The eyes of a cat are protected not only by the same types of eyelids that people have, but also by the nictitating membrane, which is sometimes called the third eyelid. This additional eyelid is a whitish pink color, and it is found under the other eyelids in the inside corner of the eye (near the nose).
The anatomy of the third eyelid is complex. It is a fold of tissue with the cornea on one side (bulbar surface) and the inner surface of the eyelids (palpebral surface) on the other, both covered by a specialized mucous membrane called the conjunctiva. A dense population of lymphoid follicles embedded in the bulbar surface come into contact with the tear film, which is a thin liquid layer, and the surface of the eye. These organs serve as the lymph nodes in the eyes, collecting debris and unwelcome dirt.
By maintaining the tear film against the cornea more effectively than the eyelids alone, the third eyelid is also thought to contribute to keeping the surface of the eye moister. Chronic irritation of the cornea and remaining conjunctiva is often the result of traumatizing or treating neoplasia that results in the loss of the third eyelid. Since humans no longer have a third eyelid, the true question should be “Why don’t people have a third eyelid?” As it stands, the third eyelid in cats is merely a small fleshy bump in the inner corner of the eye. Though the precise cause of our absence of a third eyelid is unknown, it might have something to do with the fact that humans rarely use biting or rooting through vegetation to catch prey (as would a horse or cat). Therefore, we might not benefit from having this additional layer of protection for the surface of the eye.
Like the human appendix or wisdom teeth, some people have considered the inner eyelid of cats—more correctly called the palpebra tertia, but also referred to as the nictitating membrane, third eyelid, or “haw”—to be a biological curiosity. Actually, procedures for excising this ostensibly unimportant structure to make it easier to examine the eye are described in certain veterinary publications from the early 1900s. Despite popular belief, cats’ third eyelid is crucial to preserving the integrity of their eye surface. It’s so crucial, in fact, that most mammals and birds have a third eyelid as the standard, and those that don’t—like humans and certain other primates—are the real outliers in the natural world.
A thick T-shaped cartilage plate is located in between the two layers of the conjunctiva. This T cartilage’s crossbar curves to fit the corneal surface while stiffening the third eyelid’s free edge. An accessory lacrimal gland that surrounds the stem of the T cartilage is responsible for producing a significant amount of the tear film. Between the lymphoid follicles on the surface of the third eyelid are the tiny ducts that allow tears to exit the gland of the eyelid. This allows the cells to discharge their contents into the tear film, which is then widely dispersed over the surface of the eye.
Although the precise purpose of a cat’s third eyelid is unknown, it is thought to assist in shielding the large cornea from harm when the animal moves through dense grass or pursues prey. Furthermore, compared to primates, even greater tear production and ocular surface rinsing is possible due to the presence of an accessory tear gland. Many immunologic mediators, such as secretory IgA and lactoferrin, are deposited into the tear film as this portion of the film passes over the lymphoid follicles covering the surface of the third eyelid. This bathes and immunologically protects the ocular surface from the myriad of bacteria and fungi that reside on the surface of even a normal eye.
Why don’t humans have a third eyelid?
Scientists believe that humans once had a third eyelid, believe it or not. The nictitating membrane was eventually reduced to the small pink tissue fold in the corner of your eye through evolution. This most likely occurred because, in contrast to cats, humans didn’t have to bite or stalk their prey through the undergrowth in order to catch them.
What is a third eyelid?
In medicine, the third eyelid is known as a nictitating membrane. This membrane helps to keep the eye moist and serves as an additional layer of defense for the surface of the eye. Sometimes, it’s said that the third eyelid functions something like “a windshield wiper blade,” clearing the surface of dust and pollen and redistributing tears across the cornea. It is translucent, whitish-pink in color, and retracts into the inner corner of each eye, giving onlookers a slightly confused impression.
Cats and dogs both possess a third eyelid. So do birds, reptiles, amphibians, fish, and some other mammals. When your cat is moving through tall grass, engaging in combat with other cats in the neighborhood, or facing resistant prey, his third eyelid protects his cornea. (Tip: Keep them inside so they won’t ever require that defense!)
FAQ
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