Plica semilunaris of conjunctiva

Short description: Small fold of mucous membrane in the eye.

Plica semilunaris of the conjunctiva

Front of left eye with eyelids separated to show medial canthus. (Plica semilunaris labeled at center left.)

Details
Identifiers
Latin| plica semilunaris conjunctivae
Anatomical terminology

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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, and to permit greater rotation of the globe, for without the plica the conjunctiva would attach directly to the eyeball, restricting movement.[1] 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 birds, reptiles, and fish, but is rare in mammals, mainly found in monotremes and marsupials.[2] Its associated muscles are also vestigial.[3] It is loose, thus eye movements are not restricted by it. Only one species of primate, the Calabar angwantibo, is known to have a functioning nictitating membrane.[4]

With ocular allergies, the lacrimal caruncle and plica semilunaris may be inflamed and pruritic (itchy) due to histamine release in the tissue and tear film.

## References

1. ↑ "The Conjunctiva—Structure and Function". Duane's Foundations of Clinical Ophthalmology. 2. Philadelphia: Lippincott Williams & Wilkins. 2006. Chapter 2. http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v8/v8c002.html#pli. Retrieved November 8, 2012.
2. ↑ Comparative Anatomy and Physiology of Vertebrates. London. 1866–1868.
3. ↑ The Descent of Man, and Selection in Relation to Sex. London: John Murray. 1871.
4. ↑ "The skin of primates. 33. The skin of the Angwantibo (Arctocebus calabarensis)". American Journal of Physical Anthropology 25 (3): 277–90. November 1966. doi:10.1002/ajpa.1330250307. PMID 5971502.

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The orbit of the eye

Bones|

* Frontal bone
* Zygomatic bone
* Maxillary bone
* Sphenoid bone
* Ethmoid bone
* Palatine bone
* Lacrimal bone

Muscles|

* Superior rectus muscle
* Inferior rectus muscle
* Lateral rectus muscle
* Medial rectus muscle
* Superior oblique muscle
* Trochlea of superior oblique
* Inferior oblique muscle

Eyelid|

* Levator palpebrae superioris muscle
* Tarsus
* Medial palpebral ligament
* Epicanthic fold
* Meibomian gland
* Ciliary glands
* Eyelash
* Palpebral fissure
* Canthus
* Gland of Zeis

Lacrimal apparatus|

* Krause's glands
* Lacrimal canaliculi
* Lacrimal caruncle
* Lacrimal gland
* Lacrimal lake
* Lacrimal papilla
* Lacrimal punctum
* Lacrimal sac
* Nasolacrimal duct

Other|

* Eyebrow
* Unibrow
* Conjunctiva
* Plica semilunaris
* Orbital septum
* Periorbita
* Suspensory ligament of eyeball
* Tenon's capsule

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