EYE ANATOMY AND HOW IT WORKS

Tahoma Jr. High8th Grade ScienceMaple Valley, WA

All vertebrate (backboned) animals’ eyes are built and work the same way with some slight differences.

We briefly learned that some animals can see more than just the humans’ visible spectrum. Pit

vipers have pits under their eyes with infrared (IR) sensing organs to see heat in the dark – like

the rat shown.

Here’s Mr. Sias waving “Hi” with an

IR camera at a museum in

Bozeman, MT

(would you call him a “hot-head”??)

We also learned that some insects can see ultraviolet energy, and flowers appear much different at those wavelengths.

The visible spectrum (to humans) is quite narrow – only 300 billionths of a meter wavelength separates the violet from red.

Here’s the colors we are most sensitive to.

First let’s look at how the eye moves.

There are three pairs of muscles to move

them:

•up and down•left and right

•twist or rotate a little

(you do NOT need to know the muscle names)

Location of the eyes can tell a lot. If both eyes can focus on something at the same time, they are probably a

predator (they have depth perception for accurate strikes – falcons move

at around 200 MPH!).

If the eyes are on the sides, they are usually prey and need “wrap around”

vision to best see the predator coming and get out of the way!

There are a number of parts and you must learn to identify them and understand what they do to help you see.

Let’s follow a light ray as it hits the eye.

On the outside the ray may hit the white leathery, tough part called the SCLERA or it may hit the clear part called the

CORNEA. Think of the cornea as the “window” into the eye.

CIRCLED WORDS IN RED ARE TESTABLE PARTS OF EYE

The cornea is much more dense than air, so the light ray will change direction and most of the focusing happens here.

Since the cornea needs to let light through, and blood vessels will block that light, the AQUEOUS (water) HUMOR bathes the cornea with nutrients and oxygen.

The amount of light must be controlled, so the colored muscles of the IRIS will dilate (open) or contract (close) just

like the shutter of a camera.

By doing this the iris creates an opening called the PUPIL. The pupil is actually a hole, not a “thing”.

Once through the pupil, the light ray will meet the LENS, where the fine focusing occurs. There are muscles attached around the

outside of the lens and will stretch it change its shape.

As we get older (around 40) the lens becomes stiffer and we’ll often need reading glasses.

Also, ultraviolet light can cause the lens to get cloudy causing a condition called CATARACTS.

Once through the lens, the light passes through the VITREOUS (aka vitrious) HUMOR, which is more like jelly

than water, to help the eye hold its shape.

You can easily remember which “Humor” is which by following the light ray. It hits them alphabetically: Aqueous first, Vitreous next.

If too much fluid pressure builds up in the aqueous and vitreous humors, then it can

squeeze out the blood supply to the retina.

This condition is called GLAUCOMA.

There is actually an illegal (in most states) street drug

sometimes used for treating glaucoma. However, you

should not use it unless you HAVE a prescription!

Otherwise, you’ll find yourself in a cell with Justin or Miley…

Once through the vitreous humor, the ray will hit the RETINA, which is like the film or projection screen. Its made of nerves

and is actually an extension of the brain.

Hopefully the image will be focused properly onto the retina. But often it does not.

The cornea does most of the focusing, and sometimes even with the help of the lens, the focus point that should happen

on the retina happens too soon or too late.

Myopia (near sighted) Hyperopia (far sighted)

Astigmatism (irregular curve on cornea)

Due to simple laws of physics, as light rays enter the eye they must cross over – creating an inverted and reversed image.

You are seeing UPSIDE DOWN! – your brain is automatically flipping it over to make sense of the world!

This shows how the image is not only upside down, but has to also be flipped left to

right.

Look at the dark blotch in the green circle carefully.

The retina is the “projection screen” or “film”. In humans, there are two basic types of nerves in the retina:

Rods (which are sensitive to low light levels)Cones (see color but need lots of light to work)

This is why at night you see things in shades of grey.

How do we know dogs can’t see color, or cows or deer. Because if you look at their retinas under microscopes, there are no cones! And you can stimulate cones with only red, or

green or blue light and they fire.

So there’s no use using that red flag to provoke a bull – he’ll charge anything moving…

and sometimes win!!

Yea bull !!

All these nerves have to meet up somewhere, and they join at a point called the optic disc which causes a BLIND SPOT.

You have one in each eye, but the missing information is filled in by the other eye and brain.

All these nerves bundled together are called the OPTIC NERVE which connects with the brain.

Here’s a picture of Mr. Papers’ retina in his left eye.Notice the bright blind spot (where the optic nerve connects), and darker shaded area to its right where the MACULA and

FOVEA are (best vision).The fovea is off to the side to protect it, and when the eye twists slightly to look at something carefully, it puts the image on the fovea where there’s lots of cones.

So what causes eyes to reflect back bright colors? It’s called the tapetum – a shiny layer on the retina and is used to reflect

the light a second time to help see in the dark.

2015 - this guy had a bad retina replaced with a smaller artificial one!

Well, that’s it for the eye.

Be sure to know the parts before we dissect a real one, so you know what

you’re looking at !!

“See” Ya !!