The third eyelid - a nictitating membrane, which exists in some animals.
It protects the eyes and moisturizes it without losing visibility.
Sharks, birds and some reptiles have a full third eyelid.
In humans, a rudiment of the nictitating membrane called the lunate fold.
Why third eyelid
Lunate fold (a rudiment of the third century) - isvertical fold of mucous membrane (conjunctiva). It is located at the inner corner of the eyelids and a small size. This fold is strongly developed among the Negroes and Malays, and may contain cartilage, like centuries. It produces mucus, which collects foreign matter (dust, dirt, causing damage to the cornea). Then, they move into the lacrimal duct and removed from the eye. Near the lunate fold is tearful caruncle. It does not assume specific functions. Abnormalities in the eye area can cause disorders of lachrymal passage.
Diseases of the lunate folds and lacrimal caruncle
Diseases of the lacrimal caruncle and semilunar foldIt is rare. They manifest malformations, degenerative changes, inflammation, tumors. Anomalies of the lunate folds and lacrimal caruncle manifest their shape changes. In some cases, there is atrophy of the lunate folds, it can cause watery eyes due to loss of contact points with the lacrimal fluid.
Anatomically tearful caruncle is education,Its structure consists of including sweat and sebaceous glands, adipose tissue, hair follicles. A person may develop purulent inflammation of the lacrimal caruncle. The disease usually develops quickly and begins with the sudden pain and redness in the inner corner of the eye, while the temperature increases, a malaise and headache. Lacrimal caruncle dramatically increases in size, painful to the touch. Further, in its tissues appear purulent plugs, which merge to form an abscess. When the abscess will burst, the inflammation subsides, recovery comes in the next four or five days.
When argyrosis (pigmentation) lunate foldsIt appears dirty-gray color, which covers the eyeball conjunctiva and eyelids. The pathology is caused by too long instillation of eye drops, which is composed of a silver salt (colloid silver, silver nitrate, Protargolum). Argyrosis lunate fold, tear myastsa no effect on the lacrimal function. Treatment with cosmetic purpose is inefficient, because the process is irreversible, the silver particles are removed from the tissue can not.
The lacrimal caruncle may be formedbenign and malignant tumors. Most prevailing benign tumors (papillomas, adenomas, fibromas) occur polyps, cysts. Cancers predominate from melanoblastoma sarcomas. On the crescent-fold sometimes formed moles (nevi), which are malformations. In some cases, a tendency towards growth and malignant transformation.