In the late 1800s, a psychologist named  George Stratton did something bizarre. He  

wore special goggles that flipped his vision  upside-down. For days, he saw the entire world  

inverted. But after about a week, something  incredible happened – his brain adapted. The  

upside-down world started to feel… normal. Now, the interesting part with this is that  

you and I also see the world upside  down every day … we just don’t notice. 

When something enters your visual field, the  available light bounces off that object and  

enters your eyes. It first passes through the  cornea, then through the pupil, which is adjusted  

by the iris, and reaches the lens. The lens bends  and focuses the light onto the retina, and at this  

point, the image is actually upside down. The retina is a thin layer of tissue at  

the back of the eyeball that contains  over 100 million light-sensitive cells. 

These cells convert the light coming in, into  nerve signals, and take that nerve signal outwards  

again towards the optic nerve. The optic nerve  then takes that signal straight to the visual  

cortex at the back of your brain, and that’s where  the image is processed and flipped the right way  

up. And all of this happens instantly. So if the brain is doing the “seeing,”  

what exactly are the eyes doing? How do  structures like the cornea, lens and the  

retina work together to transform simple light  into something your brain can understand? 

In this video, we’re going to break down the  anatomy of the human eye, and we’re going to  

do that by first going through the layers of the  eye, the tunics. Which are basically going to be  

the fibrous layer like the sclera and cornea,  vascular layer like the choroid, ciliary bodies  

and iris, and nervous layer, the retina. We’re also going to cover the anterior  

and posterior segments of the eyes and why  they matter. We’ll be going through the flow  

of aqueous humor, then we’ll go through the  accessory visual structures like the eyelids  

and the tear production system. I’ll also  mention some relevant clinical notes along  

the way to basically make you an expert on the  human eye anatomy by the end of this video. 

What’s up everyone, my name is Taim. I’m a medical  doctor, and I make animated medical lectures to  

make different topics in medicine visually easier  to understand. If you’d like a PDF version or a  

quiz of this presentation, you can find it on  my website, along with organized video lectures  

to help with your studies. Alright, let’s get started. 

We’ll start with the very basic concept as  if you don’t know anything about the eyeball,  

except you know eyeballs are in the sockets of the  orbits. Right? Let’s remove one of the eyeballs  

and put it on the screen. The eye a highly  specialized sensory organ whose entire job  

is to take incoming light and convert it  into electrical signals. Those signals are  

then carried by the optic nerve straight to the  occipital cortex of the brain for processing. 

So technically, your eyes aren’t doing the  “seeing”, the brain is. The eyes are just the  

hardware that captures and transmits the data. Now to understand how this happens, we need to  

break the eye down anatomically. And  the best place to start is with the  

layers of the eyeball, also called the tunics. If we slice the eye in half, you’ll see that it’s  

made up of three main layers that are basically  arranged as the fibrous layer which is the outer  

protective coat, then just underneath there’s  the vascular layer, which is basically a highly  

pigmented layer, full of blood vessels, and  finally, the inner nervous layer the retina,  

is the part that captures light and converts it  into electrical signals to send back to the brain. 

Let us start with the fibrous layer. This is the  outermost coat of the eye, and it’s made up of two  

structures, the sclera and the cornea. Now, the  sclera is the dense, white part of the eye and  

basically acts like a protective shell. It also  serves as the attachment site for the extraocular  

muscles that move the eye in different directions. This outer white coat, the sclera,  

is divided into three layers, the  episclera, stroma, and lamina fusca. 

The episcleral layer is the outermost  layer that is basically made up of a  

thin layer of dense connective tissue. The scleral stroma in the middle is the  

thickest part, made of collagen and fibroblasts,  and gives the sclera its distinctive white color.  

The innermost layer is the lamina fusca, which is  very pigmented and sits right next to the choroid.  

So those are the three layers and again it’s made  up of like non-see through connective tissue.  

And there is a little clinical correlation to  this because sometimes in certain situations  

like jaundice, yellow-colored pigments called  bilirubin can build up in the tissues over the  

sclera and give it that yellow color. Bilirubin  is basically a waste product that comes from the  

normal breakdown of red blood cells, and it is  usually processed by the liver and excreted. But  

when the liver is not clearing it properly, it  can start to build up in the blood and show up  

in the eyes before anywhere else. This can  happen, for example, in Gilbert’s syndrome,  

which is basically they produce too much  bilirubin, or in more serious conditions  

like hepatitis or liver failure, where the  liver’s ability to handle it is reduced.

Alright, that’s the sclera. If we continue  forward, we’ll see the scleral sulcus,  

which is a groove located at the corneoscleral  junction, meaning the junction between the  

sclera and the cornea, so let’s do the cornea,  which is the other one under the fibrous layer. 

The cornea is the transparent part at the very  front of the eye. It is dome-shaped, avascular,  

and highly specialized for refraction. In fact,  the cornea is responsible for most of the eye’s  

focusing power, around two-thirds of the bending  of the light happens right here. Light first  

hits the cornea and gets bent toward the center,  helping to focus it before it reaches the lens. 

Now, if we were to take a section of the cornea,  we would see that it is actually made up of 5  

layers. The outer part is made up of epithelial  tissue. The epithelial tissue is actually rich  

in a lot of nociceptors, or pain receptors. Why is  this important? Because you know whenever you have  

something irritating the cornea, what do you wanna  do? That irritation is going to be sending signals  

to cause you to wanna blink, right? So we have  pain receptors there to help us to let us know  

if there’s any irritation to the cornea. Underneath that is Bowman’s layer, which  

is a tough sheet of connective tissue that adds  structural support. Then comes the stroma, which  

is the thickest layer and made up of organized  collagen fibers that help maintain corneal  

clarity. After that is Descemet’s membrane, a  thin but strong basement membrane that supports  

the final layer, the endothelium. The endothelium  is critical because it helps pump excess fluid  

out of the stroma to keep the cornea clear. Now what’s interesting with the cornea is that it  

is considered avascular, meaning it doesn’t really  have a lot of blood supply, specifically the  

epithelial layer too, so because of that, that’s  kinda interesting, that’s why you can actually  

do corneal transplant from one person to another  without having any type of rejection because there  

are no immune molecules in that vicinity. Alright. So that was the fibrous tunic, made up of the  

sclera and the cornea. Let’s now move over to  the next part, which is the vascular layer. 

The vascular layer is made up of the choroid,  which continues as the ciliary body and then  

as the iris. Let’s do the choroid first. The choroid is a highly vascular layer,  

and when we say vascular, we’re saying it’s  loaded with blood vessels that can give blood  

supply to the retina. Second thing is  that it is also a pigmented membrane,  

and that pigmentation is important because it  has a very specific physiological role. When  

light rays enter the eye, they can bounce around  and scatter in different directions, and if that  

scattered light reaches different parts of the  retina, it can interfere with the sharpness of the  

visual signal. We do not want that, because that  can affect the quality of what you are seeing. 

So what happens is the choroid helps absorb any  excess light rays that are not being focused,  

which prevents internal reflection and  scattering. That keeps the visual signal  

clean and improves contrast. Alright so that was the  

choroid. Let’s now talk about the ciliary body. The ciliary body is the direct continuation of the  

choroid, and it eventually continues as the iris.  Now the ciliary body is interesting because it has  

two main functions. First is that it’s involved  in accommodation — which is just the fancy way  

of saying it helps change the shape of the lens  so we can focus on things up close or far away. 

Now the ciliary body itself is made up of  this base part called the ciliary ring,  

and then you have this folded region called the  corona ciliaris, and if you zoom in on that,  

you’ll see these little ridges or bumps  — those are called the ciliary folds and  

processes. Attached to those folds are these  suspensory ligaments called the zonular fibers,  

and they connect directly to the lens. So now, how does this whole system actually  

adjust the lens? That’s where the ciliary  muscle comes in. The ciliary muscle is  

part of the ciliary body and it is divided into  radial fibers and circular fibers. Some sources  

also mention longitudinal fibers, but the two  main ones to remember are radial and circular. 

These muscle fibers are under parasympathetic  control, which is very important to understand  

because that’s what actually drives accommodation.  The parasympathetic fibers come from the  

Edinger–Westphal nucleus, they travel through the  oculomotor nerve, that’s the third cranial nerve,  

then synapse at the ciliary ganglion,  and from there, the short ciliary nerves  

go on to innervate the ciliary muscle. When the parasympathetic system is active,  

the ciliary muscle contracts. That contraction  loosens and reduces the tension on the zonular  

fibers. When the zonular fibers become  loose, the lens becomes thick and round.  

And this thick lens is better at focusing on  objects that are near you as you see here. 

But there is also some sympathetic innervation,  it works through beta-2 receptors in the ciliary  

muscle, and instead of activating accommodation,  it kinda works the opposite way. It relaxes the  

muscle, makes the zonular fibers tight, and  flattens the lens, so that’s more helpful  

for far vision. So sympathetic doesn’t drive  accommodation, but it can oppose or inhibit it,  

pushing the eye toward distance focus. So again, that parasympathetic pathway  

is Edinger–Westphal nucleus → oculomotor nerve  → ciliary ganglion → short ciliary nerves → then  

ciliary muscle and also the sphincter pupillae  at the iris which we will look at very soon. 

Now the second function of the ciliary body is  that it produces aqueous humor. This is secreted  

through the non-pigmented ciliary epithelium,  which is part of the ciliary processes. That  

aqueous humor then flows from the posterior  chamber through the pupil into the anterior  

chamber. From there, it drains out via the  trabecular meshwork and into the canal of Schlemm,  

eventually reaching the episcleral veins. Alright, so the choroid continues as the  

ciliary body, and the ciliary body continues  forward and is attached to the iris. 

So now let’s do the iris. The iris is this  beautiful colored part of the eye that is actually  

a highly pigmented structure, rich in melanocytes.  The amount and type of melanin here basically  

determines your eye color. More melanin gives you  darker eyes, less melanin gives you lighter eyes. 

Now, functionally, the iris controls how much  light enters the eye, and it does that because it  

surrounds an opening called the pupil. It is able  to control this opening because within the iris,  

there are two smooth muscles that regulate the  size of that pupil. The first one is called  

sphincter pupillae. Now the way this one works  is that if you were to look at bright light,  

this muscle is going to contract like this, and  tighten the pupil opening, making it smaller.  

This is called Miosis. So sphincter pupillae  causes pupillary constriction called miosis,  

and it is controlled by parasympathetic  fibers. The same pathway we saw earlier  

through Edinger–Westphal, third cranial nerve,  ciliary ganglion then short ciliary nerves. 

Now, what if there is dim light in your field  of vision? Or very very low light, what happens  

then? You naturally want to open up the pupil to  allow for more light to enter, and you do that by  

contracting the dilator pupillae. They contract in  this direction, which dilates the pupils. Dilated  

pupils is called mydriasis. So, dilator pupillae  causes pupillary dilation called mydriasis, and is  

primarily driven by the sympathetic control. So,  sympathetic fibers travel from the chain ganglia,  

through internal carotid nerve, internal carotid  plexus, ciliary ganglion, then towards the muscle. 

So that was the iris, a very very important  structure that both looks beautiful,  

and controls the pupillary opening. So that was the vascular layer. Now let’s do  

the inner nervous layer, this is primarily going  to be the retina. So the first thing we need to  

know is that there is something called the optic  part of the retina, and that’s what we’re looking  

at here. It’s this whole region that lines the  back of the eye and it’s the part that’s actually  

involved in receiving and processing light. So when light enters the eye, it travels all  

the way to the back and hits the retina. But  more specifically, it passes through multiple  

layers of cells until it reaches the bottom  layer, which is where the photoreceptors are,  

the rods and cones. These are the cells  that actually detect light and initiate  

the visual signal. That processing happens as the  signal moves upwards from the photoreceptors to  

bipolar cells, and then to ganglion cells. After  that, the output of the ganglion cells forms the  

optic nerve. So you can see how the light travels  inward, and then the neural communication travels  

in the opposite direction, toward the brain. If we talk about the cellular layers of the  

retina from the outermost to the innermost: we  start with the photoreceptor layer where the rods  

and cones are located. Right above that, we have  the horizontal cells, which help integrate signals  

between photoreceptors. Then comes the bipolar  cell layer, which connects the input from the  

photoreceptors to the output layers. The amacrine  cells are involved in more complex processing and  

are found here too. Finally, we have the ganglion  cell layer, and the axons from these ganglion  

cells all converge to form cranial nerve II, the  optic nerve. I go through this processing in a bit  

more detail in the separate video I made about  the optic nerve if you want to check that out. 

Another structure on the retina we should mention  is the macula. The macula is basically the central  

part of the retina responsible for sharp, detailed  central vision. And in the center of the macula,  

you have the central fovea — and this is super  important because it contains only cones,  

and they’re very tightly packed here. That’s why  your sharpest, most high-definition vision — like  

reading tiny text — happens in this exact spot. Now as the signal gets processed and travels  

through the ganglion cells, all those axons  converge at one point to form the optic nerve.  

That point where they meet and converge is called  the optic disc, and this region is actually a  

blind spot because there are no photoreceptors  here. It’s just the exit point for all the nerve  

fibers leaving the retina and going to the brain. Now what else can we see in the neural layer? We  

can see the ora serrata, which is basically the  jagged peripheral edge of the retina where the  

light-sensitive part ends. Beyond this point, the  retina continues as the non-visual retina, which  

lines the ciliary body and part of the iris, but  this part doesn’t participate in capturing light. 

And the last part I want to mention here is that  at the very bottom of the retina, right next to  

the choroid, there’s a layer called the pigmented  layer of the retina. This layer contains pigmented  

epithelial cells, and it helps absorb excess  light so it doesn’t scatter inside the eye. But  

it also supports the photoreceptors by recycling  visual pigments and providing metabolic support.  

There’s actually a clinical correlation to  this. In conditions like diabetic retinopathy,  

retinal detachment, or certain inflammatory  diseases, this pigmented layer can get disrupted  

or even separated from the rest of the retina. Now, diabetic retinopathy itself does not  

directly cause a detachment in most cases, but  what can happen is that chronic hyperglycemia  

damages the retinal blood vessels, and the  body responds by forming new fragile blood  

vessels — this is called neovascularization.  These vessels can bleed, cause scarring,  

and physically pull on the retina, leading to  what is called a tractional retinal detachment. 

When the pigmented layer separates from the  neural retina, it disrupts the support and  

metabolic exchange that the photoreceptors  rely on, and that can lead to vision loss. 

So that was the neural layer. And that wraps  up all the layers of the eye, the tunics. 

Now let’s talk about the lens. The lens sits just behind the iris,  

and it is made up of a few different parts. On the  very outside, you have the capsule of the lens,  

which is a thin but tough elastic layer that  maintains the shape of the lens and serves as  

an anchor for the zonular fibers coming from the  ciliary body. Just underneath that is the cortex,  

which contains newly formed lens fibers  that are long, transparent cells packed  

with crystallin proteins. These fibers  are continuously added throughout life. 

And at the center of the lens, you have  the nucleus. This part contains the oldest,  

most compacted lens fibers. Because of how  tightly packed the proteins are, this area  

tends to be more rigid, and over time it becomes  less flexible, which is one of the reasons people  

develop presbyopia as they age, where the lens  cannot change shape as easily for near vision. 

Now, the lens helps bend and refract light rays so  they focus properly on the retina. That is one of  

its main roles. But if we look at its structure,  the lens is actually made of specialized elongated  

cells called lens fibers, packed with crystallin  proteins, and are covered on the anterior  

side by a layer of cuboidal lens epithelium. Now in certain situations, like with cataracts,  

these crystallin molecules can start  to clump together. That clumping causes  

clouding of the lens. It can happen due to  aging, but also from things like diabetes,  

smoking, and even sometimes due to congenital  conditions. Some epidemiological studies show that  

moderate dietary intake of vitamin C may protect  against certain types of age-related cataract,  

especially the nuclear subtype, likely due to its  antioxidant properties. So take your vitamins. 

Alright, now let’s move from  the lens back into the eye. 

This whole space behind the lens is called the  vitreous chamber, and it is filled with something  

called the vitreous body. The vitreous body is a  clear, gel-like substance made mostly of water,  

collagen, and hyaluronic acid. It helps  the eye keep its shape and holds the retina  

firmly in place against the wall of the eye. Sometimes you might notice little floaters,  

those are small clumps or condensations in  the vitreous body that cast shadows on the  

retina. They are usually harmless, but if  you suddenly see a lot of them with flashes  

of light, it could indicate a retinal tear or  detachment, so it is something to be aware of. 

Alright, now let’s see this  whole thing in context. 

Here is the full eye. It is covered by the  cornea in the front and the sclera around  

the rest. Now if we peel off the sclera,  you will start to see this deep red layer  

underneath — that is the choroid, part of the  vascular tunic. If we then peel off the choroid,  

we get to the retina. Now if we carefully remove  the retina, we will reach the vitreous body. 

And you can see how it is like a gel that  helps hold the shape and contour of the  

eye. If we remove the opposite side as well,  you can actually see a channel running through  

the center — that is the hyaloid canal. In  the fetus, it contains the hyaloid artery,  

which supplies the developing lens. But in  adults, it is usually just a remnant — an  

empty space running through the vitreous body. Alright, so that covers the internal components  

of the eye, including the lens, the  vitreous chamber, and the vitreous body.  

But what if we move to the anterior side? Here again just to recap, we can see the cornea,  

the iris, the lens, and the ciliary body. Now in this anterior part of the eye,  

we can also see some chambers. Between the cornea  and the iris, we can see the anterior chamber,  

and between the iris and the lens, we can see the  posterior chamber. This is significant because,  

remember we said that one of the functions of  the ciliary body is production of aqueous humor?  

It is going to produce that fluid, and this  fluid is going to flow in a specific direction. 

So let’s look at the production and flow of  aqueous humor. It is first produced in the  

ciliary body, then secreted into the posterior  chamber. As the fluid flows through the chamber,  

it will reach the pupil and go through it. Then  it will continue to flow forward and fill up the  

anterior chamber. And as it continues up, it  will eventually reach the iridocorneal angle. 

Now the way this drains is that in a healthy  adult human eye, the vast majority of aqueous  

humor leaves via the conventional route, through  the trabecular meshwork into Schlemm’s canal,  

and then into collector channels and the  episcleral veins, often estimated around 80–90%.  

The remaining 10% follows what’s called the  uveoscleral pathway, it goes through the ciliary  

body and drains into the suprachoroidal space. This whole process is super important because it  

is what helps maintain the intraocular pressure,  and any blockage in this drainage system,  

especially in the trabecular meshwork.  can lead to glaucoma, which is a group  

of conditions where the pressure inside the  eye builds up and can damage the optic nerve. 

Alright, so that’s how aqueous humor is  produced and how it flows and drains. 

Now, here we see the eye. The eye is protected  by eyelids, we have an upper eyelid and a lower  

eyelid. Eyelids are considered part of the  accessory visual structures of the eye. 

Inside the eyelids, we can see  the superior and inferior tarsus,  

which are also called tarsal plates. These  are dense connective tissue structures that  

give the eyelids their shape and firmness. They  act as a structural framework and anchor point  

for muscles involved in eyelid movement. Then, we have the palpebral part of the  

orbicularis oculi, this is the portion  of the orbicularis oculi muscle that  

lies within the eyelid itself. It’s a  circular muscle that helps you close  

the eyelids, like when you blink or squint. Next is the levator palpebrae superioris,  

this is a skeletal muscle that lifts  the upper eyelid, and it’s innervated  

by the oculomotor nerve, cranial nerve III. Along with that, there is also the superior tarsal  

muscle of Müller, which is a smooth muscle that  assists in maintaining upper eyelid elevation.  

It’s sympathetically innervated, and this  becomes clinically important in conditions  

like Horner’s syndrome, where you can get mild  ptosis because of the sympathetic denervation. 

There’s also an inferior tarsal muscle on  the lower eyelid, which functions similarly  

but is often less clinically significant. Now moving to the edge of the eyelid, we  

have eyelashes — these are the cilia, which help  catch debris and protect the eye from particles. 

Around the base of these lashes are a  number of glands. You’ve got Tarsal glands,  

which produce an oily layer of the tear  film. Ciliary glands, which are modified  

sweat glands and Sebaceous glands, which  secrete oily substances around the follicles.

A clinical note here is that these glands can  become blocked or infected, which can cause a  

stye, also known as a hordeolum. It’s usually a  tender, red bump near the eyelid margin caused by  

infection or inflammation of one of these glands. Now notice that there’s an extra protective  

layer behind the eyelids. This is called the  conjunctiva. The conjunctiva is basically a thin,  

transparent mucous membrane that covers the  inside of the eyelids and extends to cover  

the white part of the eye, the sclera. It  keeps the eye moist and helps trap debris. 

If it’s behind the eyelid, it’s called  the palpebral conjunctiva. Then we  

have the superior conjunctival fornix,  which is where the conjunctiva folds.  

Covering the sclera is the bulbar conjunctiva. And  in the space between the eyelid and the eyeball,  

we have the conjunctival sac, this is  where eye drops go when you apply them. 

Unfortunately, this conjunctiva can sometimes  get inflamed, leading to a condition called  

conjunctivitis. That’s when the vessels  in the conjunctiva become dilated and the  

eye looks red or irritated. It can happen due to  infection, allergies, or irritation, and treatment  

depends on the cause, sometimes lubricating drops,  antihistamines, or antibiotics if it’s bacterial. 

Alright, next thing. Another  accessory structure is the tear  

production system, or the lacrimal apparatus. Tears are first produced in the lacrimal gland,  

which is located in the superolateral part of  the orbit, just above the outer part of the eye.  

Tear secretion is primarily stimulated  by the parasympathetic nervous system,  

via the facial nerve, cranial nerve VII. The tears are secreted through excretory  

ducts that drain onto the surface of  the eye. There are also some smaller  

conjunctival glands that help support this by  providing mucus and additional tear components. 

From there, the tears travel medially along the  surface of the eye toward the lacrimal caruncle,  

which is the small pink structure  in the medial canthus of the eye. 

The tears then drain through the  superior and inferior lacrimal puncta,  

tiny openings at the edge of each eyelid, and  enter the lacrimal canaliculi. From there,  

they move into the lacrimal sac, and then down  through the nasolacrimal duct, which empties into  

the inferior nasal meatus inside the nasal cavity. So in context, the lacrimal gland sits on  

the upper lateral edge of the orbit, while  the nasolacrimal duct drains down medially  

into the nose. This explains why your  nose runs when you cry — the tears are  

literally draining into the nasal cavity. Tears are produced and flow across the eye  

to keep it moist and clear of debris, and they  either drain into the nasal cavity or spill over  

the eyelid if production exceeds drainage. So that was all I had for the anatomy of  

the eye. I really hope you found that helpful. I’ve made free courses for other topics here on  

YouTube if you wanna keep learning, otherwise  if you want a handmade PDF version of this  

lecture or take a quiz to test your knowledge,  or access an organized list of all my videos,  

you can find everything on my website.  Thanks for watching! See you in the next one.