The Eye –(cont’d)
“Stand for something or you will fall for anything. Today’s mighty oak is yesterday’s nut that held its ground.”
– Rosa Parks
WHAT IS it that you truly believe in? Are you willing to sacrifice for your belief? Do not allow yourself to be swayed by public opinion? Be the acorn that will develop into the mighty oak tree.
This week, we continue with our study of the eye. Can you create a sentence that will summarise the functions of the eye? Did you say that it is a light-sensitive organ that enables humans to distinguish variations of colour, shapes, sizes, brightness and distance? If you did, then you are right, because that is what it is! Last week when we looked at the structure of the eye, you were reminded that you needed to study the diagram and its associated labels, as well as their functions.
Use the diagram below to test your knowledge.
This week, we begin by looking at some of the structures that are associated with the eye. These include:
■ The eyebrow – which is responsible for preventing sweat moving down the forehead from draining into the eye. (Boys, you really should not shave those eyebrows, especially if you are very active! Ladies, the eyebrow pencil just does not serve the same purpose!)
■ The eyelids – these close to protect the eye against dust and bright light.
■ The tear gland – which produces tears to wash away dust particles, and contains the enzyme lysozyme that kills bacteria.
■ The eyelashes – which keep the front of the eye free from dust and dirt.
How do we see?(Revision of previous work)
Remember that the stimulus for the eye to function is light, so when there is no light we are unable to see. For us to be able to see, the following must take place:
1. Light rays from the object travel in a straight line to the eyes.
2. Light passes through the structures at the front of the eye, the cornea, pupil, lens, and aqueous and vitreous humours to become focused on the retina.
3. The light stimulates the light-sensitive cells of the retina.
4. The light -sensitive cells are the rods and the cones. The rods are sensitive to dim light and enable us to see in black and white. The cones are sensitive to bright light and help us to see in colour.
5. The lens vary the amount of refraction (the bending of light rays as they pass through objects of different densities) so as to focus the light rays to create a sharp image on the retina.
6. The black pigment of the choroid absorbs the light after it has passed through the retina.
ACCOMMODATION
The eye has to make adjustments to enable us to see objects that are near and those that are far. This adjustment is carried out by the changing of the shape of the lens and is known as accommodation. These changes are brought about by the relaxation , or contraction, of the suspensory ligaments and the ciliary muscles. The following summarises the changes that take place in accommodation.
DISTANT OBJECTS
1. Ciliary muscles become relaxed.
2. Suspensory ligaments become taut (not slack).
3. Lens becomes thin.
4. Less light is refracted. The light rays from distant objects are almost parallel when they enter the eye. The image is focused on the retina.
NEAR OBJECTS
Light rays coming from near objects are separating from each other (diverging).
1. Ciliary muscles contract.
2. The suspensory ligaments slacken.
3. The lens become more rounded (fat).
4. More light is refracted.
5. The image becomes focused on the retina.
As an individual ages, his or her power of accommodation lessens and the range of distances over which he/ she can see sharp images is reduced. This becomes more noticeable in dim light than in bright light. Have you ever wondered why everyone over a certain magical age (40 and over ) needs reading glasses, or has to hold the paper about a metre from the eyes in order to see the words properly? Now you know! Their lenses are losing their ability to carry out accommodation!
Next week ,we will look at the defects that can affect the eye. See you then!