Anatomy of the orbit

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Introduction0:00–0:42

To be able to see everything that surrounds us, including this video, we can count on a very special sense organ, 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 is a soft cushion of fat to prevent any friction or damage to the eyes.
Additional protection is ensured by the eyelids which close and open as needed, and the lacrimal apparatus which secretes tears to lubricate the eyes.
Now each orbit is shaped like a pyramid, so they have an apex posterior medially or towards the inside of the skull, a base anterior laterally that opens in the facial skeleton, and four walls.

Structure0:42–1:54

Superior, inferior, lateral, and medial, these walls are made up of several bones. The medial wall comprises the ethmoid bone in the center, the lacrimal bone and maxilla, or maxillary bone anteriorly, and the lesser wing of the sphenoid bone posteriorly.
The superior wall or roof of the orbit is mainly formed by the orbital part of the frontal bone anteriorly and a small posterior part by the lesser wing of the sphenoid bone.
The lateral wall is made up of the zygomatic bone anteriorly and the greater wing of the sphenoid bone posteriorly. Finally, the inferior wall or floor of the orbit is formed by the maxillary bone medially, the zygomatic bone laterally, and a tiny part by the palatine bone posteriorly.
In between the two maxillae lies the nasal bone, but it doesn't contribute to the orbit. All right, now above the orbit there's the supraorbital margin of the frontal bone.

Orbital openings1:54–3:58

Towards the medial part of the supraorbital margin, the supraorbital nerve and vessels pass through the supraorbital foramen or notch.
Moving on to the inside of the orbit, on the anterior lateral part of the roof of the orbit, there's the lacrimal fossa, a depression in the frontal bone that houses the lacrimal gland.
Now, in the apex of the orbit, there's the optic canal in the lesser wing of the sphenoid through which the optic nerve and the ophthalmic artery pass.
Laterally between the lesser and greater wings of the sphenoid there's the superior orbital fissure which allows for the passage of the superior ophthalmic vein, cranial nerves 346, the nasociliary nerve, and other branches of the first division of the trigeminal nerve.
So that's a lot of structures. Now if there's a superior, there's also an inferior orbital fissure which is formed by the greater wing of the sphenoid silaterally and the maxillary bone inferior medially.
Luckily fewer structures pass through the inferior fissure. These are the divisions of the inferior ophthalmic vein, the second division of the trigeminal nerve called the maxillary nerve, and the infraorbital vessels.
After passing through the infraorbital fissure, the maxillary nerve continues as the infraorbital nerve, and together with the infraorbital vessels, it lies on the infraorbital groove that's in the floor of the orbit.
These structures then dive into the maxillary bone to come out again through the infraorbital foramen of the maxilla below the inferior margin of the orbit.
So to easily recall the orbital openings, you can remember there's 2 of each. Two foramina, the supraorbital and infraorbital foramina, and 2 fissures, the superior and inferior orbital fissure.

Orbital axes3:58–4:38

Now if we look at them from above we can see there are two different axes inside the orbit which gives us an idea of how the orbits are aligned.
The medial walls of the orbits are parallel. However, the lateral walls of the orbits point laterally and are angled at 90 degrees compared to each other.
Therefore, our orbital axes, which represent the anatomical alignment of the orbit, are actually angled anteriorly and laterally at a 45 degree angle to each other.
This is compared to the optical axis, which is simply our line of sight or gaze, which are directly, completely straight and are parallel to each other.
Now that we've seen the bony structure of the orbit, let's take a quick break. Let's see if you can name the bones labeled from A to F and the openings from 1 to 3 on the following image.

Quiz4:38–4:54

Now, besides the orbits, the eyes are also protected from irritation or injury by the eyelids. The eyelids cover the front of the eyes, preventing excessive light exposure and spreading the tears produced by the lacrimal glands all over the cornea and the exposed part of the sclera.

Eyelids4:54–7:51

If we take a look at a sagittal cut of the eye and the eyelids, we'll see that the eyelids are made up of layers of different tissues from the inside out.
First, there's a mucous layer covering the internal part of the eyelids called the palpebral conjunctiva. This is continuous with the bulbar conjunctiva.
Which adheres to the periphery of the cornea and covers the exposed sclera until the corneoscleral junction. The parts where their palpebral and bulbar conjunctiva meet are called the superior and inferior conjunctival fornices, and they are located in the superior and inferior eyelids respectively.
Surprisingly, the islets also have a skeleton. OK, so we're not talking about bones here, but rather bands of connective tissue called the superior tarsus and inferior tarsus.
These tarsi strengthen the eyelids and also hold the tarsal glands inside, which produce an oily substance that lubricates the edges of the eyelid so they do not stick together when closed and prevents the tear film from evaporating from the surface of the eyes.
The superior eyelid has an extra layer made up by the levator palpebrae superiois which attaches to the superior tarsus and the skin of the superior eyelid, and it opens a palpebral fissure.
Which is the fancy way to call the space between your eyelids. The distal portion of the levator palpebrae superiois, called the superior tarsal muscle, inserts directly on the superior border of the tarsus to help open the eye completely.
On the same plane as the tarsi, there is the superior and inferior orbital septa, a sheet of periosteum that extends from the periosteum of the orbit downwards to insert on the levator palpebrae superiois on the upper eyelid and upwards to insert on the inferior tarsus itself on the lower eyelid.
Moving on, on each eyelid, there's the palpebral part of the orbicularis oculi muscle right before the tarsi and the orbital septa, and this muscle is in charge of gently closing the eyes.
And finally, the most superficial layers of each eyelid are the subcutaneous tissue, and last but not least, the skin. On an anterior view of these structures we can see the superior and inferior tarsi connect to the medial and lateral edges of the orbit through the medial and lateral palpebral ligaments respectively.
And in the same region, specifically where the upper eyelid meets the lower eyelid, there are the medial and lateral commissures.

Lacrimal apparatus7:51–10:36

In addition to the eyelids, the lacrimal fluid also plays an important role in protecting the eyes by lubricating them and also protecting them from infections.
The lacrimal fluid is made from the lacrimal apparatus which begins with the lacrimal gland found in the lacrimal fossa of the frontal bone.
The lacrimal fluid then drains through the excretory ducts of the lacrimal gland into a specialized bursa called the conjunctival sac, which is bound by palpebral conjunctiva and.
conjunctiva. When the eyelids close, the fluid is spread over the cornea and the sclera and directed inferiorly and medially towards the medial angle of the eye where it accumulates in the lacrimal lake.
From the lacrimal lake, the fluid drains into the lacrimal sac via the lacrimal canaliculi. The lacrimal sac is the most superior and dilated part of the nasal lacrimal duct.
Which goes on to carry the lacrimal fluid to the nose, specifically the inferior nasal meatus. This is why people have a runny nose when they cry.
All right, but let's rewind a little bit. What makes the lacrimal gland start secreting in the first place?
As you might guess, the answer to this is the autonomic nervous system. Specifically, the parasympathetic nervous system is what makes you cry.
And not only because it's a bit hard to learn, the way it makes your lacrimal glands secrete lacrimal fluid goes a little bit like this.
First, the presynaptic parasynthetic fibers travel with cranial nerve 7, or the facial nerve, and then with a branch of the facial nerve called the greater petrosal nerve.
Then these fibers hitch a ride with the nerve of the terragoid canal all the way to the teragopalatine ganglion, which can also be referred to as the senopalatine ganglion, where they synapse with the cell body of the postsynaptic fibers.
The postsynaptic fibers are then carried by branches of cranial nerve 5. Intuitively, we're talking about the lacrimal nerve, which is a branch of the ophthalmic nerve.
This goes straight to the lacrimal gland, stimulating tear production, but it also receives a small communicating branch from the zygomatic nerve, which is a branch of the maxillary nerve.
On the other side, here is how the sympathetic nervous system reaches the lacrimal gland. First, the postsynaptic fibers of the sympathetic system originate in the superior cervical ganglion, from where they are carried by nerves of the internal carotid plexus, as well as the deep petrosal nerve.
These nerves then join the parasympathetic fibers and together form the nerve of the pterygoid canal, and the sympathetic fibers then follow the same pathway via the lacrimal and zygomatic nerve to the lacrimal gland to cause vasoconstriction and potentially reduce tear production.

Quiz10:36–10:57

Now, feel free to take a quick break and pause the video to test your knowledge on the lacrimal apparatus. Can you label these missing parts A, B, C, D, and E on the following image?

Innervation10:57–14:56

Now let's look at the major nerves that pass through the orbit. There are 5 cranial nerves or branches of them.
That travel inside the orbit and these are cranial nerves 2 or the optic nerve, the superior and inferior divisions of cranial nerve 3.
Or the oculomotor nerve Cranial nerve 4 or the trochlear nerve. Cranial nerve 5 or the trigeminal nerve.
And cranial nerve 6 or the abducent nerve. Of these, the optic nerve enters the orbit through the optic canal and then runs anteriorly and laterally towards the posterior end of the eyeball connecting the retina to the brain.
Cranial nerves 34, a branch from 5 and 6 enter the orbit through its apex. Now before they enter the orbit, the oculomotor nerve divides into a superior branch and an inferior branch, and the ophthalmic branch of the trigeminal nerve divides into three branches lacrimal, frontal, and nasociliary nerves.
Both branches of the oculomotor nerve, the nasociliary nerve, along with the abducens nerve, pass through the superior orbital fissure inside an area created by the common tendinous ring.
The lacrimal and frontal nerves together with the trochlear nerve pass through the superior orbital fissure outside the common tendinous ring.
Once inside the orbit, the superior branch of the oculomotor nerve goes up to innervate the superior rectus and the levator palpebrae superiois.
The inferior branch runs downwards and divides into three branches to innervate the medial rectus, the inferior rectus, and the inferior oblique.
The trochlear nerve runs medially above the levator palpebrae superiois to innervate the superior oblique. The abducens nerve runs laterally to innervate the lateral rectus.
Now let's move on to the branches of the ophthalmic nerve, which is the 1st division of cranial nerve 5. The lacrimal nerve runs laterally to innervate the lacrimal gland.
The frontal nerve runs anteriorly and divides into two branches the supratrochlear and the supraorbital nerve. Finally, we have the nasociliary nerve, where naso means nose, and ciliary refers to the anatomical structures in or around the eye.
So this nerve gives branches called the long ciliary nerves which bypass the ciliary ganglion and go to innervate the nasal cavity and its surroundings and to the eye itself.
Specifically, these nerves bring sympathetic information to the dilator pupillae while also receiving sensory information from areas of the nose and eyes.
Now, last but not least, an integral part of the innervation of the eye is the ciliary ganglion. Now the pre-synaptic parasympathetic nerve fibers traveling within the oculomotor nerve go on and synapse on the postsynaptic neuron cell bodies within the ciliary ganglion.
Additionally, the nasal ciliary nerve fibers carrying general sensory information and sympathetic nerve fibers also travel through the ciliary ganglion.
However, these fibers don't synapse on the postsynaptic neuron cell bodies within the ciliary ganglion, but rather continue their journey through the ganglion.
Then the parasympathetic, sympathetic, and general sensory nerve fibers all exit the ciliary ganglion through the short ciliary nerves.
The parasympathetic portions go on to innervate the ciliary body and the sphincter pupillae muscle. The sympathetic fibers of either the short or the long ciliary nerves innervate the dilator pupillae muscle, and the sensory fibers have a general distribution to all eye structures.
Now let's look at the arteries that supply the structures inside or surrounding the orbit, which are all branches of the ophthalmic artery.

Vascular supply14:56–15:38

Together with the optic nerve, the ophthalmic artery enters the orbit through the optic canal right after it branches out from the internal carotid artery.
These branches have pretty much the same names as the nerves lacrimal artery, short and long posterior ciliary arteries, and so on.
There's also muscular arteries that supply blood to the extrinsic eye muscles. The central artery of the retina, one of the first branches of the ophthalmic artery, pierces the optic nerve sheath and runs inside the nerve to reach and supply the retina.

Venous drainage15:38–16:09

Venous drainage, on the other hand, is ensured by the veins that escort the arteries of the orbit. These drain either into the superior ophthalmic vein or the inferior ophthalmic vein, which then leave the orbit through the superior orbital fissure and drain into the cavernous sinus.
The central vein of the retina drains, well, the blood from the retina into one of the ophthalmic veins or directly into the cavernous sinus.
So together these two veins drain all the structures of the eyeball itself. So we know where all these muscles and nerves are in relation to each other because at some point someone decided to dissect the orbit and describe what they were finding along the way.

Superior view16:09–17:27

If you wanted to do the same from a superior view, you could remove the orbital part of the frontal bone or the roof of the orbit.
Then you'll find from lateral to medial the lacrimal nerve, artery, and gland. The frontal nerve with its branches lying above the levator palpebrae superiois and the superior rectus muscle lying below the levator palpebrae superiois, then the trochlear nerve innervating the superior oblique muscle.
After that you can dissect the levator palpebrae superiois, the superior rectus, and some of the intraorbital fat to get a better look of the deeper structures inside the orbit.
Again, from lateral to medial, you'll see the lateral rectus with the abducens nerve innervating it, the optic nerve, which is an extension of the brain, so you'll see the dura surrounding it, then the short ciliary nerves and the long ciliary nerves around the optic nerve.
And finally you'll see the medial rectus, and you should be able to get a better look at the trochleaa and the superior oblique inserting in the sclera.
OK, now, before we wrap up, let's look at two final aspects in relation to the orbit how the eyes move within the orbit and how the extraocular muscles control the movements of our eyes.

Eye movements17:27–19:36

See, the gaze can't be fixed on the same point all the time. It needs to change constantly, and these eye movements need to be independent from the head.
Otherwise we'd be moving our heads all around while playing a video game. That would look a little weird.
So the eyes can move along three different axes a vertical, a transverse, and an anteroposterior axis. The movements along each axis have specific names depending on how they modify the position of the pupil.
The rotation of the eyeball along the vertical axis allows the pupil to move laterally, called abduction, and medially, called abduction.
When the eye rotates along the transverse axis, it allows us to look up and down, which is actually called elevation and depression of the eyeball.
Now when moving along the anteoposterior axis, the pupil doesn't actually move anywhere, but it rotates kind of like the central logo of a car's steering wheel when you turn right or left.
This allows us to keep seeing straight when we tilt our heads. When the superior pole of the eyeball moves towards the nose, it's called medial rotation.
When it moves towards the temporal region, it's called lateral rotation. All right.
So from the primary position of the eyeball, remember that's when you're looking straight forward. You can look in 8 different directions.
Up or down, right or left, and in diagonal, right and up, right and down, left and up and left and down. Most of the extrinsic muscles of the eye or extraocular muscles are in charge of doing this.
There's 7 of them in total, and the only one that isn't involved in the eyeball movements is the levator palpebrae superiois, which instead elevates the superior eyelid so we can open our eyes.
The other 6 muscles originate around the apex of the orbit and travel forward to insert at different sites. Four of these, the superior rectus, inferior rectus, lateral rectus, and medial rectus originate on the common tendinous ring that surrounds the optic canal and part of the superior orbital fissure.

Extraocular muscles19:36–23:11

They run forward to insert onto the sclera at the anterior half of the eyeball. The levator palpebrae superiois originates right above the superior rectus and runs forward to insert in the superior tarsus and the skin of the superior eyelid.
The superior oblique originates medially to the levator palpebrae superiois and runs forward to the fibrous trochleaa at the medial end of the roof of the orbit.
There the muscle passes through the trochleaa which forms a fibrous sling for the superior oblique and drastically changes direction to run posterior laterally to insert beneath the superior rectus.
The inferior oblique is the only muscle that originates anteriorly on the floor of the orbit and runs posterior laterally to insert in the lateral part of the sclera, posterior to the center of the eyeball, and right beneath the lateral rectus.
Now our eyes can turn in 8 directions, but there's only 6 muscles to do that job. That means some of them have to work together to perform specific movements.
Basically, to make a movement in one direction, the muscle needs to be aligned perpendicular to the axis along which the rotation occurs.
That's not a problem for the lateral and medial recti, which perform abduction and abduction of the eye respectively because they are aligned perpendicular to the vertical axis.
However, as we discussed before, The orbital axis points anterolateral where the optical axis points straight ahead. Since the orbital and optical axes are not aligned, the rest of the muscles are somewhat oblique to all the axes of the eye.
Therefore, they rotate the eyeball along the three axes. For example, the main function of the superior rectus, which runs anterolaterally in line with the orbital axis and inserts superiorly in the sclera, is to elevate the eyeball.
And if the optical axis was in the same direction as the orbital axis, then that is the only function the superior rectus would have.
However, because the optical axis is medial to the orbital axis, the superior rectus also performs abduction and medial rotation due to its oblique orientation to the optical axis.
The inferior rectus also runs anterolaterally but inserts inferiorly, so it mainly depresses but also adducts and laterally rotates the eyeball because the superior and inferior rectus both approach the eye from the medial side.
They are both adductors. The inferior oblique elevates the eyeball just like the superior rectus, but its main function is lateral rotation.
The superior oblique runs posterior laterally and inserts superiorly so it depresses the eyeball like the inferior rectus, but it mainly medially rotates the eyeball.
Opposite to the two rectus which adduct the eye, both obliques perform abduction of the eye. If you also noticed, both superior muscles, the rectus and oblique, medially rotate the eye, where both inferior muscles, the rectus and oblique, laterally rotate the eye.
OK, so all of this seems totally overwhelming, but in clinical practice it's much simpler. In fact, you can evaluate the correct functioning of each muscle individually.

Clinical evaluation23:11–25:32

Let's analyze, for example, the movements of the right eye first. You ask the person to look to the right.
In other words, abduction. To do this, their lateral rectus contracts.
With the eye in this abducted or lateral position, the optical axis is now aligned with the orbital axis. So now the superior and inferior recti become completely perpendicular to the transverse axis of the eye and no longer have any secondary action in an oblique plane.
Therefore, when you ask the person to look up, their superior rectus is fully responsible for elevating the eyes. And when you ask them to look down, the inferior rectus depresses the eyeball.
If the person cannot elevate their eye when abducted, you know that there is dysfunction of the superior rectus because you are isolating that muscle.
Then again starting from the primary position you ask the person to look to the left, which for them is to adduct the eyeball which is evaluating the action of the medial rectus.
Now after abducting the eye, the oblique muscles become perpendicular to the transverse axis of the eye. So now they are in charge of solely elevating and depressing the eyes.
For example, the inferior oblique, due to its attachment, will pull its posterior eye attachment downward, causing elevation of the eye.
So to evaluate the inferior oblique, you ask the person to look upwards, and to evaluate the superior oblique, you ask them to look downwards.
So by asking an individual to either abduct or abduct their eyes first before testing elevation and depression of the eye, we can isolate the movements of these extraocular muscles with overlapping functions to determine if there is any dysfunction.
The oculomotor nerve or cranial nerve 3 innervates all the extraocular muscles except for 2. These exceptions are the superior oblique that runs through the trochleaa and is innervated by the trochlear nerve or cranial nerve 4.
And finally, the lateral rectus that abducts the eye is innervated by the abducent nerve or cranial nerve 6. All right, as a quick recap, the orbits are pyramidal shaped cavities that house and protect the eyes along with the eyelids and the lacrimal fluid secreted by the lacrimal apparatus.

Review25:32–26:46

Except for the optic nerve and the ophthalmic artery, most nerves and blood vessels enter the orbit through the superior orbital fissure.
The ciliary ganglion and the ophthalmic nerve provide innervation for the structures in the orbit beside the extrinsic muscles.
The ophthalmic artery and veins respectively supply and drain blood to and from the structures inside the orbit. The eyeball can move along 3 axes, which results in 6 possible movements abduction and abduction.
Elevation and depression And medial and lateral rotation. The extrinsic eye muscles perform different combinations of these movements, and their correct function can be clinically tested.
Most of them are innervated by the oculomotor nerve except for the superior oblique nervated by the trochlear nerve and the lateral rectus innervated by the abducens nerve.