eyelids and drain into the nasal cavity, thereby producing
a runny nose.
The eye has external and internal muscles (
Contraction of different combinations of the six external
muscles enables the eye to move rapidly in many directions
(also see Figure 6.15). Internal muscles of the
the size of the
, regulating the amount of light that
enters the eye. Other internal muscles change the shape of
the lens to focus light refracted by the lens onto the retina.
The Eyeball Is Composed
of Three Structural Layers
The eyeball is a layered structure that consists of the outer
, a middle
lar layer) that supplies blood to the eye’s tissues and se-
cretes fluids, and an inner
incoming light to nerve impulses
). Inside the eye are
two chambers, the
in front of the
behind it. The anterior cavity is
filled with a liquid called
, which helps maintain the
shape of the eyeball and nourish-
es the lens and
humor has a composition similar
to that of cerebrospinal fluid. The
posterior cavity is filled with a
jelly-like substance called the
, which prevents the
eyeball from collapsing and holds
the retina in place.
The retina has layers of con-
. The photorecep-
tors consist of two types,
, which allow us to see color
and to see in different light levels.
change from one form
(AK-we-us HO-mer) The
watery fluid that fills
the anterior cavity of
the eye between the
cornea and the lens.
re-us) A soft, jellylike
substance that fills the
posterior cavity of the
eyeball between the
lens and the retina.
receptor that detects
light shining on the
retina of the eye.
M uscles o f th e ey e • Figure 8.7
The eye has exterior and interior muscles to allow it to move and
to control the entry of light.
Exterior Eye Muscles
moves the upper eyelid up and down.
Superior rectus muscle
moves the eye up.
Inferior rectus muscle
moves the eye down.
Circular muscles constrict the pupil, and radial muscles dilate it.
Pupil constricts as
of iris contract
Pupil dilates as
of iris contract
Interior eye muscles
Inferior oblique muscle
moves the eye down, while
superior oblique muscle
(not shown) moves the eye up.
236 CHAPTER 8
Somatic Senses and Special Senses