Human Anatomy & Physiology
An eyelid is a thin fold of skin that covers and protects the eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily. The human eyelid features a row of eyelashes which serve to heighten the protection of the eye from dust and foreign debris, as well as from perspiration. "Palpebral" (and "blepharo") 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 lacrimal apparatus is the physiologic system containing the orbital structures for tear production and drainage. It consists of:
(a) the lacrimal gland, which secretes the tears, and its excretory ducts, which convey the fluid to the surface of the eye;
(b) the lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct, by which the fluid is conveyed into the cavity of the nose, emptying anterioinferiorly to the inferior nasal conchae at the nasolacrimal duct.
(c) the nerve supply of lacrimal apparatus done by carotid plexuse of nerves along artery internal and external sympathetically but parasympathetic from lacrimal nucleus of the facial nerve