In today's video, with the help of the cadavers here in the lab,
we're going to take a look inside a real human eyeball, as well as discuss
how on earth such a complex structure can even evolve in the first place.
It's going to be incredibly cool.
Let's do this.
You are looking at a real human eye.
Now, the outermost layer of the eye is what's known as the fibrous layer
or fibrous tunic, and it's made up of the white,
which is more accurately known as the sclera as well as the cornea, which
we're going to be able to see much better from this cross-sectional view.
So I’m actually gonna place this half to the side for now,
but as we're looking at this, you can see that fibrous tunic.
There's the sclera, the tough outer layer
as it's connecting, it's continuous with the cornea.
So this is the cornea that I'm poking with the probe here.
And then it's going to go back down and line up with the rest of the sclera.
And then they're going to go round,
and eventually in the back, we meet the optic nerve.
So this is where all the neurons
that are going to be heading towards the brain are going to be located.
Now, if we look at this red portion,
this red portion is what's known as the vascular tunic,
and it's just deep — from superficial to deep, right, this is the outermost layer.
So this is most superficial section of the eye
— it goes, fibrous tunic, then it goes vascular tunic, which is where
all the blood supply goes — comes into the eye from.
And then the internal tunic is this “film”.
So if you look closely, you can see this white film
that's actually the internal tunic or the retina.
That's the photoreceptive layer
that all the rods and cones are going to be placed in, although
most of what you're seeing here would be just rods.
The cones are going to be more densely
located in the back
in what's known as the fovea, although we won't be able to see that today.
But it's just cool to see that the eye is layered,
as most things are in within human anatomy.
But I'm going to go ahead and grab this other half again
so we can see
this structure here. This is the lens.
Now, the lens actually broke a bit during the dissection process, but that's okay.
But just know that this would be more solid.
This is just a crystalline
network of proteins that will help further refract the light.
So what I can do with my probe is I can slide it between the cornea.
And this right here is actually the iris.
So if you look closely, you might be able to see the iris,
which is actually two different muscles called the dilator and sphincter
pupillae muscles.
And then there's a hole in the center of them called the pupil.
So when you see the black of the pupil, you're actually looking into the eye
and seeing the darkened pigment of the internal eye.
But in this space where my probe is,
there would be what's a humor called the aqueous humor.
And the aqueous humor helps provide nutrients to that cornea.
be refracted through the aqueous humor, go through the pupil,
which the dilator and sphincter pupil muscles are going to be making
it wider or smaller, depending on how much light needs to be let in.
And then the light is going to go towards this lens.
The lens will then refracted once again into this space.
Now, this space would normally be filled
with another humor called the vitreous humor.
The vitreous humor is going to be
very jelly-like so upon dissection, you can like squeeze it out.
And that light is then going to go towards the back of the eye
to that photoreceptive retina.
And again, there's one small area that we can't really see in this dissection,
but it's called the fovea And the fovea is where all of the cones,
or at least the vast majority of your cones
are going to be located, and cones detect color.
The rest of the retina, even though not all of the retina is actually
visual, are going to have rods in them and rods detect low light sensitivity.
So they're really kind of like black and white vision, more or less.
The retina is also going to have pigment cells in them.
And that's what gives
a lot of the darkened aspect to the eye along with the vascular tunic.
And that's important because if the inside of the eye
was bright, like the sclera, light would just be bouncing
around inside of the eye and it would be impossible
to know where the light came from and you wouldn't see anything.
So you actually have to be able to absorb the light in order to see it
when it hits the proper photoreceptors.
But again, all the rods and cones and all of that, well, it's
not the rods and cones...
They're going to be connecting to bipolar
cells and ganglion cells and amacrine cells.
But all the cells inside of the retina are then going to be sending their signals
that are then going to go through that optic nerve
and then just make
their way to the optic chiasm and all the way to the back of the brain.
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I want to apologize right off the bat because I'm a terrible artist.
But what I'm attempting to show you here is the evolutionary progression
from what's known as an ‘eyespot’, all the way to
what's known as a camera-style eye, which is what we have today.
So let's first orient you as to what the colors are showing
so you have a better understanding of what you're looking at.
The orange is what's known as a photoreceptive organelle,
and this photoreceptive organelle is able to detect the presence of light
and it will then send a signal through the pink, which is representing
neurons or nerve fibers — kind of depends on how you want to look at it —
and they're traveling through the black, which is representing a micro-organism,
at least at this point.
Eventually, this black is going to transition
into the outer portion of an eyeball.
But at this very basic level, you're essentially looking at a microorganism
that has the ability to detect the presence of light, and that's about it.
But that's actually very useful because if you're able to determine that
there is light or isn't light, this is the beginnings of a circadian rhythm.
This is what is throughout all of the animal kingdom, and it's
extremely essential to a wide variety of bodily processes.
But this is still very basic.
All you're able to say is, “do we have light, or do we not have light?”
That's why this next design is really, really cool, and I'll show you what
the green means in a second.
But what's actually happening here is that photoreceptive organelle
has now become what's known as a ‘eyecup’.
And this is going to actually help
determine the direction that light is coming from.
And that's what the green is representing.
So let's say a photon comes from this direction
and hits this aspect of that photoreceptive organelle.
Well, then obviously you can determine that the light came from that direction.
And so that's what all these other green lines are showing you.
But it's important to understand that this is a very broad understanding.
This is not very precise.
It's more like the light is coming from that direction or from that direction.
It's not like it's coming from there or there.
Instead, that's when we get to this one.
And as you can see here, the that outer aspect of the organism
has created a really small aperture.
So we're really starting to get into this camera-style eye.
And as you can see is that photon now comes in here
if it's able to make its way through this tiny gap
and hit this aspect of that photo receptive layer, which, you know,
you might be able to consider this a retina at this point.
So I'll come back to that in just a second.
But if you're able to hit this specific aspect, you have a lot
better of an understanding of where that light is actually coming from.
Now, the reason why I say it's hard to know if this is a retina is because,
understanding the exact transition points from one structure to another,
it's it's a very difficult thing to do inside of evolutionary biology.
In fact, it's hard to even say when does this become an eye?
We know it's an eyespot, but some say the eyecup
can actually be considered the first eye because you're able
to determine spatial information while others say no,
you need more complex structures to really be considered an eye.
So, I mean, it's just up for debate.
But regardless, this is what's known as a ‘pinhole’.
By creating the pinhole, we have much more precision.
So we went from an eyespot, to an eyecup, to a pinhole,
and then we transitioned to this.
And what you'll notice is there is this dark blue line that is coming across.
This is the beginnings of a cornea.
The cornea is a refractory piece of tissue that is going to help bend light
to get to specific areas on that retina.
But it's also important to understand that it's now also closed off
the outer aspect of this, I guess, eye, at this point?
So if we quickly go back to here when there's no cornea,
you have to understand that this organism is in the water at this point,
which means this hollow cavity here is really a water filled cavity.
But by closing it off with a cornea now, the body can create
or the organism can create something called humor.
Humor is a jelly-like substance that is going to assist in
how light is being transmitted and going throughout
this photoreceptive structure.
So we're going to see that you actually have different kinds of humor in our eye.
But from there, this is where we develop what's known as a lens,
and the lens is what's in red.
So that again, that dark blue is representing a basic cornea.
But now we have a lens.
This is another refractory piece of tissue.
And as you can see, I've drawn some light that's coming in from this direction.
And what will happen is it is being bent by the cornea and the lens
and being directed to a specific area in the back of the retina.
This is going to be extremely important, especially when you consider
just the diversity of rods and cones and just how there's really precise
areas for vision.
But it's really cool to see that what's happening is now it's like,
cool, we're going to bend light.
It's not it's not just that we want light coming from directions,
we want to get light to a very specific spot.
Really, really cool.
But what you also can't see, because again, I'm a terrible artist,
is what's also in here is going to be the beginnings of what's known as the iris.
The iris is a muscular tissue that's able to dilate and constrict, and
there's a hole in the center of the iris, which is what we call the pupil.
By dilating, it can let more light in.
By constricting, it closes off more light.
So again, now we're getting a very precise aperture here.
And when we go back to this one,
this aperture — this opening is really just created
by the organism itself, the wall of the organism.
Well, it's important to understand that this organism there,
they move — they’re fluid.
So it's not like this is set in stone, but still it's pretty much going to be
essentially the same size.
But now with an iris,
you're able to control how much light is getting in and out,
and then you bend it through the lens and the cornea.
And then last we get to our type of eye, which is known as a camera-style.
And again, you can actually start seeing this is a camera-style eye, but
this is just a —you know — a better camera.
So what's happened is the cornea is now much smaller.
So what's happened is you can see the cornea very broad here,
but now it's getting smaller.
It's also getting thicker.
It's going to actually have its own humor
inside of it.
But then the lens is also become more ovular (elliptical).
I'm not sure I did a great job representing this,
but here it's more circular.
Now the lens is more oval shaped.
And again, this is going to help just the precision
of refracting that light to get it to the back of the retina.
So you can see the progression actually
from an eyespot to a camera-style lens is very logical and rational.
Now, some say that this is not possible through the evolutionary process
— that it's just too complex.
But what's fascinating is that scientists,
through the study of say, like genetics, proteomics,
just overall microbiology, studying cranial fossils
of many different organisms, have discovered that this entire process
can happen in as little as 500,000 years, which,
geologically speaking, is the blink of an eye.
So really, really cool.
I often tell my students you'd be surprised with what nature can do
when there are environmental pressures accompanying mutations.
Thanks for watching, everybody.
There are a ton of different topics we can discuss with the eye
so be sure to let us know in the comments below what you'd like to hear from us.
Maybe it's amblyopia, myopia,
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the list goes on and on.
So again, just let us know what you'd be interested in seeing.
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