Chapter 15
The Special Senses
551
15
are, but they do not provide sharp images or color vision.
Tis is why colors disappear and the edges of objects appear
fuzzy in dim light and at the edges of our visual field.
Cones
, by contrast, are our vision receptors for bright light
and provide high-resolution color vision.
Lateral to the blind spot of each eye, and located precisely at
the eye’s posterior pole, is an oval region called the
macula lutea
(mak
9
u-lah lu
9
te-ah; “yellow spot”) with a minute (0.4 mm) pit
in its center called the
fovea centralis
(see Figure 15.4). In this
region, the retinal structures abutting the vitreous humor are
displaced to the sides. Tis allows light to pass almost directly
to the photoreceptors rather than through several retinal layers,
greatly enhancing visual acuity (the ability to resolve detail). Te
fovea contains only cones, the macula contains mostly cones,
and from the edge of the macula toward the retina periphery,
cone density declines gradually. Te retina periphery contains
mostly rods, which continuously decrease in density from there
to the macula.
Only the foveae (plural of fovea) have a sufficient cone den-
sity to provide detailed color vision, so anything we wish to view
critically is focused on the foveae. Because each fovea is only
about the size of the head of a pin, only a thousandth of the
entire visual field is in
hard focus
(foveal focus) at a given mo-
ment. Consequently, for us to visually comprehend a scene that
is rapidly changing (as when we drive in traffic), our eyes must
flick rapidly back and forth to provide the foveae with images of
different parts of the visual field.
Te neural layer of the retina receives its blood supply from two
sources. Vessels in the choroid supply the outer third (contain-
ing photoreceptors). Te inner two-thirds is served by the
central
artery
and
central vein of the retina
, which enter and leave the
eye through the center of the optic nerve (see Figure 15.4a). Ra-
diating outward from the optic disc, these vessels give rise to a
rich vascular network. Tis is the only place where small blood
vessels are visible in a living person
(Figure 15.7)
. Physicians may
observe these tiny vessels with an ophthalmoscope for signs of
hypertension, diabetes, and other vascular diseases.
is appealing, in response to fear, and during problem solving.
(Computing your taxes should make your pupils get bigger and
bigger.) On the other hand, boredom or repulsive subject matter
causes pupils to constrict.
Although irises come in different colors (
iris
5
rainbow),
they contain only brown pigment. When they have a lot of pig-
ment, the eyes appear brown or black. If the amount of pigment
is small and restricted to the posterior surface of the iris, the
unpigmented parts simply scatter the shorter wavelengths of
light and the eyes appear blue, green, or gray. Most newborn
babies’ eyes are slate gray or blue because their iris pigment is
not yet developed.
Inner Layer (Retina)
Te innermost layer of the eyeball is the delicate
retina
(ret
9
ĭ-
nah), which originates as an outpocketing of the brain. It contains
millions of photoreceptors that transduce (convert) light energy,
other neurons involved in processing light responses, and glia.
Te retina consists of two layers: an outer
pigmented layer
and an
inner
neural layer
(Figure 15.6)
. Although the pigmented and
neural layers are very close together, they are not fused. Only the
neural layer of the retina plays a direct role in vision.
Pigmented Layer of the Retina
Te outer
pigmented layer
,
a single-cell-thick lining, abuts the choroid, and extends anteri-
orly to cover the ciliary body and the posterior face of the iris.
Tese pigment cells, like those of the choroid, absorb light and
prevent it from scattering in the eye. Tey also act as phago-
cytes participating in photoreceptor cell renewal (described on
p. 559), and store vitamin A needed by the photoreceptor cells.
Neural Layer of the Retina
Te transparent inner
neural
layer
extends anteriorly to the posterior margin of the ciliary
body. Tis junction is called the
ora serrata
, literally, the saw-
toothed margin (see Figure 15.4).
From posterior to anterior, the neural layer is composed of
three main types of neurons:
photoreceptors
,
bipolar cells
, and
ganglion cells
(Figure 15.6). Signals are produced in response
to light and spread from the photoreceptors (abutting the pig-
mented layer) to the bipolar cells and then to the innermost
ganglion cells, where action potentials are generated. Te gan-
glion cell axons make a right-angle turn at the inner face of the
retina, then leave the posterior aspect of the eye as the thick
optic nerve. Te retina also contains other types of neurons—
horizontal cells and amacrine cells—which play a role in visual
processing.
Te
optic disc
, where the optic nerve exits the eye, is a weak
spot in the
fundus
(posterior wall) of the eye because it is not
reinforced by the sclera. Te optic disc is also called the
blind
spot
because it lacks photoreceptors, so light focused on it can-
not be seen. Nonetheless, we do not usually notice these gaps in
our vision because the brain uses a sophisticated process called
filling in
to deal with absence of input.
Te quarter-billion photoreceptors found in the neural layer
are of two types: rods and cones.
Rods
are our dim-light and peripheral vision receptors. Tey
are more numerous and far more sensitive to light than cones
Macula
lutea
Central
artery
and vein
emerging
from the
optic disc
Optic disc
Retina
Figure 15.7
Part of the posterior wall (fundus) of the right
eye as seen with an ophthalmoscope.
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