The eye (part 1)
Monacia Williams, Contributor

Welcome to another in our series of lessons. By now, your revision should be well on its way and you should be counting down the days to your first paper in the examinations. I hope that from now on your entire focus will be on your studies.

We have been looking at the nervous system over the last few weeks and today we continue with the study of the eye. This is a popular topic on the examination paper so you should spend some time trying to understand the underlying concepts and studying them. The eye is one of the body's most important receptors; try to imagine what it would be like to live in darkness all the time. This should prompt you to have both consideration and respect for the visually impaired.

Humans have both eyes pointing forwards but because they are not at the same place on the head, they view the environment from slightly different angles. This is known as binocular vision and helps us to judge distances. The question on the eye usually begins with a request for the diagram of the eye to be labelled. This means you must know the diagram and its labels as well as the functions of the different parts.

Let us begin. Starting from the front of the eye, let us work our way to the back of the eye.

1. Conjunctiva - thin transparent skin, continuous with the lining of the eyelids. Serves to protect the cornea. Infection of this membrane causes the disease known as conjunctivitis.

2. Cornea - transparent front part of the sclera. It refracts (bends? light rays to a point of focus on the retina. The cornea is responsible for most of the bending of the light rays.

3. Aqueous humour - colourless fluid found at the front of the eye.

4. Iris - coloured disc composed of muscle. Serves to control the amount of light entering the eye. The iris contains circular and radial muscles which contract and relax to change the size of the pupil in dim and bright light. This feature forms the basis of the pupil reflex, an example of a cranial reflex.

5. Pupil - hole in centre of iris which allows light to enter the eyeball.

6. Lens - transparent, elastic, biconvex structure which makes fine adjustments to focus light on the retina. Adjustments are made by changing the shape of the lens. Fat lens bend light more than thin lens.

7. Suspensory ligament - attaches lens to the ciliary body, holding it in place. Changes in its tension changes the shape of the lens.

8. Ciliary muscle - circular ring of muscle fibre which alters lens shape during accommodation. Contraction releases tension on the ligaments so they become loose, relaxation increases tension so the ligaments become taut.

9. Vitreous humour - transparent, jelly-like substance which supports the back of the eye.

10. Sclera/Sclerotic coat - tough, white fibrous coat which protects the eyeball.

11. Choroid - contains blood vessels to supply retina with food and oxygen, as well as a black pigment to prevent reflection of light to the inside of the eye.

12. Retina - contains the light-sensitive cells - the rods and the cones. The rods are sensitive to dim light and allow for black and white vision. The cones are sensitive to bright light and allow for coloured and detailed vision. These are the receptor cells. When the light stimulates them they send impulses to the brain along the optic nerve.

13. Fovea/yellow spot - contains cones only and is the most sensitive part of the retina. Most of the light rays are focused here.

14. Blind spot - point where the optic nerve leaves the eye. There are no light-sensitive cells here.

15. Optic nerve - carries impulses from the retina to the brain.

A section through the human eye

As you can see, the eye has a lot of labels to learn. Learning them may be difficult but definitely not impossible. So what are you waiting for? Begin the process! Try your hand at labelling the diagram of the eye given below.

Figure 2. A section through the human eye

See you next week as we continue our study of the eye.

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com