Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

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Many more eyes and
other anatomical models
available online
via this link

The structure of the eye

Use the mouse, or tap, to label the various parts of the eye in the horizontal cross-sectional diagram below:

eye - lots of labels to know!

What is the name of the tough white protective layer?

> sclerotic

Why is there also a black layer?

> to prevent reflection inside eye (which would degrade the image)

List (in order) 6 structures or substances that light passes through, on its way from the front to the back:

> conjunctiva / cornea / aqueous humour / lens / vitreous humour /retina

Which parts "bend" the light as it passes through them?

> cornea > aqueous humour > lens

Assuming that the diagram is viewed from above, which eye is shown (left or right)?

> right

The response of the eye to changing light intensity

Complete the diagrams below (by drawing in circles of appropriate sizes) to show the response of the iris of the eye to different lighting conditions. This is an example of a reflex action.

Two opposed sets of muscles in the iris - the radial muscles and the circular muscles - contract to alter the size of the pupil.

main iris muscle set involved is
eye diagram showing iris

main iris muscle set involved is
another eye diagram showing iris

What is the difference between the iris and the pupil?

>The iris is a disc with a hole in the centre > The pupilis the hole!

Interestingly, the colour of the iris is due to the dark pigment melanin, whatever the "colour" of the eyes.

The pupil usually looks black because not much light is reflected out of the eye. Cats' eyes are exceptions, as they have a layer to reflect light and so maximise their sensitivity to dim light.

What is so special about the yellow spot (at the back of the eye)?

> zone of maximum sensitivity

What cells is it composed of, and what is their function?

> cones > give a clear picture, and sensitive to colour

What is the rest of the retina composed of?

> rods (and a few cones)

What are the advantages and disadvantages of these cells?

> work in dim light > vision not as clear

At the blind spot, what structures are missing, and what is there instead?

> light sensitive cells > optic nerve (and blood vessels)

The image which forms inside the eye is apparently upside down. Which of the following statements about image formation are correct?

(use 3 ticks)

- the book is always wrong

- brain interprets information, so orientation is meaningless/

- the image is upside down to prevent damage to your eyes

- inversion depends how far away the object is

- the image is usually smaller than the object itself/

- it shows a person who is cross-eyed

- the rays of light are brought to a focus on the retina /

Activity: Finding out about the blind spot

This may not work straight away with the image on the screen. Try viewing the screen at different distances, and make sure you keep your head level!

Blind spot expt; dot and cross 8 cm apart

1) Close or cover your left eye, and use your right eye to look at the circle, and see what happens to your image of the + sign.

2) Still with your right eye only, look at the + sign, and see what happens to the circle.

3) Repeat 1 & 2 above, but this time look with your left eye instead.

What do you conclude from this?

> The blind spot on one eye is opposite to (i.e. NOT covered by) the one on the other side

Accommodation - how the eye forms sharp images of near and distant objects

Sometimes the eye is compared to a camera, although it should probably best be compared to a video camera.
Unlike a cheap camera, the eye can be made to bring rays of light from different distances into sharp focus. This focussing (focusing) is also called accommodation.
Unlike an expensive camera, focussing in our eye does not involve moving the lens in and out (not to be confused with zooming)! Instead, the eye uses the natural elasticity of the lens to alter its shape (and hence its focal length). This action, which is completely automatic, involves the contraction of a ring of muscle (in the "ciliary body") against the pressure of fluids inside the eye.

You are expected to know the roles of the cornea, lens, ciliary body, suspensory ligaments in the process.

Complete the 2 diagrams below to show the changes in the parts of the eye as described below, and show the tension in the suspensory ligaments. You could also try to add lines to show the rays of light, and the image.

To see a distant object

distant object blank space 300 pixels wide

ciliary muscle ring relaxed
suspensory ligaments taut
lens pulled thin

To see a near object

near object Lens must be fatter to focus on a near object

ciliary muscle ring contracted
suspensory ligaments slack
lens bulges fatter

Draw in the missing sections in the eye diagrams above.

Defects of vision

Focussing problems: If the rays of light are not brought to a focus properly, either because of the size of the eyeball, or because of the refracting power of the lens, then the resulting defects of vision ("short sight" or "long sight") can usually be corrected by wearing glasses or contact lenses.

LASIK (Laser-Assisted In Situ Keratomileusis) is a surgical procedure which can reduce dependency on glasses or contact lenses. A consultation at Lasik Austin or one of the many other LASIK centers throughout the U.S. can determine if a patient is a good candidate for the procedure.

If the eyeball or lens is not symmetrical, then astigmatism is the result, and special lenses are needed .

In middle age, the lens may lose its elasticity, so it will not change shape to accommodate for close vision, and "reading glasses" are probably the solution.

Other problems: If the lens becomes cloudy, the result is called a cataract. This may be corrected by removal of the lens. Henceforth (more powerful) glasses are needed to make up for the lost refractive power. Cloudiness of the cornea may be dealt with by a corneal transplant - a fairly simple operation.

Blindness may have many causes, such as problems with the retina, optic nerves, and the part of the brain dealing with visual information.

Colour blindness is presumably caused by problems with the cones. Red-green colour blindness (the most common form) affects the ability to distinguish between certain colours, and is much more common in males than females - see later notes on Genetics.

Binocular (two-eyed) vision

The fact that we have two eyes gives us several advantages.

- It gives us stereoscopic (3-D) vision in the region of eyesight where each eye's field of vision overlaps with the other.

- It also enables us to judge distances. See activity below

- In some animals, these advantages are sacrificed for a wider field of view.

- The blind spot of one eye is covered by the normal view of the other.

- As in many other cases, we can "get by" with only one of a pair of organs, but this does not mean one is a spare!

Activity: Finding out about eyes and judging distance

Place an object like a pen top or rubber on the bench, sticking upwards, at about arm's length away from you.
Cover one eye, then try with one action to touch the tip of it with your finger tip. Try again with the other eye only, then with both eyes open.
What do you conclude from this?

> 2 eyes are necessary for judging distance

Nowadays, eye tests for adults are charged for on the National Health Service, but there are some exceptions.
These not only monitor changes in acuity of vision, but also sight-threatening and general health-threatening conditions, e.g. glaucoma, and diabetes, can be detected in these examinations.


Many other anatomical models and charts available by following this link.

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