Sentence:  There is a "third" inner eyelid, highly developed and of beautiful mechanism: this is the "nictitating" membrane, or “<WSD>winker</WSD>” ("nictito", I wink), a delicate, elastic, translucent, pearly-white fold of the conjunctiva. While the other lids move vertically and have a horizontal commissure, the <WSD>winker</WSD> sweeps horizontally or obliquely across the ball, from the side next the beak to the opposite. 
Meaning of the word marked with <WSD> tags:  The winking membrane of a bird's eye; the winking muscle.