CHAPTER 20
Eye
391
melanin (
retinal pigment epithelium
). In general, the
retina
can be
divided into an
optic
(
neural
)
retina
and a
non-optic
(
nonneural
)
retina
. The
neural retina
, often referred to as simply “retina,”
contains neural elements and has the visual functions described
below. The
nonneural
retina
has no neural elements and no visual
function. It is the anterior continuation of the pigmented layer,
which covers the surface of the ciliary body and the posterior
surface of the iris.
Unlike the rest of the eye, the retina develops as a part of
the central nervous system (CNS). It contains F ve types of neural
elements:
photoreceptor cells
(
rods
and
cones
),
bipolar cells
,
horizontal cells
,
amacrine cells
, and
ganglion cells
(±ig. 20-15).
The retina can be divided into 10 layers, some of which contain
the nuclei of cells and others of which contain cell processes and
synapses (±igs. 20-13 and 20-15).
The retina is not homogeneous throughout its extent. Over
most of the retina, the focused light of the visual image must
pass through all of the neural layers, as well as small blood
vessels, before reaching the
photoreceptors
. This degrades the
image to a certain extent. However, in the
fovea centralis
, a small
region near the posterior pole of the eye, the superF cial layers are
displaced to the side, and light strikes the photoreceptors directly
(±igs. 20-1 and 20-13A). The visual image in this region is per-
ceived with the greatest detail. The area immediately surrounding
the fovea centralis is the
macula lutea
(±ig. 20-13B). This is the
thickest region of the retina and contains a high concentration of
both
cones
(for
color vision
) and
rods
(for
low-light vision
[±ig.
20-14]). More peripherally, the retina becomes thinner. There are
fewer ganglion cells, fewer cones, and a relatively higher propor-
tion of rods (±ig. 20-13C). These changes cause visual acuity to
be reduced but sensitivity to low light levels to increase.
Optic Nerve
The
ganglion cells
are the output cells of the retina (±ig. 20-15C).
The axons of ganglion cells travel in the nerve F
ber layer to the
optic disk
, where the nerve F
bers exit the eye and form the
optic
nerve
(±ig. 20-17A). There are no photoreceptors in the optic
disk; this absence produces a small
blind
spot
in the visual F eld.
The appearance of the optic disk when viewed through an oph-
thalmoscope is an important diagnostic aid.
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