Anatomy of the eye

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Introduction0:00–0:40

To be able to see everything that surrounds us, including this video, we can count on two very special sense organs: the eyes.
The eyes can be easily injured, so each of them is protected by a hard bony structure called the orbit. The orbits also protect the muscles, vessels and nerves of the eyes.
And between each eye and the orbit protecting it, there’s a soft cushion of fat to prevent any friction or damage to the eyes.
Additional protection is ensured by the lacrimal apparatus, which secretes tears to lubricate the eyes, and the eyelids, which close and open as needed.Okay, now, if you look at a sagittal cut of the eyeball, you’ll see it’s shaped like two spheres fused, a bigger posterior one and a smaller anterior one, both with liquid inside.

Fibrous layer0:40–2:26

To keep this shape, the eyeball needs a solid structure: the fibrous layer, which is made of dense connective tissue and forms the skeleton of the eyeball.
This layer has two parts: the one that makes the outer layer of the smaller sphere, called the cornea; and the one that makes the outer layer of the bigger sphere, called the sclera.
The cornea is transparent and located at the anterior end of the eyeball. It allows light to pass through to the interior of the eyeball.
Have you ever needed to use eye drops? Were you able to do that without instinctively closing your eyes once the drop touches your eye?
The involuntary blinking is actually because of the corneal reflex! See, when something touches or irritates the cornea, it is sensed by the ophthalmic nerve, a branch of cranial nerve V.
This sensory signal then reaches the brain stem, and signals the facial nerve, cranial nerve VII, to contract the orbicularis oculi to close our eyes.
Now, the sclera is opaque and makes the white shell of the eyes. It occupies the majority of the posterior eyeball and serves for attachment of the extrinsic muscles of the eye.
Also, the sclera is pierced by the optic nerve at the posterior end of the eyeball. The place where the cornea and the sclera meet, is called the corneoscleral junction.So, the bigger posterior sphere is made of another two layers of tissue: the vascular layer that provides blood supply, located beneath the sclera; and the internal layer called the retina, that contains the photoreceptors and supporting cells.The vascular layer, in turn, is made of three parts.

Vascular layer2:26–3:28

From posterior to anterior, these are: the choroid, the ciliary body, and the iris. The choroid is made of two layers of blood vessels that provide blood supply to both the loosely attached sclera above, and the firmly attached retina beneath.
The ciliary body is a thickening of the vascular layer, made of blood vessels and smooth muscle, and attaches the choroid layer to the circumference of the iris.
The internal surface of the ciliary body has projections called ciliary processes, that serve as attachments for the suspensory ligament of the lens which keeps the lens in place, while also secreting aqueous humor that fills the anterior chamber or “smaller sphere” of the eyeball.
Finally, there’s the iris, which is only responsible for the beautiful diversity of eye colors. Okay… that’s a big fat lie.
The iris is far more important than that, as it lies on the anterior surface of the lens and regulates the amount of light that passes through the pupil, a small opening in its center.
Just beneath the vascular layer, there’s the inner layer, called the retina, where the beautiful journey of the visual pathway begins.

Inner layer (Retina)3:28–5:08

The retina has two different parts: the optic part right below the choroid layer, where the light impacts and is sensed by the photoreceptors; and the non-visual retina, found underneath and covering the ciliary body and the posterior iris.
Now, the optic part can be clinically analyzed in order to detect abnormalities that could be affecting the vision. This can be done by looking through an instrument called ophthalmoscope at the fundus of the eyeball, formally known as the ocular fundus, which is defined as the internal aspect of the posterior part of the eyeball where light enters and is focused.
The retina of the fundus contains the optic disc, which is a circular area that corresponds to the exit of the optic nerve and the central retinal vein from the posterior eyeball, and the entrance of the central retinal artery, a branch of the ophthalmic artery which supplies the retina.
This circular area contains no photoreceptors, so it’s a veritable ‘blind spot’ in our vision. On a fundoscopic examination, you can also see the retinal arterioles and venules diverging from the optic disc, and to differentiate the two, remember that the arterioles are narrower.
Another important area of the fundus is the macula of the retina, lateral to the optic disc, which contains only photoreceptor cones.
This area is important because it specializes in acuity of vision - so without it, everything would be blurry. Actually, the most acute vision is sensed at the central area of the macula, called the fovea centralis, which is more depressed than the rest of the retina.Alright now, inside the eye, there are a number of transparent elements that light passes through in order to reach the retina.

Anterior & Posterior segments5:08–6:40

First, there’s the cornea, then the aqueous humour, followed by the lens, and finally the vitreous humor. In contrast to the cornea, which is the structure that primarily bends light, the lens, found behind the iris, is in charge of fine tuning our focus, just like the lens of a camera.
The cornea and the lens help us delineate the two segments of the eye, which are the anterior and posterior segment. The anterior segment is between the cornea and the lens, and the posterior segment is found between the lens and the retina.
The anterior segment contains the aqueous humor, which is fluid and provides nutrients for the cornea and the lens. The anterior segment is further divided into an anterior chamber, between the cornea and the iris; and a posterior chamber, between the iris and the lens.
The aqueous humor is produced by the ciliary processes in the posterior chamber, and it flows through the pupil and into the anterior chamber.
In the anterior chamber, the aqueous humor is reabsorbed by the scleral venous sinus, or the canal of Schlemm, at the angle where the iris meets the cornea, called the iridocorneal angle.
On the other side, the posterior segment, also called the vitreous chamber, contains a more dense fluid called vitreous humor, which is also transparent, and it provides support to the lens and holds the retina in place.So, that’s a lot of information to take in.

Quiz6:40–6:53

Let’s see if you can name the elements labeled from A to F on the following image of the layers of the eyeball.Let's take a bit of a closer look at some of the ways we are able to modify the path of light as it enters our eyes.

Intrinsic eye muscles6:53–7:52

The cornea, being the first and the most powerful refractive medium, makes the light rays converge to pass through the pupil.
It does this in a constant manner, which means it always changes the direction of light at the same angle because the curvature of the cornea remains the same.
On the other side, the lens is an elastic and biconvex structure, which means both its anterior and posterior surfaces are convex.
This means the lens can be stretched or bulged to modify its curvature, and by doing this, it fine tunes our ability to focus.
Unfortunately, the lens can’t stretch or bulge by itself, so it needs a partner: the smooth muscle within the ciliary body, called the ciliary muscle.
Imagine this muscle like a contractile ring, where the smaller sized lens is placed at the center of it, and is connected circumferentially to the ciliary body by the suspensory ligament of the lens.The ciliary muscle is under autonomic control in order to contract and relax quickly, which in turn causes the ciliary body to shrink or dilate, allowing our lens to change shape so we can rapidly change our vision from near to distant objects and vice versa.

Accommodation of the lens7:52–8:55

The absence of parasympathetic stimulation, as well a potential inhibitory effect due to sympathetic innervation, causes the ciliary muscle to relax, increasing the diameter of the ciliary body which tenses the suspensory ligament so as to pull the lens in each direction.
This results in thinning of the lens, which allows for more distant objects to come into focus. In contrast, parasympathetic innervation causes contraction of the ciliary muscle, via cranial nerve III or the oculomotor nerve, and this relaxes the suspensory ligament, decreasing the diameter of the ciliary ring.
Overall this diminishes the tension over the lens, which is now bulged in order to better focus on closer objects. This process is called accommodation.But the ciliary muscle isn’t the only muscle inside the eyes - or so-called intrinsic eye muscle.

Constriction & dilation8:55–10:44

There’s also the iris. Just like the ciliary muscle which allows us to focus on distant and closer objects, the iris allows us to see well either in a dark or light environment because it regulates the amount of light that passes through the pupil.
It can do this because it’s actually made of two rings of muscle. If you look at it from the front, you’ll see the one that immediately surrounds the pupil is circularly arranged, called the sphincter pupillae.
The second muscle, called the dilator pupillae, is radially arranged around the sphincter pupillae. Now, let’s say you’ve been studying anatomy for quite a few hours now and need some air.
So you decide to go out to the park to clear your mind. When you come out from the relative darkness of your home and bombarded by the light of day, your parasympathetic system sends a signal to the sphincter pupillae to contract immediately, therefore contracting the pupil and preventing excessive light from entering your retina.
This is because while studying anatomy in the relative darkness indoors, your eyes have become used to the low levels of light, so the sudden onset of excessive light coming into the retina causes discomfort.
The consequent decrease in the pupil diameter is called miosis. Alright now, after hanging out for a while in the sunny park, you realize you have to go back home to continue studying.
So exactly when you get inside your house again, your sympathetic system immediately sends a signal to the dilator pupillae to contract.
This is because you are now used to having an abundance of light entering the retina, so in the new low light environment your body reacts by trying to dilate the pupil to allow more light into focus.
However, it could take up to 20 minutes for the muscle to contract and for you to be able to see clearly. The dilation of the pupil is called mydriasis.Alright, as a quick recap...The eyeball is made of three layers: the fibrous layer, made of the cornea and the sclera; the vascular layer, that comprises the choroid, ciliary body, and iris, with the pupil in its center; and finally the inner layer, or retina, which can be clinically analyzed through an ophthalmoscope.

Review10:44–11:37

The refractive media of the eye include the cornea, aqueous humor, lens, and vitreous humor. These media belong either to the anterior or posterior segment of the eye.
The anterior and posterior segments are separated from one another by the lens. The anterior segment is further divided by the iris into an anterior and posterior chamber.
The accommodation of the lens allows us to see closer objects clearly, and the constriction or dilation of the pupil allow us to adapt to light and dark environments,