469
CHAPTER TWELVE
Nervous System III
Sclera
Retinal pigment
epithelium
Receptor cells
Layer of
connecting
neurons
Rod
Cone
Pigmented
choroid
coat
Retina
Impulses
to optic
nerve
Bipolar neuron
Ganglion cell
Nerve fibers
Vitreous humor
Horizontal cell
Amacrine cell
Light waves
Pupil
Circularly arranged
smooth muscle fibers
of the iris
Radially arranged
smooth muscle fibers
of the iris
Sympathetic
motor nerve
fiber
In dim light
In normal light
In bright light
Parasympathetic
ganglion
Parasympathetic
motor nerve fiber
FIGURE 12.31
Dim light stimulates the radial
muscles of the iris to contract, and the pupil dilates.
Bright light stimulates the circular muscles of the iris to
contract, and the pupil constricts.
The Inner Tunic
The inner tunic of the eye consists of the
retina
(ret
ı˘-nah),
which contains the visual receptor cells (photoreceptors).
This nearly transparent sheet of tissue is continuous with the
optic nerve in the back of the eye and extends forward as the
inner lining of the eyeball. It ends just behind the margin of
the ciliary body.
The retina is thin and delicate, but its structure is com-
plex. It has distinct layers, including retinal pigment epithe-
lium, neurons, nerve F bers, and limiting membranes
(f gs.
12.32
and
12.33)
.
There are F ve major groups of retinal neurons. The nerve
fibers of three of these groups—the
receptor cells, bipolar
neurons,
and
ganglion cells
—provide a direct pathway for
impulses triggered in the receptors to the optic nerve and
FIGURE 12.32
The retina consists of several cell layers.
brain. The nerve F bers of the other two groups of retinal
cells, called
horizontal cells
and
amacrine cells,
pass later-
ally between retinal cells (see F
g. 12.32). The horizontal and
amacrine cells modify the impulses transmitted on the F
bers
of the direct pathway.
In the central region of the retina is a yellowish spot called
the
macula lutea
that occupies about 1 square millimeter. A
depression in its center, called the
fovea centralis,
is in the
region of the retina that produces the sharpest vision.
Just medial to the fovea centralis is an area called the
optic
disc
(f g. 12.34)
. Here the nerve F bers from the retina leave
the eye and become parts of the optic nerve. A central artery
and vein also pass through at the optic disc. These vessels are
continuous with capillary networks of the retina, and together
with vessels in the underlying choroid coat, they supply blood
to the cells of the inner tunic. The optic disc lacks receptor
cells, so it is commonly referred to as the
blind spot
of the eye.
The space enclosed by the lens, ciliary body, and ret-
ina is the largest compartment of the eye and is called the
posterior cavity.
It is F
lled with a transparent, jellylike fl
uid
called
vitreous humor,
which with some collagenous F bers
comprise the
vitreous body.
The vitreous body supports the
internal structures of the eye and helps maintain its shape.
In summary, light waves entering the eye must pass
through the cornea, aqueous humor, lens, vitreous humor,
and several layers of the retina before they reach the photo-
receptors (see F g. 12.32).
Table 12.6
summarizes the layers
of the eye.
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