The plica semilunaris is a small fold of
bulbar conjunctiva on the medial canthus of the
eye. It functions during movement of the eye, to help maintain tear drainage via the
lacrimal lake
The lacrimal lake is the pool of tears in the lower conjunctival cul-de-sac, which drains into the opening of the tear drainage system (the '' puncta lacrimalia''). The volume of the lacrimal lake has been estimated to be between 7 and 10 μ ...
, and to permit greater rotation of the
globe
A globe is a spherical model of Earth, of some other celestial body, or of the celestial sphere. Globes serve purposes similar to maps, but unlike maps, they do not distort the surface that they portray except to scale it down. A model glo ...
, for without the plica the conjunctiva would attach directly to the eyeball, restricting movement. It is the
vestigial remnant of the
nictitating membrane (the "third eyelid") which is drawn across the eye for protection, and is present in other animals such as
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s,
reptiles, and
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
, but is rare in
mammals, mainly found in
monotremes and
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...
s. Its associated muscles are also vestigial.
It is loose, thus eye movements are not restricted by it. Only one species of
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
, the
Calabar angwantibo
The Calabar angwantibo (''Arctocebus calabarensis''), also known as the Calabar potto, is a strepsirrhine primate of the family Lorisidae. It shares the genus '' Arctocebus'' with the golden angwantibo (''Arctocebus aureus''). It is closely rela ...
, is known to have a functioning nictitating membrane.
With ocular allergies, the
lacrimal caruncle
The lacrimal caruncle, or caruncula lacrimalis, is the small, pink, globular nodule at the inner corner (the medial canthus) of the eye. It consists of tissue types of neighbouring eye structures. It may suffer from lesions and allergic inflammati ...
and plica semilunaris may be inflamed and pruritic (itchy) due to histamine release in the tissue and tear film.
References
External links
Human eye anatomy
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