very crux of our ability to see.
The photoreceptors are light sensitive cell types that reside in the retina - there are 2 types:

For a great animated flash movie about the eye and it's components, visit brainpop.com - vision

 


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