Vision or sense of sight is the most important of all body senses. We all respond to the light stimulus. Human beings have two eyes situated in the orbits on the front side of the head.
Each eye is in the form of a ball called eyeball which measures about 2.5 cm (1 inch) in diameter. Each eyeball can be rotated with the help of six distinct sets of muscles.
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Accessory Structures of the Eye:
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Eyebrows, eyelids, eyelashes and the tear glands are the accessory structures of the eye.
The eyebrows protect the eyeball from foreign objects, perspiration and direct rays of sunlight.
Eyelids are folds of skin and muscles. The upper and lower eyelids have many important functions. They protect the eyes from excessive light and foreign particles, cover the eye during sleep and spread lubricating secretions over the eyeballs. The upper eyelid is more movable than the lower one.
Projecting from the border of each eyelid is a row of thick, short hairs called eye lashes. At the base of hair follicles of eyelashes are found sebaceous glands. These glands secrete a lubricating fluid into the hair follicles called sebum.
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Tear glands or lacrimal glands (collectively known as lacrimal apparatus) are a group of glands that manufacture and pour tears (lacrima: tears). A lacrimal gland is a compound gland located at the upper sideward portion of each eyelid.
Each lacrimal gland gives rise to 6 to 12 excretory lacrimal ducts which empty their secretion (tears) onto the surface of conjunctiva. From here, it passes to lacrimal canals and then into the lacrimal duct. From here a nasolacrimal duct conducts the secretion into the nasal cavity in the nose.
Tear or lacrimal secretion is a watery fluid containing salts, some mucus and bacteriocidal enzyme called lysozyme. After being secreted it spreads over the surface of the eyeball and serves as a lubricant.
It also cleans the front surface of the eyeball by washing away dust particles. It also moistens the eyeball. I he enzyme lysozyme contained in the tears kills the germs also. Tears also help in communicating emotions.