Video: Eyelids and tunica conjunctiva
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Meet Amelia – a happy-go-lucky lady who has a lot to smile about. The man of her dreams has just proposed to her and she couldn't be more thrilled. But planning a wedding could not be more stressfu... Read more
Meet Amelia – a happy-go-lucky lady who has a lot to smile about. The man of her dreams has just proposed to her and she couldn't be more thrilled. But planning a wedding could not be more stressful. So much to organize – it's just exhausting. All this stress takes its toll, and on the morning of the wedding, disaster strikes. Amelia wakes up to find a large, red, nasty-looking sty on her eye. The wedding photographs will be ruined. What in heaven's name is a sty in any case?
Unfortunately, I can't do much about Amelia's wedding photographs, but I can explain what is happening with her eye. Stay with me now, it's time to explore the eyelids and the conjunctiva.
Now before we start, I already know what you're thinking. A whole video tutorial dedicated to the eyelids? Have we gone mad? Well, believe it or not, despite being one of the smaller and somewhat delicate parts of the body, the eyelids and their thin underlying membrane – the conjunctiva – pack a huge amount of anatomical detail. They also play an extremely important role in the health and general well-being of your eyes. But before we begin all that, let's take a few moments to remind ourselves of the general anatomy of the orbit, or eye socket, as well as its contents before we look a little deeper.
So, as you can see, we're looking at a sagittal section of the orbit, or eye socket, in this illustration. The center of the orbit is, of course, dominated by this structure, which most of us would call the eyeball, but in anatomical terminology, is also referred to as the ocular bulb or globe of the eye. In our section here, we can see the orbit nicely positioned between the orbital roof, which is largely comprised of the orbital process of the frontal bone, and the orbital floor found underneath. We know this is largely composed of the orbital surface of the maxilla and we can even see a portion of the maxillary sinus just here.
The ocular bulb sits in a bed of adipose tissue known as retrobulbar fat. This acts as a protective cushion to the eye protecting the rather delicate eyeball from bashing off the walls of the orbit as we move our heads. Within the retrobulbar fat, we can see a number of structures attaching to the ocular bulb here, the most obvious being the optic nerve, which relays visual information from the retina to the occipital lobe of the brain.
We can also see a number of the extraocular muscles, which control eye movement. These include the superior rectus muscle, which helps us look up; the inferior rectus muscle, which draws our eyes down like when we look at our phones; and finally, the inferior oblique muscle, which is a pro at rolling our eyes.
Apart from these muscles which move the eye itself, I want to draw your attention to this muscle here which is the levator palpebrae superioris. The word palpebra comes from the Latin and means eyelid. This muscle inserts into the superior eyelid and elevates it, but we'll talk more about this later.
Focusing on the anterior portion of the ocular bulb now, we can see this large bubble-like structure here which is, of course, the lens of the eye. This helps us to focus our vision on the objects around us – near and far – through a process known as accommodation.
Moving in a little closer, we can see that the lens is flanked by the structure here – the ciliary body. The ciliary body works to change the shape of the lens which is what allows our eyes to change focus between near and far objects. Anterior to the lens, we then have the iris which we know is the pigmented part of our eye and this gives it its color. It also separates the anterior and posterior chambers of the eye. Finally, we can see the anterior most part of the ocular bulb here, which is the cornea.
Now you'll have noticed that we've just quickly touched on the structures of the orbit and the eyeball. If you want to learn more about the specific anatomy of each of these structures, rest assured, you'll find all you need to know by checking out our extensive range of other study units on the orbit and its contents. But, as you know, we're focusing on the eyelids and the conjunctiva today, and I don't know about you, but I don't want to waste any time, so let's get straight into it beginning first with the eyelids, also known as the palpebrae.
So, your eyelids, or palpebrae, are two folds of specialized skin which cover the anterior or exposed portion of the ocular bulbs. Their primary function, of course, is to shield and protect the eyes from dust and other particles, injury, as well as from exposure to excessive light. They also assist in keeping the corneal surface of the eye moist, but we'll find out more about what exactly this means in just a few moments.
We do, of course, have two eyelids over each eye – a larger superior one which is relatively mobile and serves as an attachment point to the levator palpebrae superioris which we met earlier and we also have the much smaller inferior eyelid which is less mobile. The eyelids are separated by what is known as the palpebral fissure and joined at their extremities as the medial and lateral canthi.
Each eyelid can be subdivided into two parts – a small medial part which lacks eyelashes known as the lacrimal part and a larger lateral part which does have eyelashes and is called the ciliary part. Each eyelid is only two to three millimeters in thickness and features a number of tissue layers. We're going to look at each of these layers now from superficial to deep.
The skin on the surface of the eyelids is the thinnest skin over the entire body. It is relatively loose and is elastic to accommodate their movement. Rooted within the distal edge of this tiny layer of the skin are the cilia which, to you and me, are eyelashes. These short, yet, relatively thick hairs protrude from the anterior palpebral margin of the eyelid curving away from the palpebral fissure which prevents them from interlocking with those of the opposite eyelid.
And, believe it or not, the function of our cilia is not to make our eyes look beautiful. No. Of course, like all things in anatomy rather than fabulousness, our cilia, or eyelashes, are primarily there to protect our eyes from foreign particles which might be blown into our eyes as well as provide shade to the cornea which helps prevent evaporation of the tear film. Also, interestingly enough, the cilia also act as a type of whisker-like apparatus in the sense that they are associated with sensory receptors that cause reflex shutting of the eyelids when stimulated.
Just deep to the skin within the eyelid itself, you'll encounter a small amount of subcutaneous adipose tissue. This tissue contains two types of glands, both of which are associated with the follicles of eyelashes. The first type of gland in the subcutaneous tissue is the ciliary gland, sometimes referred to as the glands of Moll. Ciliary glands are modified apocrine sweat glands that are found at the margin of the eyelid just adjacent to the root of the cilia. These glands secrete a lipid-based compound whose function is not completely understood. However, it is thought it helps prevent evaporation of the tear film over the eye.
The sebaceous glands are also very small and these ones specific to the eyelid are known as the glands of Zeis. They're located in periocular skin, caruncle, and eyebrow skin follicles. These glands secrete sebum – an oily substance that is thought to service the hair follicle itself preventing them from drying out and turning brittle.
Let's speak now about the skeletal muscle tissue within the eyelid, which is largely comprised of fibers from this muscle – the orbicularis oculi muscle. This muscle is a sphincter that surrounds the entire eye. Its fibers are divided into two parts – the orbital part and the palpebral part. This is the orbital part. When this muscle contracts, it closes the eyes tightly. The palpebral part of the orbicularis oculi is just a bit closer to the margin of the eyelid. This is also a sphincter around the eye and when it contracts, the eyelids close gently.
Just deep to the orbicularis oculi muscle, we next encounter the tarsal plate. The tarsal plate, or tarsus, is a long plate of firm, dense connective tissue that gives some structure and form to the eyelid. As you can see in the illustration, each tarsal plate is crescent-shaped in appearance and conforms to the shape of the anterior ocular bulb. Each tarsal plate is connected to medial and lateral borders of the orbit by means of the medial and lateral palpebral ligaments which help anchor them in position.
The superior tarsal plate is also secured by some fibers from the aponeurosis of the levator palpebrae superioris muscle as well as this muscle which is also known as the superior tarsal muscle or Muller's muscle. Along with the levator palpebrae superioris, the superior tarsal muscle assists in elevating the upper eyelid. However, unlike the skeletal muscle of the levator palpebrae superioris, the superior tarsal muscle is made up of smooth muscle tissue that is under autonomic or involuntary control.
A corresponding, but less well-developed inferior tarsal muscle is also found in the inferior eyelid. It too is made up of smooth muscle under involuntary control and assists in retracting the lower eyelid. It is a very small muscle that we can just barely see inferior to the orbit inserting on the inferior tarsal plate.
Embedded within each tarsal plate are small glands arranged in rows perpendicular to the margin of the eyelid. These are known as tarsal, or meibomian glands, and as you can see in our illustration here, are composed of a single long tube or duct which is flanked along its length by several invaginations. Each gland opens into the free palpebral margin via a minute orifice seen here.
The tarsal glands produce and secrete a sebaceous substance, sometimes known as meibum, which covers the margins of the eyelids which in turn creates an oily layer over the tear film of the eye through the blinking action of the eyelids. This reduces evaporation of the tear film as well as lubricates the free edge of the eyelid preventing them from sticking together when closed.
The meibum also creates a barrier of sorts which limits the tear film or the lacrimal fluid from spilling over the palpebral margins when the eye is open. When production of lacrimal fluid increases, like when we begin to cry, the barrier formed by meibum causes our eyes to well up. However, should excessive production persist, when you feel like a proper cry, the lacrimal fluid spills over the barrier onto the cheeks as tears.
And with that, we've mentioned all of the major components of the eyelid. It's time to move on now to our next structure, which is the conjunctiva.
The conjunctiva is a thin mucous membrane that covers the sclera of the anterior orbit before being reflected onto the entire posterior surface of the eyelid. This forms a space known as the conjunctival sac which opens anteriorly at the palpebral fissure. The conjunctiva produces mucin, which is a kind of slimy substance which functions to keep the surface of the eyes and the eyelids lubricated eliminating friction as the eyelids move across the sclera and the cornea. It also plays a very important role in protecting the ocular surface from microbial infection.
At its most basic level, we can break the conjunctiva up into two main parts. The portion of the membrane covering the eyeball, or the ocular bulb, is called the bulbar conjunctiva, while that which lines the eyelids is called the palpebral conjunctiva. Let's take a closer look at each of these for a second beginning first with the palpebral conjunctiva.
This part of the conjunctiva can be further divided into three regions which are the marginal region which, true to its name, is found at the free palpebral margin adjacent to the openings of the tarsal glands; the tarsal region, which is tightly attached to the dense connective tissue of the tassel plates; and finally, the orbital region, which is somewhat looser and contains tiny accessory lacrimal glands known as Ciaccio’s glands or glands of Wolfring. These regions are not really distinguishable at a gross level; however, at a microscopic level, there are differences in the cellular makeup of their tissue, but that's a story best kept for a histology tutorial.
The bulbar conjunctiva, as I mentioned earlier, covers the anterior sclera as far as the sclerocorneal junction, where it is also referred to as the limbic region of the conjunctiva. It is loosely attached to the eyeball which allows this part of the conjunctiva to be extremely mobile.
As you can see in the illustration, the bulbar and the palpebral conjunctiva are continuous with each other and the point at which the two parts of the membrane meet is called the fornix. The word fornix means arch in Latin. The junction is called a fornix because, at this point, the membrane folds on itself and forms a curve. Here we can see the superior conjunctival fornix where the bulbar and palpable conjunctiva on the other part of the eye meet.
Although not visible in our illustration, there are also tiny accessory lacrimal glands located at the conjunctival fornix which are known as Krause’s glands. These assist the larger lacrimal gland to produce lacrimal fluid which forms the aqueous or water layer of the tear film. Here we're looking at the inferior conjunctival fornix. This is where the bulbar and the palpebral conjunctiva on the lower part of the eye meet and it too contains Krause’s glands.
Okay, well, that's us done and dusted for today. We've flown through this interesting tutorial, haven't we? Before I leave you for today, let's discuss some clinical points related to the eyelids and the conjunctiva.
And, of course, I think it's only appropriate that we go back to our story about Amelia and her sty. So, to ask the question again, what is a sty? A sty, also known as an external hordeolum, is a common disorder of the eyelid. Essentially, it's a local infection of the sebaceous glands, or the glands of Zeis, which we looked at earlier. Most often, it is caused by what is commonly known as a staph infection – meaning that it’s pesky staphylococcal bacteria is the troublemaker here – and like many staph infections, sties tend to occur during periods of immunosuppression, and often, are stress-related. For instance, like when you're planning your big day. The ciliary glands of Moll can also be affected.
As we know, a hordeolum presents as a localized, somewhat painful swelling of the eyelid with a small, noticeable yellowish head or lesion developing at the palpebral margin. The lesion usually ruptures after a couple of days and discharges what we would call a gunky pus-like substance, which usually relieves any pain. Most often treatment for a sty is relatively conservative. Hot compresses and antibacterial ointment will normally clear sty within two to three days. However, a sty should not be confused with another condition known as chalazion. This also causes swelling of the eyelid; however, it is due to blockage of tarsal glands rather than an infection.
And that concludes our video tutorial on the eyelids and the conjunctiva. Before I leave you, let me quickly summarize what we've learnt today.
So, this tutorial was all about the intricate anatomy of the eyelids and the conjunctiva. We first began by refreshing our memory about the general anatomy of the orbit, or the eye socket, identifying structures such as the ocular bulb, commonly known to most of us as the eyeball; the optic nerve which relays visual information from the retina to the brain. Next, we identified some of the extraocular muscles, specifically, the superior rectus muscle, the inferior rectus muscle, and finally, the inferior oblique muscle. Another muscle we briefly mentioned was this one here, which is the levator palpebrae superioris, and we discovered this muscle was of particular relevant for today's tutorial as it works to raise the upper eyelid.
Looking closer at the anterior part of the eyeball, we identified several other structures such as the lens, seen here interposed between the posterior chamber and the vitreous body of the eye; the ciliary body, which changes the shape of the lens to help us focus our vision; the iris, which is the pigmented part of the eye; and finally, the cornea, which is the anterior most part of the eyeball.
After refreshing our minds about the general anatomy of the orbit, we moved our attention to the focus of our tutorial beginning first with the eyelids. We started superficially looking at the thin elastic skin of the eyelids. We then identified various structures such as the cilia or the eyelashes of the eyelid which are found along the anterior palpebral margin of the eyelid. And we also spoke about the subcutaneous tissue of the eyelids, which contains ciliary and sebaceous glands.
Deeper within the eyelids, we discovered one of the muscular components of the eyelids: the orbicularis oculi muscle, which we saw can be divided into two parts – an orbital part, which surrounds the eyelids, and a palpebral part, which directly cover the eyelids.
Continuing on, the next structure we examined were the tarsal plates of the eyelids, which is a plate of dense connective tissue that gives form to the eyelids. Attached to the superior border of the superior tarsal plate is the superior tarsal muscle, and likewise, with the inferior tarsal plate where we have the inferior tarsal muscle. Within each tarsal plate are small tarsal glands, also known as the meibomian glands, which open into the free palpebral margin of the eyelid – and that completes our summary of the eyelid.
Next up was the conjunctiva, which we said was a thin mucous membrane covering the anterior sclera of the eyeball and the posterior surfaces of the eyelids. We then discussed the two main divisions of the conjunctiva, which were a bulbar part, which covers the eyeball or the ocular bulb; a palpebral part which lines the eyelids, and remember we said that this could be further divided into three smaller parts – the marginal, tarsal, and orbital regions – and the point at which the superior bulbar and palpebral conjunctivae meet is known as the superior conjunctival fornix while its counterpart on the inferior aspect of the eye is known as the inferior conjunctival fornix.
And with that, we've reached the end of our tutorial.
In case you're wondering, Amelia’s sty didn't stop her from marrying the man of her dreams. She had the perfect day! And they all lived happily ever after.