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Accessory structures

Accessory structures protect, lubricate, move, and in other ways aid in the function of the eye. They include eyebrows, eyelids, lacrimal apparatus, and extrinsic eye muscles.

The eyebrows prevent perspiration from entering the eyes and help shade of the eye. The eyelids consist of 5 tissue layers. They protect the eyes from foreign objects and help lubricate the eyes by spreading tears over their surface. During sleep the eyes are protected from drying out by being closed. Lacrimal glands produce tears that flow across the surface of the eye. Excess tears enter the lacrimal canaliculi and reach the nasal cavity through the nasolacrimal canal. Tears lubricate and protect the eye. The extrinsic eye muscles attached to the outside covering (sclera) of the eye move the eyeball. They act on concert to move both eyes up, down, around, and from side to side so that our two eyes will center on exactly the same point.

Ex. 20. Read the following text and write down 2-3 questions that you would like to be asked. Begin some of your questions with the following phrases:

Could you tell me …

Is it true that …

I’d like to know …

Would you explain why (how, where, what…) …

Eye placement

Why do we have two eyes, facing forward and not on the sides so that we can see most everything around us like other animals? There are advantages and disadvantages to both types of eye placements. Our two eyes are face forward, what is called stereovision, because they work together to make sense out of an image and with depth perception (seeing in 3D). Other animals with eyes on the sides, like fish, cannot see straight ahead as well as we can. Most of the times they use their peripheral vision to see straight or else they have to turn their heads to the side. Their depth perception is not as good as ours, but their main purpose for sight is to protect them from danger all around. Each component that make up the eye work together so that you enjoy a good sight.


The eyes are unique instruments. Thousands of times a day, the eyes move and focus on images near and far. The eye consists of conjunctiva, sclera, cornea, lens, retina, iris, pupil, anterior chamber (aqueous humor), canal of Schlemn, posterior chamber (vitreous humor), fovea (focal point), choroids, and optic nerve. The conjunctiva runs along the inside of the eyelid. It contains tiny blood vessels. The cornea refracts light as it enters the eye. The pupil is the opening through which light passes to the back of the eye. The anterior chamber is filled with aqueous humor. This fluid is manufactured in the posterior chamber of the eye. Behind the iris and anterior chamber the lens is. The lens can change shape in order to focus light rays on the retina. The bulk of the eyeball is formed by the posterior chamber. It is filled with a colorless substance known as the vitreous humor. The retina is located behind the vitreous chamber. It processes the light images. Fovea located in the center of the retina provides the most acute vision. The optic nerve takes the electrical impulses and transmits them to the brain. The optic nerve interprets these messages into what we perceive as sight.

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