Eye-The Sense of Sight
Human beings have two eyes situated in deep sockets or on the front side of the head. Human eyes are photoreceptors (respond to light). Each eye is in the form of a ball called an eyeball which measures about 2.5cm in diameter. Each eyeball can be rotated with the help of six distinct sets of muscles i.e, superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique.
Accessory Structures of the Eye
Eyebrows, eyelids, eyelashes, and the tear glands are the main accessory organs of the eye.
The eyebrows protect the eyeball from foreign objects, sweat, water, direct rays of sunlight, and other debris from falling down into the eyes.
Eyelids are folds of the skin and muscles. The upper and lower eyelids protect eyes from excessive light rays and foreign particles. They cover the eyes during sleep and spread lubricating oily secretion called sebum over the eyeballs. Eye is responsible for spreading the tear film evenly across the surface. 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 eyelashes. At the base of hair follicles of eyelashes are found sebaceous glands. These glands secrete a lubricating fluid into the hair follicles called sebum.
Tear glands are also known as lacrimal glands. Lacrimal glands are paired almond-shaped exocrine glands.
Lacrimal glands are a group of glands that manufacture and pour tears. A tear or lacrimal gland is a compound gland. A lacrimal gland is situated at the upper sideward portion of each eyelid. Lacrimal glands give rise to 6 to 12 excretory lacrimal ducts which empty their secretion on to the surface of the conjunctiva. From here lacrimal canals pass into the lacrimal duct. From here nasolacrimal duct conducts the secretion into the nasal cavity of the nose.
Function of Tears
- It provides moisture to the eyeball.
- It also cleans the front surface of the eyeball by washing away the foreign dust particles.
- It serves as a lubricant.
- The enzyme lysozyme contained in the tears kills the germs.
Conjunctiva is a thin membrane that covers the entire front part of the eyeball and lines the inside of the eyelids.