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In anatomy, the term "canthus" refers to the corners of the eye where the upper and lower eyelids meet, forming a small notch. The medial, or nasal, canthus is located at the inner corner of the eye, where it extends towards the nose. At the external corner of the eye is the lateral canthus. The canthi are connected by a transversal plane called the bicanthal plane, which creates the uppermost border of the facial region known as the midface. This region includes the lower eyelids, nose, cheeks, and upper lip.
The medial canthus contains the caruncle, a pinkish part of the eyelid that houses small accessory tear glands called Ciaccio's glands. These glands receive their name from an Italian scientist, Giuseppe Vincenzo Ciaccio, who first discovered their existence in 1874. Typically, two to five accessory glands of this type are found in the upper eyelid. The glands produce tears, which keep the eyes lubricated and free of debris. Also located in the inner corner of the eye are the canaliculi, which are tiny ducts that siphon fluid, usually in the form of tears, away from the eye's surface and into the lacrimal sac.
A large percentage of the world's population possesses what is known as an epicanthus or epicanthic fold, which is a fold of skin on the upper eyelid that partly obscures the medial canthus. Epicanthic folds are most often found on individuals of Asian descent, although Native Americans, Inuits, and members of Oceanic ethnic groups also commonly have epicanthic folds. When an epicanthus is present, the eye takes on a narrower appearance that is often described as "almond-shaped." The reason for the existence of the epicanthus is not entirely clear, although a common hypothesis states that it provides extra protection from ultraviolet rays.
Canthi can be affected by a number of medical conditions. Individuals with a genetic syndrome known as Waardenburg's syndrome often present with a facial feature known as dystopia canthorum or telecanthus. Dystopia canthorum results in the canthi being spaced widely apart, though the pupils and the rest of the eyes are set at a normal distance. The medial canthus is also susceptible to the growth of cancerous tumors. Many medical conditions related to chromosome abnormalities, such as Down Syndrome and fetal alcohol syndrome, may result in the formation of an epicanthus on individuals who are not genetically predisposed to epicanthic folds.
Since the region described as the midface is the most susceptible to signs of aging, some individuals may choose to undergo a form of cosmetic surgery known as canthoplasty. Canthoplasty involves the reshaping of the lower eyelid by tightening the muscles that support the lateral canthus. This lifts the lower lid of the eye and gives a more youthful appearance. Apart from the cosmetic benefits, canthoplasty may also be required in cases where the eyelids droop due to facial paralysis or a prior surgical procedure on the eye.