Veterinarian Paul Miller of the University of Wisconsin-Madison explains.
The inner eyelid of cats--more properly called the palpebra tertia but also known as the nictitating membrane, third eyelid or "haw"--has been regarded by some as a biological curiosity much like the human appendix or wisdom teeth. In fact, some veterinary articles in the early 1900s describe methods for removing this supposedly irrelevant structure so as to facilitate examination of the eye. Despite these perceptions, the third eyelid of cats plays an important role in maintaining the health of their eye surface. In fact, it is so important that among mammals and birds the norm is for a species to have a third eyelid and those lacking one--such as humans and some of our fellow primates--are the true oddities in nature.
The anatomy of the third eyelid is complex. It is a fold of tissue covered by a specialized mucous membrane (the conjunctiva) that faces the inner surface of the eyelids (palpebral surface) on one side and the cornea on the other side (bulbar surface). Embedded in the bulbar surface is a dense population of lymphoid follicles that are in contact with the surface of the eye and the tear film, a thin layer of liquid. These structures function as the lymph nodes of the eye, trapping unwanted dirt and detritus.
Between the two layers of conjunctiva is a dense T-shaped cartilage plate. The crossbar of this T cartilage stiffens the free edge of the third eyelid and is also curved so as to conform to the corneal surface. The stem of the T cartilage is surrounded by an accessory lacrimal gland, which produces a substantial portion of the tear film. The tiny ducts through which tears leave the gland of the third eyelid exit between the lymphoid follicles on the surface of the third eyelid, allowing these cells to dump their contents into the tear film and be widely distributed over the surface of the eye.
In cats, as in most species, the third eyelid is large enough to completely cover the cornea and acts much like a windshield wiper blade by removing debris from the surface and redistributing tears over the cornea. When the cat is alert, the bulk of the third eyelid is hidden within the eye socket and only a small portion is visible in the inner corner of the eye. When relaxed, during sleep or during blinking, however, retraction of the eyeball by a set of skeletal muscles causes the third eyelid to passively move across the ocular surface from the inner, lower corner of the eye to the upper, outer corner. Movement of the third eyelid in cats is also partially regulated by the sympathetic nervous system as well as by smooth muscle cells within the third eyelid. The former fact has been used extensively in the study of how certain drugs affect the sympathetic nervous system.
The exact function of the third eyelid in cats is not completely known but it is believed to help protect a very large cornea from injury as cats move through tall grass or capture prey. Additionally the presence of an accessory tear gland allows for even greater tear production and rinsing of the ocular surface than is found in primates. As this portion of the tear film flows over the lymphoid follicles covering the surface of the third eyelid, a variety of immunologic mediators, including secretory IgA and lactoferrin, are dumped into the tear film to bathe and immunologically protect the ocular surface from the stew of bacteria and fungi that inhabit the surface of even a normal eye.
The third eyelid is also believed to help keep the surface of the eye moister by holding the tear film against the cornea better than the eyelids do by themselves. Loss of the third eyelid through trauma or in the treatment of neoplasia frequently results in chronic irritation of the cornea and remaining conjunctiva. In view of this, the real question is not "Why do cats have a third eyelid?" but "Why don't people have a third eyelid?" In humans, the third eyelid has been reduced to a rudimentary fleshy bump in the inner corner of the eye. Although the exact reason why we lack a third eyelid is unknown, it may be related to the fact that humans do not typically capture prey by biting (as would a cat) or by rooting through vegetation (as would a horse). Thus, there may be no advantage for us in having this extra measure of protection for the surface of the eye.