The Eye: Three Main Parts

The eye is a complex and sensory organ that is specialized for the gathering of visual information. The eye is made up of three main parts: eyeball (globe), orbit (eye socket), accessory (adnexal) structures.

• Accessory structure

The eye’s accessory structures include the eyelids, conjunctiva, caruncle and lacrimal glands. The eyelids are folds of skin that cover and protect the eye. It contains glands, which produce an oily secretion which in turns covers the tear layer and prevents tears from evaporating and the eyelids from sticking together. The conjunctiva is a clear mucous membrane that lines the inner surface of the eyelids and the outer surface of the eye. The conjunctiva secretes mucus to lubricate the eyeball and keep it moist. The caruncle is the small, pinkish portion of the innermost corner of the eye that contains oil and sweat glands and conjunctival tissue. The lacrimal gland (tear gland) is located at the upper, outer corner of each eye. The lacrimal gland secretes tears to keep the surface of the eye and lining of the eyelids moist and lubricated. Tears help to reduce friction, remove dust and debris from the eye and prevent infection.

• Orbit (eye socket)

The orbit (eye socket) is made up of bone formed from the skull that contains the eyeball and the connective tissues. The bone and connective tissues cushion and protect the eye. Muscles attached to the eyeball make it move in different directions.

• The eyeball

The eyeball (globe) is rich in blood vessels. Human eyes are roughly spherical, filled with a transparent gel-like substance called the vitreous humor. It supports the internal structures and maintains the shape of the eye.

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The eyeball consists of three concentric layers (tunics), whose names reflect their basic functions: a fibrous tunic, consisting of the sclera behind and the cornea in front; a vascular pigmented tunic, comprising the choroid, ciliary body, and iris; and a nervous tunic, the retina.