Cats and dogs both possess a third eyelid. So do birds, reptiles, amphibians, fish, and some other mammals. Your cat’s third eyelid functions as a shield for his cornea while moving through tall grass and during skirmishes with neighborhood felines or resistant prey.
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.
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.
Like the majority of animal species, cats have a third eyelid that is big enough to cover the entire cornea. It functions similarly to a windshield wiper blade, clearing debris from the surface and redistributing tears across the cornea. Only a small portion of the third eyelid is visible in the inner corner of the eye when the cat is alert; the majority of the lid is concealed within the eye socket. However, when the eyelid is relaxed, as in during sleep or blinking, the third eyelid passively moves across the ocular surface from the inner, lower corner of the eye to the upper, outer corner due to the retraction of the eyeball by a set of skeletal muscles. Both the sympathetic nervous system and the smooth muscle cells inside the third eyelid of cats have a role in regulating the movement of the third eyelid. The latter fact has been widely employed in research on the effects of specific medications on the sympathetic nervous system.
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.
Like most other mammals and birds, cats have a third eyelid that covers the entire surface of the cornea. This eyelid functions similarly to a windshield wiper, clearing away debris and redistributing tears across the cornea. The third eyelid is primarily concealed within the eye socket and only partially visible in the inner corner of the eye when the cat is alert. However, during sleep or during periods of relaxation, a group of skeletal muscles retracts the eyeball, enabling the third eyelid to glide from the inner, lower corner of the eye to the outer, upper corner of the eye. The sympathetic nervous system and the smooth muscle cells inside the third eyelid of cats both play a role in controlling the movement of the third eyelid.
The reason behind this lack of a third eyelid in humans, as opposed to a simple fleshy bump in the inner corner of the eye, is unclear, but it might have something to do with the fact that humans do not usually hunt or gather food by biting animals like cats or sifting through vegetation like horses do. Therefore, we might not benefit from having this additional layer of protection for the surface of our eyes.
A thick cartilage plate in the shape of a T lies between the conjunctiva’s two layers. This T cartilage’s crossbar curves to hug the corneal surface while stiffening the third eyelid’s free edge. A gland that generates a large portion of the tear film’s water component surrounds the stem of the T cartilage. The gland replenishes the tear film and provides the immune system with access to the ocular surface by releasing these watery tears through tiny ducts between the lymphoid follicles on the inner surface of the third eyelid.
A cat has an enigmatic third eyelid, called the palpebra tertia, which is also referred to as the nictitating membrane or “haw,” in addition to its upper and lower eyelids that meet together when the eye closes. Like the human appendix or wisdom teeth, the third eyelid, which is tucked away in the inner corner of the cat’s eye, is a biological curiosity that was once believed to be unimportant. Early 1900s veterinary publications even included instructions on how to remove the extra eyelid so that the eye could be examined more easily. However, the third eyelid really contributes significantly to preserving the health of the eye. The true oddities are those without a third eyelid, like humans and certain other primates. In actuality, the majority of mammals and birds have them.
Although the purpose of the third eyelid is unclear, it is thought to assist in shielding the cornea from physical harm, particularly when the cat is pursuing prey or navigating through dense vegetation. Additionally, it enables cats to cry more than primates do, possibly in an effort to better lubricate the larger cornea of their eyes. The third eyelid’s surface is covered in lymphoid follicles that secrete antibodies and other immunological mediators into the tear film, saturating the ocular surface and shielding it from the myriad bacteria and fungi that live on any eye’s surface.
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