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Structure and Function of the

Human Eye

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Vertebrates have single-lens eyes

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Fig. 49.9

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• Sclera: a tough white layer of connectivetissue that covers all of the eyeball except

the cornea.

 – Conjunctiva: external cover of the sclera— keeps the eye moist.

• Cornea: transparent covering of the front of 

the eye.

 – Allows for the passage of light into the

eye and functions as a fixed lens.

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Parts of the Eye

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• Choroid: thin, pigmented layer lining the

interior surface of the sclera. – Prevents light rays from scattering and

distorting the image.

 – Anteriorly it forms the iris. • The iris regulates the size of the pupil. 

• Retina: lines the interior surface of the

choroid. – Contains photoreceptors.

• Except at the optic disk (where the optic

nerve attaches).Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Focusing Light

• The lens and ciliary body divide the eye intotwo cavities.

 – The anterior cavity is filled with aqueoushumor produced by the ciliary body.

 – The posterior cavity is filled with vitreoushumor. 

 – The lens, the aqueous humor, and thevitreous humor all play a role in focusing lightonto the retina.

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Focusing Light

• Accommodation isthe focusing of lightin the retina.

• Mammals focus bychanging the shapeof the lens.

• The lens is flattenedfor distant objects.

• The lens is roundedfor near objects.

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• Photoreceptors of the retina.

 – rod cells - light sensitive but do not

distinguish colors.

 – cone cells - not as light sensitive as rods but

provide color vision

• Most highly concentrated on the fovea  – an area of 

the retina that lacks rods.

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Vision

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Vision

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• Rhodopsin (retinal + opsin) is the visualpigment of rods.

• The absorption of light by rhodopsin initiates a

signal-transduction pathway

• Receptor potential is hyperpolization .

Fig. 49.13

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• Color reception is more complex than therhodopsin mechanism.

 – Three types of cone cells each withunique photopsin (green cones, redcones, blue cones)

• Brain’s analysis of color depends on relativeresponses of each type of cone.

 – Colorblindness is due to a deficiency, or absence, of one or more photopsins.

• Inherited as an X-linked trait.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Vision

• Rods and Cones

synapse with

nuerons called

bipolar cells

• Bipolar cells

synapse with

galgion cells of 

optic nerve