The inner corner of the eye is formally called the medial canthus. “Canthus” refers to the angle where the upper and lower eyelids meet, so the outer corner has a name too: the lateral canthus. But the medial canthus is by far the more anatomically interesting of the two, because it houses a cluster of specialized structures crammed into a remarkably small space, from the fleshy pink mound that most people notice to the hidden openings that drain your tears.
What You See When You Look at the Inner Corner
If you pull your lower eyelid down slightly and glance toward the inner corner in a mirror, three distinct features become visible. The most obvious is the lacrimal caruncle, that small, pinkish mound of tissue sitting right in the corner. Beside it, slightly closer to the eyeball, is a thin crescent-shaped fold of tissue called the plica semilunaris. And just above and below the caruncle, on the edges of the upper and lower eyelids themselves, are two tiny openings called puncta. Each of these has a different job.
The caruncle is not just a decorative bump. It contains sebaceous glands, hair follicles (which is why fine hairs sometimes grow there), and striated muscle fibers from the orbicularis oculi, the muscle you use to blink and squeeze your eyes shut.1PubMed Central. Histopathological study of lesions of the caruncle: a 15-year single center review Its glands contribute a small amount of oily secretion that mixes with the tear film. People sometimes wonder why that corner of the eye occasionally collects whitish or yellowish gunk; part of the answer is that the caruncle is essentially a tiny secretory organ sitting right where tears naturally pool.
The plica semilunaris is the more subtle of the two. It is a fold of conjunctiva, the clear membrane that covers the white of your eye and lines the inner surfaces of the eyelids. At first glance it looks like it does nothing. But it has a deep evolutionary history, which we will get to shortly.
The Hidden Tendon Holding Your Eyelids in Place
Beneath the visible surface of the medial canthus lies a structure you cannot see but would immediately notice if it failed: the medial canthal tendon. This tough band of connective tissue anchors the inner ends of both eyelids to the bone of the nose and the wall of the eye socket. It is supported by a complicated three-dimensional web of smaller structures, including a muscle called Horner’s muscle, several ligamentous extensions, and parts of the levator muscle that raises your upper eyelid.2PubMed. Medial canthal support structures: the medial retinaculum: a review
This tendon does more than keep your eyelids from flopping around. It helps maintain the position of the eyelids against the eyeball, which is critical for blinking effectively and for directing tears toward the drainage system. When the medial canthal tendon is torn, typically from facial trauma, the results can be severe: the eyelid margin pulls away from the eye, tear drainage stops working, and the eye surface loses protection.3FPSAM Video Journal. Complex Medial Canthal Tendon Laceration: Sequence of Repair Surgeons take these injuries seriously because a botched repair can leave someone with permanently distorted eyelid positioning.
There has been ongoing debate about the anatomy of this tendon, specifically whether it has a distinct “posterior limb” extending deeper into the orbit. A dissection study of Asian cadaveric specimens found that the supposed posterior limb was not a separate structure at all but rather part of Horner’s muscle.4PubMed. The posterior limb in the medial canthal tendon in asians: does it exist? This matters for surgeons planning reconstructive procedures, since the repair strategy depends on understanding exactly what is anchored to what.
How Tears Drain Through the Inner Corner
Most people know that tears are produced somewhere near the eye, but fewer realize that the inner corner is where tears exit. The drainage pathway starts with those two tiny puncta on the eyelid margins near the medial canthus. Each punctum opens into a narrow channel called a canaliculus. The upper and lower canaliculi merge and empty into the lacrimal sac, a small pouch tucked into a bony groove on the side of the nose. From there, fluid travels down the nasolacrimal duct and empties into the nasal cavity, which is why your nose runs when you cry.
The dimensions of these channels vary from person to person, influenced by age, sex, and individual anatomy.5Modern technologies in ophtalmology. Features of the anatomical structure of the lacrimal ducts taking into account morphometric parameters This variability helps explain why some people are more prone to blocked tear ducts than others. The whole system is surprisingly delicate. Research on tear drainage function has confirmed that the integrity of the puncta and the canaliculi is essential for the system to work properly; damage to either one disrupts the flow.6PubMed. Lacrimal drainage function after cheese wiring of lacrimal passage intubation
Newborns commonly have a membrane blocking the lower end of the nasolacrimal duct, which is why many babies have persistently watery or crusty eyes in the first months of life. The membrane usually opens on its own. In adults, blockage of the nasolacrimal duct is one of the most common reasons for a visit to an eye specialist for tearing problems. When the duct stays blocked, tears stagnate in the lacrimal sac, creating conditions for bacterial infection, a condition called dacryocystitis. The hallmark sign is a painful, swollen lump just below the inner corner of the eye, near the side of the nose. Chronic cases can lead to measurable changes in the surrounding bone, including thickening of the tissue lining the bony fossa where the lacrimal sac sits.7PubMed. Lacrimal Fossa Bony Changes in Chronic Primary Acquired Nasolacrimal Duct Obstruction and Acute Dacryocystitis
The Plica Semilunaris and Your Vestigial Third Eyelid
That small crescent of tissue next to the caruncle, the plica semilunaris, is widely considered a vestigial remnant of the nictitating membrane, or “third eyelid,” found in many other vertebrates. In birds, this membrane sweeps horizontally across the eye from the medial canthus, pulled by a dedicated muscle behind the eyeball.8PubMed Central. The Ultrastructure of the Nictitating Membrane of the Little Penguin (Eudyptula minor, Aves) Sharks, amphibians, and reptiles have versions of it too. In these animals, the membrane serves as a transparent or translucent shield that protects the cornea while still allowing some vision, useful for diving, flying into wind, or subduing prey.
Humans lost the functional version long ago, but the plica semilunaris persists as a quiet anatomical echo. It does still serve a minor practical purpose: it helps the eye move smoothly when you look side to side, and it assists in draining debris and tears toward the medial canthus. In extremely rare cases, a more developed nictitating membrane persists in humans. One documented case involved a nine-year-old girl who was born with a fleshy membrane extending from the medial canthus across roughly a third of the cornea.9PubMed Central. Persistent unilateral nictitating membrane in a 9-year-old girl: A rare case report The membrane had been present since birth and was not growing, suggesting it was a developmental throwback rather than a tumor or other pathology.
Why the Inner Corner of the Eye Is So Sensitive
If you have ever had even a tiny eyelash or speck of dust lodge in the inner corner, you know the area is exquisitely sensitive to touch. The nerve responsible for this is the infratrochlear nerve, a branch of the ophthalmic division of the trigeminal nerve. It provides sensation to the skin over the medial canthus, the medial portions of the upper and lower eyelids, the side of the nose just above, the medial conjunctiva, and the lacrimal caruncle.10PubMed. Neuroanatomy, Infratrochlear Nerve
This rich nerve supply is part of why the inner corner tends to be the spot where irritation feels most intense. It also means that surgical procedures in the medial canthal area require careful local anesthesia, and that injuries there can produce disproportionate pain relative to their size. People who get injectable cosmetic fillers or Botox near the inner brow or nose bridge are having needles placed in territory innervated by this nerve, which is one reason the area bruises and stings more than, say, the cheek.
Growths and Tumors of the Caruncle
Because the caruncle contains such a mix of tissue types, including glands, hair follicles, muscle, and pigment-producing cells called melanocytes, it can give rise to a surprising variety of growths.1PubMed Central. Histopathological study of lesions of the caruncle: a 15-year single center review Most caruncle lesions are benign. The most common ones include small cysts, papillomas, and nevi (moles). People sometimes notice a bump in the inner corner of the eye that has been there for years without changing, and these are usually harmless.
Malignant tumors of the caruncle are rare, but they do occur and can be easy to miss. Warning signs include rapid growth, new pigmentation, an ulcerated or bleeding surface, or a change in color. The types of cancer that can arise there include basal cell carcinoma, squamous cell carcinoma, sebaceous carcinoma, melanoma, and lymphoproliferative disease. Each has somewhat distinctive features: melanoma tends to show increased pigment, sebaceous carcinoma can appear yellowish, and lymphoproliferative lesions often have a salmon-pink hue.11PubMed Central. Malignant lesions of the caruncle Because the caruncle sits in such a confined space near critical structures, treatment typically involves surgical removal with careful attention to margins. In advanced cases, the surgery can be quite extensive. The take-home message is that any new or changing lump in the inner corner of the eye deserves an evaluation, even though the odds strongly favor something benign.
Epicanthic Folds and the Medial Canthus
An epicanthic fold, sometimes called an epicanthal fold, is a crescent-shaped fold of skin that runs vertically over the inner corner of the eye, partially or fully covering the medial canthus and caruncle. It is a normal anatomical variant found in a large percentage of people of East Asian descent and is also common in young children of all backgrounds, often becoming less prominent as the nasal bridge grows during early childhood. The fold does not affect vision or eye function; it is simply a feature of how skin drapes over the underlying bone and soft tissue near the nose bridge.
Because the fold covers the medial canthus, it can sometimes create the visual impression that the eyes are more widely spaced or that they turn inward, even when the alignment of the eyes is perfectly normal. Pediatricians refer to this as pseudostrabismus, and it is one of the most common reasons parents bring a young child in for a concern about “crossed eyes” that turns out to be nothing. The eyes are straight; the fold just makes them look otherwise.
In cosmetic surgery, a procedure called medial epicanthoplasty involves modifying or removing the epicanthic fold to expose more of the medial canthus. It is one of the most commonly requested eyelid procedures in parts of East Asia. Surgeons performing this procedure need an intimate understanding of the anatomy described above, particularly the medial canthal tendon and the tear drainage openings, because careless dissection in this area can damage the puncta or distort the tendon’s pull on the eyelid.
Why the Inner Corner Collects Debris
If you have ever woken up with crusty material in the inner corner of your eye, you have encountered rheum, the technical name for what most people call “sleep” or “eye boogers.” During the day, blinking pushes the tear film across the eye surface toward the medial canthus, carrying with it mucus, shed cells, dust, and oily secretions from the glands in your eyelids and caruncle. Most of this gets flushed down the tear drainage system. But when you sleep and stop blinking, the material accumulates at the lowest point of the tear film’s natural pooling area, which is the inner corner. It dries into the familiar crusty residue overnight.
A small amount of rheum in the morning is completely normal. Excessive or unusually colored discharge, particularly if it is green or yellow, thick, or accompanied by redness and swelling, suggests infection. Bacterial conjunctivitis tends to produce thicker, more purulent discharge than viral conjunctivitis. Either way, the discharge gravitates toward the medial canthus because that is where the anatomy funnels everything. People sometimes mistake normal accumulation for a sign of infection, and vice versa, so the character of the discharge matters more than its location.
The Outer Corner for Comparison
The lateral canthus, at the outer corner of the eye, is structurally simpler than its medial counterpart. It does not house tear drainage openings, does not have a caruncle, and lacks a vestigial third-eyelid fold. Its main structural element is the lateral canthal tendon, which anchors the outer eyelid to the orbital rim. This tendon is thinner and less robust than the medial canthal tendon, which is one reason surgical reconstruction of the medial canthus after trauma or tumor removal is generally more complex than repairs on the lateral side.
The lateral canthus does get its own set of clinical problems, most commonly the fine lines people call crow’s feet, which form because the orbicularis muscle contracts repeatedly against relatively thin skin with minimal underlying fat. But in terms of anatomical complexity, the medial canthus wins by a wide margin. It packs tear drainage, structural anchoring, vestigial anatomy, specialized secretory tissue, and dense nerve supply into a space smaller than a fingertip.