All dogs have a third eyelid which is called the “nictitating membrane,” or “haw.” Its job is to keep a dog’s eye lubricated and protected, and it generally goes unnoticed at the inner corner of the eye in most breeds. In a few other breeds, however, the breed standard makes note of the presence of the haw: From the Basset Hound AKC standard: “The eyes are soft, sad, and slightly sunken, showing a prominent haw…” From the Clumber Spaniel AKC standard: ” Some haw may show but excessive haw is undesirable.” From the Otterhound AKC standard: “The haw shows only slightly.”
Deborah S. Friedman, D.V.M. a diplomate with the American College of Veterinary Ophthalmologists says about the haw that it acts as a “windshield-washer” for the cornea. “It clears debris and mucus off of the cornea,” she explains. Also, the gland of the third eyelid produces about one-third of the dog’s tears. The third eyelid contains lymphoid tissue which acts as a lymph node and produces antibodies to fight infection. Finally, it protects the cornea from injury.
The human eyelid also protects and nourishes the human cornea, but ours perform similar functions with two eyelids instead of three. Dogs with a prominent, visible third eyelid (nictitating membrane) are said to have “Haw Eyes.”