Animals have a third eyelid, is that so?

Recently, a rather noticeable extra eyelid was found in dogs, which became especially noticeable when they fell asleep, usually turning over on the mat. This is a fleshy curtain, visible at the corners of each eye, closest to the nose. It is also often called a flashing (literally "flining") membrane.

The anatomy of the third century is complicated. The third eyelid is a fold of tissue covered with a specialized mucous membrane (conjunctiva), which faces the inner surface of the eyelids (palpebral surface) on one side and the cornea on the other side (bulbar surface). A dense population of lymphoid follicles is built into the bulbar surface, which are in contact with the surface of the eye and the tear film, a thin layer of fluid. These structures function as lymph nodes of the eye, protecting it from dust, dirt, grains of sand, small debris.

You may have noticed that these "third" eyelids in your pets appear from time to time, perhaps in moments of sleep or when they enjoy caressing. But what does this unusual structure actually do? And why don't we have one? In humans, this eyelid is rudimentary, forming a semilunar fold in the corner of the eye.

The third eyelids usually move along the eye in the horizontal direction, not vertically, like the upper and lower eyelids. In fact, this is a specialized fold of the conjunctiva - a thin wet membrane that covers other eyelids and open whites of the eyes (sclera).

They are found in many species of mammals, but they are not unique to them. The third eyelid can also be in birds, reptiles, amphibians and fish.

The structure also varies; many species provide support to the cartilage skeleton, while others have glands that secrete tears. This change is probably designed to help animals adapt to many different environments - to the sea, air and even to the woody habitat on trees!

In several different studies, the third eyelid has been studied to understand its role in hedgehogs, kangaroos and brown bears.

Studies have shown that the third eyelid functions in the same way as the upper and lower eyelids. It protects the eyes and sweeps away the garbage. It also distributes tears over the surface of the eye, keeping it moist and preventing the formation of ulcers. This is especially important for brachycephalic dogs (with a flat muzzle), such as pugs and King Charles Spaniels, whose bulging eyes are not as well protected compared to other breeds.

And finally: Where is this third eyelid?

It is located in the inner corner of the eye, formed by the fold of the conjunctiva.