Vision Involves Photoreceptors in the Eyes
LEARNING OBJECTIVES
1.
Identify
the accessory structures of the eye and
their functions.
2.
Describe
the interior structure of the eyeball.
3.
Explain
how receptors detect light.
4.
Describe
several common vision problems.
5.
Outline
the processes involved in vision, includ-
ing the neural pathways involved.
o
far we have talked about senses in which the
stimulus either touches or chemically binds to
the receptors. The remaining special senses—
sight, hearing, and balance—deal with in-
tangible physical phenomena—light, sound, and gravity,
respectively. The structures that change these stimuli into
nerve impulses are quite complex. Let’s look first at our
window to the world, the eye.
Accessory Structures Protect the
Eyeball and Muscles Allow It to Move
Because sight is our dominant sense, the human eye is
a complex sense organ. Beginning at the surface of the
face, the eye has several accessory structures that protect
it (
Figure 8.6
). The eyebrows and eyelashes protect the
eyeballs from foreign objects. The eyelids cover, protect,
and help prevent the eyes from drying out via the blink re-
flex. The lacrimal apparatus secretes and drains the fluid
that moistens the eye.
Lacrimal fluid
, also referred to
as
tears,
is a watery solution that contains salts, mucus,
and
a
bacteria-killing
enzyme
called
lysozyme
. Typically,
the
tears drain into the nasal cav-
ity, and when we cry, parasym-
pathetic activity stimulates the
lacrim al glands
to produce ex-
cessive tears, which spill over the
lysozyme
(LT-so-zTm)
A bactericidal enzyme
found in tears, saliva,
perspiration, nasal
secretions, and tissue
fluids.
A ccessory stru c tu re s o f th e ey e • Figure 8.6
The eye has several external structures to protect it, including eyebrows, eyelashes,
and eyelids. Lacrimal structures also protect the eye by keeping it moist.
Eyebrow
Upper eyelid
Eyelash
Lower eyelid
Pupil
Iris
Conjunctiva
(over sclera)
External structures of the eye
Lacrimal
glands
secrete tears into
lacrimal
ducts.
Superior and
inferior
lacrimal
canals drain tears
into nasal
lacrimal
duct
which drains into
nasal cavity.
Lacrimal structures of the eye
vision involves Photoreceptors in the Eyes 235
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