HUMAN BIOLOGY

276 Chapter 14

accommodation
Adjustments of the eye’s
lens that focus light pre-
cisely on the retina.

cornea The transparent
structure that covers the
eye’s iris.

eyes Sensory organs
that contain arrays of
photoreceptors.

iris The pigmented eye
region; light enters through
the pupil in the iris.

lens Eye structure that
focuses light on the retina.

retina Layer of tissue
containing the eye’s
photoreceptors.

vision Awareness of the
characteristics of visual
stimuli.

visual cortex Brain region
that receives nerve impulses
from the optic nerve.

Vision: an overview

n Vision requires a system of photoreceptors and brain

centers that can receive and interpret the patterns of

nerve impulses.

The sense of vision is an aware-

ness of the position, shape, bright-

ness, distance, and movement of

visual stimuli. Our eyes are sensory

organs that contain tissue with a

dense array of photoreceptors.

The eye is built to detect light

The eye has three layers (Table 14.1),

sometimes called “tunics.” The

outer layer consists of a sclera and

a transparent cornea. The middle

layer consists mainly of a choroid,

ciliary muscle, and iris. The key fea-

ture of the inner layer is the retina

(Figure 14.14).

The sclera is the dense, fibrous

“white” of the eye. It protects most

of the eyeball, except for the region

formed by the cornea. Moving

inward, the thin, darkly pigmented

choroid lies under the sclera. It pre-

vents light from scattering inside the eyeball and contains

most of the eye’s blood vessels.

Behind the transparent cornea is the round, pigmented

iris (after irid, which means “colored circle”). The iris has

more than 250 measurable features such as pigments and

fibrous tissues. Each person has a unique combination

of these features, so the iris can be used as identification

much as fingerprints are. Look closely at someone’s eye,

and you will see a “hole” in the center of the iris. This pupil

is the entrance for light. When bright light hits the eye,

circular muscles in the iris contract and shrink the pupil.

In dim light, radial muscles contract and enlarge the pupil.

Behind the iris is a saucer-shaped lens, with onionlike

layers of transparent proteins. Ligaments attach the lens

to smooth muscle of the ciliary body; this muscle functions

in focusing light, as we will see shortly. The lens focuses

incoming light onto a dense layer of photoreceptor cells

behind it, in the retina. A clear fluid, aqueous humor (body

fluids were once called “humors”), bathes both sides of the

lens. A jellylike substance (vitreous humor) fills the chamber

behind the lens.

Wall of eyeball (three layers)

Sensory tunic Retina. Absorbs, transduces light energy

(inner layer) Fovea. Increases visual acuity

Vascular tunic Choroid. Blood vessels nutritionally

(middle layer) support wall cells; pigments

prevent light scattering

Ciliary body. Muscles control lens

shape; fine fibers hold lens upright

Iris. Adjusting iris controls incoming light

Pupil. Serves as entrance for light

Start of optic nerve. Carries signals to

brain

Fibrous tunic Sclera. Protects eyeball

(outer layer) Cornea. Covers iris; focuses light

Interior of eyeball

Lens Focuses light on photoreceptors

Aqueous humor Transmits light, maintains pressure

Vitreous body Transmits light, supports lens and eyeball

Table 14.1 Parts of the eye

retina

fovea

optic

disk

(blind

spot)

optic

nerve

choroid

iris

lens

pupil

cornea

aqueous

humor

ciliary muscle

vitreous humor

sclera

Figure 14.14 Animated! The eye is specialized to receive

light and focus it on photoreceptors. (© Cengage Learning)

© Bo Valentino/Shutterstock.com

14.8

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