An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyelid features a row of eyelashes along the eyelid margin, which serve to heighten the protection of the eye from dust and foreign debris, as well as from perspiration. 'Palpebral' (and 'blepharal') means relating to the eyelids. Its key function is to regularly spread the tears and other secretions on the eye surface to keep it moist, since the cornea must be continuously moist. They keep the eyes from drying out when asleep. Moreover, the blink reflex protects the eye from foreign bodies.
The appearance of the human upper eyelid often varies between different populations. The prevalence of an epicanthic fold covering the inner corner of the eye account for the majority of East Asian and Southeast Asian populations, and is also found in varying degrees among other populations. Separately, but also similarly varying between populations, the crease of the remainder of the eyelid may form either a 'single eyelid', a 'double eyelid', or an intermediate form.
Eyelids can be found in other animals, some of which may have a third eyelid, or nictitating membrane. A vestige of this in humans survives as the plica semilunaris.
The eyelid is made up of several layers; from superficial to deep, these are: skin, subcutaneous tissue, orbicularis oculi, orbital septum and tarsal plates, and palpebral conjunctiva. The meibomian glands lie within the eyelid and secrete the lipid part of the tear film.
The skin is similar to areas elsewhere, but is relatively thin and has more pigment cells. In diseased persons these may wander and cause a discoloration of the lids. It contains sweat glands and hairs, the latter becoming eyelashes as the border of the eyelid is met. The skin of the eyelid contains the greatest concentration of sebaceous glands found anywhere in the body.
In humans, the sensory nerve supply to the upper eyelids is from the infratrochlear, supratrochlear, supraorbital and the lacrimal nerves from the ophthalmic branch (V1) of the trigeminal nerve (CN V). The skin of the lower eyelid is supplied by branches of the infratrochlear at the medial angle. The rest is supplied by branches of the infraorbital nerve of the maxillary branch (V2) of the trigeminal nerve.
In humans, the eyelids are supplied with blood by two arches on each upper and lower lid. The arches are formed by anastomoses of the lateral palpebral arteries and medial palpebral arteries, branching off from the lacrimal artery and ophthalmic artery, respectively.
The human eyelid features a row of eyelashes along the eyelid margin, which serve to heighten the protection of the eye from dust and foreign debris.
Any condition that affects the eyelid is called eyelid disorder. The most common eyelid disorders, their causes, symptoms and treatments are the following:
The eyelid surgeries are called blepharoplasties and are performed either for medical reasons or to alter one's facial appearance.
Most of the cosmetic eyelid surgeries are aimed to enhance the look of the face and to boost self-confidence by restoring a youthful eyelid appearance. They are intended to remove fat and excess skin that may be found on the eyelids after a certain age.
Eyelid surgeries are also performed to improve peripheral vision or to treat chalazion, eyelid tumors, ptosis, extropion, trichiasis, and other eyelid-related conditions.
Eyelid surgeries are overall safe procedures but they carry certain risks since the area on which the operation is performed is so close to the eye.
An anatomical variation in humans occurs in the creases and folds of the upper eyelid.
An epicanthic fold, the skin fold of the upper eyelid covering the inner corner (medial canthus) of the eye, may be present based on various factors, including ancestry, age, and certain medical conditions. In some populations the trait is almost universal, specifically in East Asians and Southeast Asians, where a majority, up to 90% in some estimations, of adults have this feature.
The upper eyelid crease is a common variation between people of Caucasian and East Asian ethnicities. Westerners commonly perceive the East Asian upper eyelid as a 'single eyelid'. However, East Asian eyelids are divided into three types – single, low, and double – based on the presence or position of the lid crease. Jeong Sang-ki et al. of Chonnam University, Kwangju, Korea, in a study using both Asian and Caucasian cadavers as well as four healthy young Korean men, said that 'Asian eyelids' have more fat in them than those of Caucasians.
|Year||Ethnic group||Gender||Prevalence of double eyelid|
Blepharoplasty is a cosmetic surgical procedure performed to correct deformities and improve or modify the appearance of the eyelids. With 1.43 million people undergoing the procedure in 2014, blepharoplasty is the second most popular cosmetic procedure in the world (Botulinum toxin injection is first), and the most frequently performed cosmetic surgical procedure in the world.
East Asian blepharoplasty, or 'double eyelid surgery', has been reported to be the most common aesthetic procedure in Taiwan and South Korea. Though the procedure is also used to reinforce muscle and tendon tissues surrounding the eye, the operative goal of East Asian blepharoplasty is to remove the adipose and linear tissues underneath and surrounding the eyelids in order to crease the upper eyelid. A procedure to remove the epicanthal fold (i.e. an epicanthoplasty) is often performed in conjunction with an East Asian blepharoplasty.
The use of double sided tape or eyelid glue to create the illusion of creased, or 'double' eyelids has become a prominent practice in China and other Asian countries. There is a social pressure for women to have this surgery, and also to use the alternative (taping) practices. Blepharoplasty has become a common surgical operation that is actively encouraged, whilst other kinds of plastic surgery are actively discouraged in Chinese culture.