The structure of the eye | Processing the Environment | MCAT | Khan Academy

In this video,
we’re going to talk about the structure of the eye. And we’re going to
do that by drawing a cross-sectional
diagram of the eyeball. The first thing
we’re going to draw is the white part of the eye,
which is known as the sclera. So I’m just drawing that in. And I’m going to
label is sclera. And the sclera is a
thick, fibrous tissue that basically forms the
substance of the eyeball. So the white part of
the eye is the sclera, and it serves to
protect the eye, and it also serves as an
attachment point for muscles so that you can move your
eyeball around when you’re looking at different things. So this white part
is the sclera. The next thing we’re going
to look at– so let’s imagine that this is the
front of the eyeball, and this is the
back of the eyeball. So when you’re
looking at somebody, you’re looking at them this way. So the first thing
you look at is something known as the cornea. The cornea is
actually transparent, so you can’t see it. But it basically serves to
protect the front of the eye, and it also serves to bend
light just a little bit. So let me label this “cornea.” The cornea is
actually protected. It’s really sensitive,
so has to be protected by a thin
layer of cells. It’s a thin layer
of epithelial cells. And it is known as
the conjunctiva. So this thin layer
of epithelial cells protects the cornea
from friction. So when you rub your eyeball,
you don’t scratch your cornea, and that’s because it’s actually
protected by the conjunctiva. It also helps
moisturize the cornea and protects it
from dust and debris so that the cornea is protected. So when you’re looking
at someone’s eye, you look at the cornea. And the next thing you actually
look at– you can’t even see– is the aqueous humour. So inside the cornea–
imagine over here– basically, it’s a chamber, and it’s
filled with a fluid, and that fluid is known
as the aqueous humour. The aqueous humour is
basically just water and salt, it fills in this anterior
chamber of the eye. So “humour” means chamber,
“aqueous” is water, so it’s the water chamber, the
anterior chamber of the eye. So the next thing that
you would see– so let’s imagine a light ray comes in. And as I mentioned,
the cornea kind of bends the light
ray just a little bit. The next thing this light ray
would hit would be the lens. The lens is back here. It’s a bi-convex
lens, which means that it’s curved on both ends. And so when the light
ray hits the lens, it bends a little bit more. So it’ll bend a little bit more. And the lens actually
can change shape, so it can either get thinner
or thicker, depending on whether an object is
nearby or far away. And so the thing that actually
makes the lens thinner or thicker is known
as the ciliary body. And the ciliary body is
composed of a few things. The first are these ligaments
that are connected to the lens, and they’re known as
suspensory ligaments. And the suspensory
ligaments are connected are connected to the
lens, and they’re also connected to the
ciliary muscles. It’s on both sides of the lens. And the ciliary muscle and the
suspensory ligaments change the shape of the lens, and
they form this bigger structure known as the ciliary body,
which also secretes this aqueous humour over here. So the cornea bends
the light, and the lens bends it a little bit further. And what’s interesting is
when you dive under water, everything becomes
really blurry. And the reason is when
light is emitted– I’ll just draw the sun
over here, happy sun. So when the sun emits a light
ray, it actually comes in, and it’s passing
through air normally, and when it hits the cornea,
it gets bent a certain amount. But when you’re underwater,
the air is replaced by water, and so the light
ray actually gets bent a slightly
different amount. So let’s say this is how
much it would get bent if there was air outside,
and this how much it gets bent when there’s water. And so that’s why everything
is a little bit blurry, because the light
rays aren’t converging where they should be. So when you wear goggles when
you’re underwater– so let’s imagine that you’re wearing
goggles underwater– what the goggles serve to do
is they add a little layer of air in front of the cornea
so that when the light hits the cornea, it bends the correct
amount instead of bending this abnormal amount,
which it normally would if there’s water
right outside here. The next thing that
we want to look at is something known as the iris. The iris is actually the part
of the eye that is colored. So if somebody has blue eyes
or if they have green eyes, that’s because the iris
is pigmented differently. The iris is actually
two different muscles that contract and expand. And when they
contract and expand, the size of the hole
right here actually gets bigger and smaller. Let me just label the iris. And this hole is
known as the pupil. Label that here. I know there are a lot of
words, but just bear with me. They should all make sense after
we draw the entire diagram. So the pupil is just a
term for the hole that is controlled by–
the size of the hole is controlled by the iris. If it’s really dark
outside, you want the hole to be really big
so that you can get the maximum amount of light rays
entering the back of the eye. And when it’s really
bright outside, you want the iris to contract
so that the pupil constricts, and there’s less
light entering the eye so that you can actually focus
in on what you’re looking at. So the light ray hits
the lens and gets bent, and now it’s passing through
something known as the vitreous humour. So the vitreous humour makes
up this posterior chamber of the eye. From here to here is
the anterior chamber. And from here to
the back of the eye is the posterior
chamber of the eye. And the posterior chamber
is composed of the vitreous humour. So “humour” means
chamber, and “vitreous” is a jelly-like substance
within the posterior chamber. So it’s composed of water and
some salt and some protein. And the main protein is albumin
in this part of the eye. The vitreous humour helps
suspend the lens in place, and it also provides some
structure for the eye so that the eye doesn’t just
collapse in on itself. It’s also transparent,
so the light is able to just
flow right through. So once the light ray comes
towards the back of the eye, it will hit a structure
known as the retina. The retina coats the
entire back of the eyeball. And so the retina is
composed of a bunch of different cells known as
photo receptors that actually take this light ray and convert
it into a neural impulse that the brain can understand. The retina is tinted red,
it’s reddish in color, and that’s why when you
take a photo of someone at night when the flash
goes off, what happens is the flash goes
off, and the light way enters the back of the eye
and actually bounces off of the retina and gets
picked up by the camera. So it causes something
known as the red-eye effect. And some cameras
actually have something that reduces the red-eye, a
red-eye reduction feature. And so what happens there is you
have two flashes that go off. The first flash simply
serves to constrict the iris so that the pupil
actually gets smaller. And then the second
flash goes off, and when the second flash goes
off to actually take the photo, the pupil is lot smaller,
so there’s less light actually getting
reflected off the retina. So it reduces the
red-eye effect. So the retina sends fibers
through the back of the eye so that the fibers can
actually go to the brain. And these fibers form something
known as the optic nerve. So this is the optic nerve. And let me just
label this “retina.” So the retina sends fibers
through the optic nerve, which goes to the brain,
and then you’re able to make sense of
what you’re looking at. The next thing that we want to
talk about, the next structure, is just inside of the retina,
and it’s just like this. It’s a membrane
known as the choroid. And the choroid is basically
a network of blood vessels that nourishes the retinal
cells and nourishes other cells within the eye. So there’s little
kind of blood vessels that come off of
the choroid that nourish all the
cells in the retina and other parts of the eye. The choroid is also
pigmented black. So that’s why, when we
look at somebody’s eye, if you look through the pupil,
it’s actually really dark. It’s black. And that’s because you’re
looking at the choroid, which is actually pigmented black. And in other species, like
cats, they don’t have pigment, so the choroid is
actually shiny. And what that serves to do is it
gives them better night vision. So you can imagine
a light ray coming in, kind of off to
the side over here. And in humans, if this light ray
wasn’t absorbed by the retina, it would be absorbed
by the choroid, because the choroid is black,
and black absorbs light. But in cats, it’s shiny, so
the light ray would actually reflect off the choroid and
hit the retina again and be absorbed by the retina. So it gets two chances
to absorb the light ray and actually
enhances night vision in other animals, like
cats, for example. So the choroid is this
network of blood vessels, and it’s pigmented black. And I just wanted to point out
one more feature of the retina. There’s a little
dimple over here. And this dimple is
filled with cones. And these cones
actually allow you to see in really rich detail,
a rich amount of detail when you’re looking
at something. And it’s known as the fovea. And the fovea is actually
in the very center of a broader region
known as the macula. So the macula is just
an anatomical name for a particular
region of the eye. In the very center
of the macula is the fovea, which
is rich in cones and lets you see in really
high levels of detail. So these are all the
structures of the eye.


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