Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
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’s 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 posteromedially, or towards the inside of the skull; a base anterolaterally, that opens in the facial skeleton; and four walls: medial, superior, lateral, and inferior.
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.
Towards the medial part of the supraorbital margin, the supraorbital nerve and vessels pass through the supraorbital foramen or notch.
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 III, IV, VI, the nasociliary nerve, and other branches of 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 superolaterally, and the maxillary bone inferomedially.
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.
So, to easily recall the orbital openings, you can remember there’s two of each: two foramina, the supraorbital and infraorbital foramina; and two fissures: the superior and inferior orbital fissure.
Now, if we look at them from above, we can see there are two different axes inside the orbit which give 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 axes, 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? 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.
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 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 the palpebral and the bulbar conjunctiva meet, are called the superior and inferior conjunctival fornices, and they are located in the superior and inferior eyelids, respectively.
Surprisingly, the eyelids 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 eyelids 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 superioris, which attaches to the superior tarsus and the skin of the superior eyelid and it opens the palpebral fissure, which is the fancy way to call the space between your eyelids.
The distal portion of the levator palpebrae superioris, 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’s 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 superioris on the upper eyelid, and upwards to insert in 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 the lateral commissures.
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 by 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 bulbar 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 lacrimal canaliculi. The lacrimal sac is the most superior and dilated part of the nasolacrimal 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! Alright, 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 auto nomic 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 parasympathetic fibers travel with cranial nerve VII, 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 pterygoid canal, all the way to the pterygopalatine ganglion, which can also be referred to as the sphenopalatine ganglion, where they synapse with the cell body of the postsynaptic fibers.
The postsynaptic fibers are then carried by branches of the cranial nerve V: 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’re 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 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.
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? Now let’s look at the major nerves that pass through the orbit.
There are five cranial nerves - or branches of them - that travel inside the orbit, and these are cranial nerves II, or the optic nerve, the superior and inferior divisions of cranial nerve III, or the oculomotor nerve, cranial nerve IV, or the trochlear nerve, cranial nerve V, or the trigeminal nerve and cranial nerve VI, or the abducens 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 III, IV, a branch from V, and VI enter the orbit through its apex.
Now, before they enter the orbit, the oculomotor nerve divides into a superior branch and inferior branch, and the ophthalmic branch of the trigeminal nerve divides into three branches: the 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 superioris.
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 superioris, 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 first division of cranial nerve V. 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.