Video: Eyeball

Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the eyeball. So, what we’re going to do essentially on this tutorial is looking at the image that you see now on the screen. And what we have here is a cut of the eye – we just cut it in half – and we’re looking at a superior view of the eye. Now, before I continue on with this tutorial, I would like to list the topics that we’re going to be discussing. So, we’re going to be describing the different tunics and also chambers of the eye that can be seen here on this image and then other important structures of the eye will be discussed as well in good detail. Now, on the following slides we’re going to be focusing on the tunics of the eye.

Now, we’re going to be describing them from the outside to the inside of the eye starting off with this one that you see here highlighted in green. This one is known as the bulbar conjunctiva. So, this is the most external tunic and the conjunctiva lines the outside of the eyelids and also covers the sclera. Now, the bulbar conjunctiva will be helping lubricate the eye by producing mucus and also tears. But keep in mind that it produces a smaller amount of tears than the lacrimal gland. This structure also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.

We covered the most external tunic of the eye to then go and talk about the other tunics and we were going to list them before we talk about them in more detail. Now, the most external one just below the bulbar conjunctiva, we find a tunic known as the fibrous tunic which comprises then the cornea and the sclera which you can see here the cornea and also here the sclera.

Now, a middle tunic which is then comprised of a vascular tunic comprising then the iris, the ciliary body, and the choroid. You can see here the iris, the ciliary body, and also this layer here, the choroid. But we’re going to be highlighting them on other slides. The most internal tunic will be then a nervous tunic, also known as the retina, as you can you see here this yellow layer.

Let’s start off with the very first one here on the list, the external tunic, which is comprised as you remember by the cornea and the sclera. And starting with the very first one here that I’m highlighting, this is the cornea, and the cornea is the transparent front part of your eye that covers the iris, the pupil and the anterior chamber which we will also be highlighting here but I can show you here, this is the anterior chamber of the eye. The cornea with the anterior chamber and also the lens will be refracting light and the cornea will be accounting for approximately 2/3 of the eye’s total optical power.

We’re going to add a few other points related to the cornea that are important to know. You can see here a zoomed in image. Now, the cornea has a diameter of about 11.5 mm and thickness of 0.5 to 0.6 mm in the center and 0.6 to 0.8 mm at the periphery. You can also here on the image how it thickens a little bit here at the periphery and it’s a bit thinner at the center. This area of the eye has no blood supply. It gets oxygen directly through the air. Now, the oxygen first will be dissolving in the tears and then diffuses throughout the cornea to keep it alive and healthy. Albumin is the most abundant found in the human cornea.

Also an important point that I would like to just highlight here on this tutorial that our cornea has about 5 layers which we will just list here on this tutorial but would like to go over in a little bit more detail on a separate tutorial but just as a reminder, the anterior to posterior layers of the human cornea include the corneal epithelium, the Bowman’s layer which is the anterior limiting membrane. We also find the corneal stroma or substantia propria, the Descemet’s membrane or known as the posterior limiting membrane and, finally, the corneal endothelium.

Now, the cornea is one of the most sensitive tissues in the body so there is innervation to be known as it is densely innervated with sensory nerve fibers via the ophthalmic division of the trigeminal nerve. One fun fact is that research suggests that density of pain receptors in your cornea is about 300-600 x greater than in your skin and 20-40 x greater than in your dental pulp which as you can see any injury that happens in the cornea can become quite painful.

We’re going to move on to another structure that I’m highlighting here in green which I pointed out before, this one is the sclera which is also known as the white of the eye. This is an opaque fibrous protective and outer layer of the eye which contains collagen and elastic fibers. The sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve, and you can see also here on this image. Now, it is believed that the conspicuous sclera of the human eye makes it easier for one individual to figure out where the other one is looking at increasing then the efficacy of this particular form of nonverbal communication.

We’re now ready to move on to the next structures. Now, we’re going to talk about the middle tunic which comprises then or is comprised of the iris, the ciliary body, and the choroid. We’re going to start off with the very first one here on the list that you see here highlighted in green which is the iris, a very beautiful structure of your eye. The iris is a thin circular structure of your eye which is responsible for controlling the diameter and also the size of the pupil, for that reason, the amount of light reaching the retina. So, the color of the iris gives the eye the actual color.

The iris consists of 2 layers and we’re going to list them here on – you can see here now a zoomed-in image. Now, these 2 layers are one that is found in the front which is the pigmented fibrovascular known as the stroma and beneath the stroma, you’re going to find then pigmented epithelial cells.

Moving on to the next structure that you see here highlighted in green which is known as the ciliary body. The ciliary body is a circumferential tissue inside the eye which is then comprised of the ciliary muscle and ciliary processes. It is coated by a double layer, the ciliary epithelium. Now, this structure of the eye has different functions which we will be covering here now. Now, it is involved in accommodation, aqueous humor production, resorption and maintenance of the lens zonules, and also anchors the lens in place.

We’re now ready to move on to the next structure that you see here highlighted in green. This one is the choroid. This one is a vascular layer of the eye containing then connective tissue and lies between the retina and the sclera. Now, the structure of the choroid is generally divided into 4 layers that I’m just going to briefly list here on this tutorial. We’re going to be listing them the ones that are the furthest away from the retina to the closest ones. Starting off with the Haller’s layer which is the outermost layer, so the outermost layer of the choroid which consists of larger diameter blood vessels. There is the Sattler’s layer which is a layer of the medium diameter blood vessels. There is a layer known as the choriocapillaris which is filled with capillaries, and the Bruch’s membrane which is the innermost layer. The choroid also has a few functions that we’re going to be pinpointing here on this tutorial. Now, this structure will be providing oxygen and also nourishment to the outer layers of the retina. And along with the ciliary body and iris, the choroid forms the uveal tract.

We’re going to move on to then the internal tunics as you can see here highlighted in green which consists mainly of the retina. And the retina is a thin layer of tissue that lines the back of your eye on the inside as you can see here on this image. It is located near the optic nerve which you see here on the image, a bit of the optic nerve, this yellow structure. Now, I wanted to add here then the retina to then show you the functions associated to the structure. Now, the main function of the retina is to receive light focused from the lens then convert the light into neural signals and then send these signals onto your brain. Essentially what the retina is doing is processing a picture from the focused light and then the brain – your brain – is left to decide what the picture is.

Now, the retina uses special cells called rods and cones to process the light. Now, rods are able to see then black, white and shades of gray and tell us the form or shape that something has. Rods cannot tell the difference between colors but they are very sensitive allowing us to then see when it’s very dark. Now, cones, they are able to sense color and they need more light than rods to work very well. Now, cones are most helpful in normal or bright light. Now, the retina has 3 types of cones. Each cone type is sensitive to one of the 3 different colors – red, green or blue.

We’re going to be talking about the rods and cones in a little bit more detail on a separate tutorial but I wanted to mention them here when we talked about the retina. These 3 types of cones are important because they help you see different ranges of color. Together, these cones can then sense combinations of light waves that enable our eyes to see millions of colors. Let’s take this opportunity to make a clinical point that color blindness or color vision deficiency is the inability or decreased ability to see color or perceive color differences under normal lighting conditions. Color blindness definitely affects a significant percentage of the population. You probably know someone who is color blind and there is no actual blindness but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to then the optic nerve.

Now, it is time for us to continue on and talk about the different chambers of the eye. The very first one that I’m going to show you here highlighted in green is the anterior chamber. Now, this one is a fluid-filled space inside the eye just between the iris and the cornea’s innermost surface, the endothelium, as you can see here. Now, aqueous humor is the fluid that fills the anterior chamber. Now, the normal depth of the anterior chamber of the eye is about 3.5 mm to 2.5 mm. Less than 2.5 mm depth can be considered a risk involved in angle-closure glaucoma. Hyphema and also glaucoma are 2 main pathologies that are associated to the anterior chamber.

I would also like to add this structure here – this very small structure that you see here highlighted in green – which is known as the venous sinus of sclera, also known as Canal of Schlemm. Now, this structure is a circular channel in the eye that collects aqueous humor from the anterior chamber then delivers it into the bloodstream via the anterior ciliary veins.

Now that we talked about the anterior chamber, it is time for us to highlight the posterior chamber as you see here highlighted in green. Now, this one is a narrow space behind the peripheral part of the iris and in front of the suspensory ligament of the lens and the ciliary processes. Now, the posterior chamber consists of a small space directly posterior to the iris but anterior to then the lens. One important point is that the posterior chamber should not be confused with the vitreous chamber.

Now that we covered the two chambers that we see here on this image, it is time for us to move on and talk about other important structures that we find here on this image of the eyeball. So, we’re going to be talking about a few muscles, the optic nerve, blood supply and also the lens apparatus.

Let’s start off with this image now of the eye from a superior view where we see the highlighted structure here which is the central retinal artery. This artery is going to be branching off of this artery here – a very important one – which is known as the ophthalmic artery. Now, going back to this image of the eyeball on a zoomed-in version here where you can see that it runs – the highlighted structure which is the central retinal artery – it runs within the optic nerve’s dural sheath to the eyeball – as you can see here the optic nerve.

The central retinal artery is approximately 160 micrometers in diameter. This small artery is very important because it will be supplying all the nerve fibers that form the optic nerve so that carries the visual information including those that reach over the fovea. One important clinical point here to make is that if the central retinal artery gets occluded in one eye, there is then complete loss of vision in that particular eye even though the fovea is not affected.

And speaking of which we’re going to be highlighting here, this structure which is the, which is the fovea centralis. This is the depression in the inner retinal surface about 1.5 mm wide. This photo receptor layer of which is comprised of entirely cones and which is specialized for maximum visual acuity. Now, approximately half of the nerve fibers in the optic nerve carry information from the fovea while the remaining half carry information from the rest of the retina.

Now, an important point I would like to make here still on the fovea centralis. Now, the high spatial density of the cones along with the absence of blood vessels at the fovea will be accounting for the high visual acuity capability of the fovea. Now, the central fovea or fovea centralis consists of very compact cones, thinner and also more rod-like in appearance than the other cones that you find elsewhere. Now, these cones are very densely packed in a hexagonal pattern starting at the outskirts of the fovea; however, rods gradually appear and the absolute density of the cone receptors will be progressively decreasing.

The next structure that we’re going to highlight here that you see a bit of the zoomed-in magnification glass to show you then the zonular fibers. Now, the Zonule of Zinn is a ring of fibrous strands connecting the ciliary body with the crystalline lens of the eye. Now, the Zonule of Zinn is split into two layers – a thin layer which lines the hyaloid fossa and a thicker layer which is a collection of zonular fibers. Now, together, the fibers are also known as the suspensory ligament of the lens. Now, the zonules are about 1-2 micrometers in diameter then a relaxation of the ligament allows the lens to then become a bit more convex.

Next structure highlighted here is known as the crystalline lens, which is a transparent biconvex structure in the eye that along with the cornea helps to refract light to be then focused on the retina. Now, the lens is flatter on its anterior side than on the posterior side as you can see a bit here. It’s a bit flatter on the anterior portion than the posterior part as you can see. Now, the refractive power of the lens in its natural environment is approximately 18 dioptres roughly 1/3 of the eye’s total power.

Now, the lens will have very important functions that we’re going to be listing here on this tutorial. Now, by changing shape, the lens can function to then change the focal distance of your eye. In that way, it can then focus on objects at various distances. For that reason, allowing a sharp real image of the object of interest to be formed on then the retina. Now, this adjustment of the lens is known as accommodation. Accommodation, in other words, is if you have your camera and then you have the different lenses to focus on an object by movement of the lenses especially those more professional cameras that have those extended lenses – as you move them, you can then focus by doing that. That’s just some visuals for you to understand what accommodation is.

Now, the next structure we’re going to be highlighting here is known as then the capsule, a capsule of the lens. The lens capsule is a clear membrane-like structure that is quite elastic quality that keeps then under constant tension. And as a result, the lens usually has a more round or more globular configuration and shape it must assume for the eye to then focus at a near distance.

The next structure we’re going to be highlighting here – as you can see – this is known as the vitreous body. Now, the vitreous body is transparent, colorless, gelatinous mass that fills the space between the space of the eye and the retina lining then the back of your eye as you can clearly see here on this image. And like the fluid that we find on the anterior parts of the eye which we talked about before – the aqueous humor which is continuously replenished – the gel in the vitreous chamber is then stagnant. For that reason, if blood cells or other byproducts of inflammation get into the vitreous body, they will remain there unless removed surgically.

Next, we’re going to be highlighting this structure here – this long structure – which is known as the hyaloid canal, also known as the Cloquet’s canal or Stilling’s canal. Now, this is a small transparent canal running through the vitreous body from the optic nerve as you can see here all the way to the lens – the structure that we talked about before. The hyaloid canal is formed by the invagination of the hyaloid membrane which encloses the vitreous body. The hyaloid canal also has functions. The hyaloid canal contains lymph and its purpose is to then facilitate changes in the volume of the lens. And as the lens expands in positive accommodation, its volume will be increasing. This results in compression of the hyaloid canal so that the volume of your eye remains constant.

The next structure that we’re going to be highlighting that I mentioned before this one is known as the optic nerve or also known as the cranial nerve II or the second cranial nerve. This is a paired nerve. It extends from the optic disc to the optic chiasm and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus. Now, the optic nerve will be then transmitting visual information from the retina to your brain which includes then brightness perception, contrast or visual acuity, red-green color perception, and also visual fields. Now, the eye’s blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.

The next structures that we’re going to be talking about that you see here on this, on this image of the eyeball that I wanted to show you here also on this image on the right side where you see this structure here also highlighted which is the lateral rectus muscle. And on this image, we’re seeing it from the left lateral view of the eye – you see here then the skull, nasal bone, you see the maxilla, the frontal bone and of course the eye seen from a lateral view. So, you can see here, this is the superior view of a cut of the eye that we’ve been exploring throughout this tutorial and the highlighted here a bit or a cut of the lateral rectus muscle.

A few words on the lateral rectus muscle which is a muscle of the orbit, it is one of the 6 extraocular muscles that will be controlling the movements of your eyes. This muscle will be then innervated by this structure that you see here highlighted in green which is the abducens nerve also known as the sixth cranial nerve. A quick word on or just a reminder on the origins and also insertions of the lateral rectus muscle. For origin point, it will be then originating from the Annulus of Zinn at the orbital apex and then inserting at 7 mm temporal to the limbus. Now, as for the function associated to the lateral rectus muscle if you look at the location, this muscle will bring the pupil away from the midline of your body so it adducts the eyeball.

There is another muscle that you can see here on this image of the superior view of the eye – notice here also this highlight here – which we are now seeing here highlighted in green from now an anterior view of your eye. And this structure is then the medial rectus muscle. The medial rectus muscle is a muscle of the orbit as well. It is the largest of the extraocular muscles. It is tested clinically by asking the patient to look medially. This muscle as all good muscles will be innervated by a nerve that you see here highlighted in green, the oculomotor nerve, more specifically the inferior division of the oculomotor nerve as you can see here – so this is the inferior division. And, as you probably know, the oculomotor nerve is cranial nerve III.

A quick word on the origins and insertions of the medial rectus muscle. Now, this muscle will be coming from the Annulus of Zinn at the orbital apex and then inserting 5.5 mm medial to then the limbus. As for the function, as you probably guessed, the function of this muscle is to then adduct the eyeball. So its function is to bring the pupil closer to the midline of your body.

Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.

Now, good luck everyone, and I will see you next time.

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