Did you know: Dogs have 3 eyelids!
That's right, they have 3 eyelids. They have the obvious upper and lower eyelid like we do, as well as a 3rd eyelid on the very inside of their eye that is also known as the nictitating membrane.
The 3rd eyelid serves many purposes. It helps to clear debris off of the cornea, acts as a shield to protect the cornea from trauma, it produces tears which helps keep the dog's eye lubricated, and contains lymphoid tissue which acts as a lymph node and produces antibodies to fight infection.
Source (Image): Merck Manual