nictitating membrane
Translucent protective eyelid
A nictitating membrane is a translucent third eyelid that moves horizontally across the eye, found in birds, reptiles, amphibians, fish, and some mammals.
It protects and moistens the eye while allowing partial vision—a bird of prey can shield its eyes during a strike without going blind.
Cats and dogs have vestigial nictitating membranes visible in the inner corner of the eye.
Humans lack one entirely.
Translucent inner ocular shield
- hawEquine left-turn command
- conjunctivaMucous membrane lining eyelids
- corneaEye's clear outer dome
- scleraEyeball's outer membrane
eyelid · ocular · orbit · cartilage · muscle · tissue · eye socket · tear duct
Large predatory fish
- birdsEgg-laying flying vertebrates
- camelsHumped desert mammals
- crocodilesLarge aquatic reptiles
- reptilesCold-blooded scaly vertebrates
amphibians · frogs · cats · dogs · polar bears · seals · beavers · manatees
Guarding against harm
- lubricationCoating to reduce friction
- moistureSmall amounts of liquid
- debris removalClearing scattered remains
- shieldHandheld defensive armor
barrier · underwater vision · sweep · reflex · secretion · hydration · blinking · closure
Allowing light through
- semi-transparentPartially permitting light passage
- opaqueImpassable to light
- see-throughAllowing vision through
- clearWithout obstruction or fog
filmy · thin · diaphanous · pellucid · gossamer · sheer · visibility
Water-inhabiting
- divingEntering water deliberately
- swimmingLocomotion through water
- desertTo leave behind completely
- sandstormWind-driven desert grit
predation · hunting · survival · underwater · burrowing · flight · environmental · evolutionary
third eyelid
Translucent inner ocular shield
- hawEquine left-turn command
- conjunctivaMucous membrane lining eyelids
- corneaEye's clear outer dome
sclera · eyelid · ocular · orbit · cartilage · muscle · tissue · eye socket · tear duct · vitreous
sharks
Large predatory fish
- birdsEgg-laying flying vertebrates
- camelsHumped desert mammals
- crocodilesLarge aquatic reptiles
reptiles · amphibians · frogs · cats · dogs · polar bears · seals · beavers · manatees · aardvarks
protection
Guarding against harm
- lubricationCoating to reduce friction
- moistureSmall amounts of liquid
- debris removalClearing scattered remains
shield · barrier · underwater vision · sweep · reflex · secretion · hydration · blinking · closure
transparent
Allowing light through
- semi-transparentPartially permitting light passage
- opaqueImpassable to light
- see-throughAllowing vision through
clear · filmy · thin · diaphanous · pellucid · gossamer · sheer · visibility
aquatic
Water-inhabiting
sandstorm · predation · hunting · survival · underwater · burrowing · flight · environmental · evolutionary · nocturnal
- animal anatomyskull, teeth, ears, eyes, nose
- cat anatomywhiskers, claws, tail, paws, ears
- animal visioncat eyes, compound eyes, cornea, depth perception, eagle eye
- bird anatomybeak, feathers, wings, tail feathers, wing feathers
- reptile anatomyskeleton, bones, lungs, skull, ribs
- vestigial structuresappendix, coccyx, tailbone, wisdom teeth, plantaris