nictitating membrane
Concise Oxford English Dictionary © 2008 Oxford University Press:
nictitating membrane
▶noun Zoology a whitish membrane forming an inner eyelid in birds, reptiles, and some mammals.
– origin C18: nictitating based on med. L. nictitat- ‘blinked’, frequentative of nictare.
'nictitating membrane' also found in these Oxford entries:

