Editing Tears (section)

=== Nictitating membrane ===
Some mammals, such as [[cat]]s, [[camel]]s, [[polar bear]]s, [[Pinniped|seals]] and [[aardvark]]s, have a full translucent third eyelid called a [[nictitating membrane]], while others have a [[Vestigiality|vestigial]] nictitating membrane.<ref>{{Cite journal |last=Barasa |first=A. |date=2003-06-01 |title=Morphology and structure of the nictitating membrane cartilage in mammals |url=https://europepmc.org/article/med/14717063 |journal=Morphologie |volume=87 |issue=277 |pages=5–12 |issn=1286-0115 |pmid=14717063}}</ref> The membrane works to protect and moisten the eyelid while maintaining visibility. It also contributes to the aqueous portion of the tear film and possibly immunoglobulins.<ref>{{Cite web |title=Nictitating Membrane - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/nictitating-membrane#:~:text=The%20nictitating%20membrane,%20or%20third,or%20may%20not%20be%20pigmented. |access-date=2022-10-18 |website=www.sciencedirect.com}}</ref> Humans and some primates have a much smaller nictitating membrane; this may be because they do not capture prey or root vegetation with their teeth, so that there is no evolutionary advantage of the third eyelid.<ref>{{Cite web |title=Why do cats have an inner eyelid as well as outer ones? |url=https://www.scientificamerican.com/article/why-do-cats-have-an-inner/ |access-date=2022-10-18 |website=Scientific American |language=en}}</ref>[[File:Crying-girl.jpg|thumb|A toddler producing tears due to emotional stress or pain|150px]]