HUMAN BIOLOGY

SenSory SyStemS 277

hoW do the eyes detect light?

• In the eye’s outer layer, the curved cornea covers the iris
  and helps focus incoming light.


• In the middle layer, the choroid prevents light scattering,
  the iris controls incoming light, and the ciliary body and
  lens aid in focusing light on photoreceptors.


• Photoreception occurs in the retina of the inner layer.
  Adjustments in the position or shape of the lens focus
  incoming visual stimuli onto the retina.

taKe-Home message

The retina is a thin layer of neural tissue at the back of
the eyeball. It has a pigmented basement layer that covers
the choroid. Resting on the basement layer are densely
packed photoreceptors that are linked with a variety of
neurons. Axons from some of these neurons converge to
form the optic nerve at the back of the eyeball. The optic
nerve is the trunk line to the thalamus—which sends
signals on to the visual cortex in the brain. The place
where the optic nerve exits the eye is a “blind spot” because
there are no photo receptors there.
The surface of the cornea is curved. This means that
incoming light rays hit it at different angles and, as they
pass through the cornea, their trajectories (paths) bend
(Figure 14.15A). There, because of the way the rays were
bent at the curved cornea, the rays converge at the back
of the eyeball. They stimulate the retina in a pattern that
is upside-down and reversed left to right relative to the
source of the light rays. Figure 14.15B gives a simplified dia-
gram of this process. The brain corrects the “upside-down
and backwards” orientation.

Eye muscle movements fine-tune the focus

Light rays from sources at different distances from the eye
strike the cornea at different angles. As a result, they will
be focused at different distances behind it, and adjustments
must be made so that the light will be focused precisely
on the retina. Normally, the lens can be adjusted so that
the focal point coincides exactly with the retina. A ciliary
muscle adjusts the shape of the lens. As you can see in
Figure 14.16, the muscle encircles the lens and attaches to it
by ligaments. When the muscle contracts, the lens bulges,
so the focal point moves closer. When the muscle relaxes,
the lens flattens, so the focal point moves farther back.
Adjustments like these are called accommodation. If they
are not made, rays from distant objects will be in focus at
a point just in front of the retina, and rays from very close
objects will be focused behind it.

F i g u r e 14.16 Adjusting the lens focuses light on the

retina. Adjustments of the ciliary muscle focus light from

near or distant sources on the retina by changing the

tension of fibers that ring the lens. (© Cengage Learning)

contracted

ciliary

muscle

relaxed

ciliary

muscle

fibers

slack

fibers

taut

DISTANCE

VISION

CLOSE

VISION

F i g u r e 14.15 Light entering the eye bends as it travels toward the retina. A How light can bend. b How light rays

reverse as they travel toward the retina. The pattern of light rays that converge on the retina is upside-down and

reversed left to right. (© Cengage Learning)

A b

Sometimes the lens can’t be adjusted enough to place

the focal point on the retina. Sometimes also, the eyeball

is not shaped quite right. The lens is too close to or too

far away from the retina, so accommodation alone cannot

produce a precise match. Eyeglasses or contact lenses can

correct these problems, which we will consider more fully

in Section 14.10.

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