Vision Involves Photoreceptors in the Eyes
the accessory structures of the eye and
the interior structure of the eyeball.
how receptors detect light.
several common vision problems.
the processes involved in vision, includ-
ing the neural pathways involved.
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
). 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.
, also referred to
is a watery solution that contains salts, mucus,
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
A bactericidal enzyme
found in tears, saliva,
secretions, and tissue
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.
External structures of the eye
secrete tears into
canals drain tears
which drains into
Lacrimal structures of the eye
vision involves Photoreceptors in the Eyes 235