Eyelids are folds of skin that lie above and below the eye and delimit the eye socket towards the front. They can be used to close the eye. Eyelids serve primarily to protect and keep the eye moist.
What is the eyelid?
An eyelid is a thin fold that borders the eye socket forward and consists of skin, connective tissue, muscles and glands. In humans, there is an upper eyelid above the eye and a lower eyelid below the eye. The eyelids are movable and can close the eye. This is mainly to protect the eye. The line between the closed upper and lower eyelids is called the palpebral fissure. In many animal species, there is an additional third eyelid, called the nictitating membrane. This is only rudimentarily developed in humans.
Anatomy and structure
The eyelids sit above and below the eyeball and can close completely in front of the eye to protect it from light, air, or foreign matter. The upper eyelid is slightly larger than the lower eyelid. Both meet on the inside of the eye and form what is called the corner of the eyelid. This is also where the lachrymal caruncle (or tear caruncle) is located. On the eyelids are the eyelashes, which additionally protect the eye from sweat or dust. The eyelid is composed of an inner and an outer lid. The inner one consists of connective tissue and the so-called tarsus, a connective tissue plate, which is interwoven with tight collagen fibers. This is also where the muscles that regulate the eyelid fissure width are located. On the inner side, the eyelid is covered by a conjunctiva. The outer lid contains special ring muscles that are responsible for closing and opening the eye. On the upper side, it is bounded by body skin.
Function and tasks
Eyelids have two functions: They protect the eye and keep it moist. The eyelids make it possible to close the eye, protecting it from light, dirt, cold air, fog or foreign substances. The eyelashes, which are attached to the eyelid, also trap sweat, rain or dust so that they do not enter the eye. To protect the eye, the eyelids close reflexively when objects move toward the eye. This eyelid-closing reflex also occurs when the eye is startled, when sudden loud noises are heard, or when bright light is shining. The second function of the eyelids is to moisten the eye: blinking, a brief closing and opening of the eyelids, distributes tear fluid evenly over the eyeball. This keeps the sensitive cornea on the eye moist and cleans it. The moist surface also causes the eyelids to glide well and the eye to close at lightning speed. This is important for the protective function of the eye. To keep the eyes moist, we blink ten to twelve times per minute in dry air, usually without realizing it. Humans and many animals also close their eyes to sleep, keeping out visual impressions that might disturb rest.
Diseases and ailments
When movement disorders of the eyelid are present, the affected person cannot fully open the eye, and the upper eyelid hangs down partially or completely. Conversely, it also happens that the eyelid elevator acts too strongly and the upper eyelid is pulled up abnormally far. Such disorders may be congenital or acquired due to various factors. Involuntary eyelid twitching is also a form of movement disorder. However, it is usually harmless and disappears on its own. These twitches are often caused by stress, fatigue, mineral deficiency or alcohol consumption. Movement disorders can also affect blinking, which then occurs too rarely or too frequently. The eyelid is also prone to inflammation: Inflammation can develop at the edge of the eyelid, known as blepharitis. It occurs mainly in connection with conjunctivitis of the eye. The eyelid glands can also be inflamed and then lead to a hordeolum (or colloquially “sty”). Chronic inflammation of the eyelid glands is called chalazion or hailstone. Diseases and discomfort can also occur on the skin of the eyelid. For example, it is susceptible to herpes viruses and other pathogens that cause skin inflammation. Fat storage rounds or pigment disorders on the eyelid skin also occur. Various tumors, cysts and abscesses can occur on the eyelid. Parasites can also affect the eyelids and lead to various diseases. In trisomy 21 (Down syndrome) and other hereditary diseases, a slanted position of the eyes and a nasal fold of the eyelid is typical.Treacher Collins syndrome, a hereditary disease with facial malformations, can cause severe malformations of the eyelids. In rare cases, eyelids may even be completely absent due to genetic defects.
