Light-sensitive receptors are called photoreceptors.
range from simple light-sensitive cells
scattered randomly on the body surface
of many invertebrates (dermal
light sense) to the exquisitely developed
camera-type eye of vertebrates.
Eyespots of astonishingly advanced
organization appear even in some unicellular
forms. That of the dinoflagellate Nematodinium
bears a lens, a
light-gathering chamber, and a photoreceptive
pigment cup—all developed
within a single-celled organism
(Figure 35-30). The dermal light receptors
of many invertebrates are of much
simpler design. They are far less sensitive
than optic receptors, but they
are important in locomotory orientation,
pigment distribution in chromatophores,
photoperiodic adjustment of
reproductive cycles, and other behavioral
|Figure 35-30 Eyespot of the dinoflagellate Nematodinium.
More highly organized eyes, many
capable of excellent image formation,
are based on one of two different principles:
either a single-lens, camera-type eye such as those of cephalopod molluscs
and vertebrates; or a multifaceted
(compound) eye as in arthropods.
Arthropod compound eyes
of many independent visual
units called ommatidia (Figure 35-31).
The eye of a bee contains about 15,000
of these units, each of which views a
separate narrow sector of the visual
field. Such eyes form a mosaic of
images of varying brightness from the
separate units. Resolution (the ability
to see objects sharply) is poor compared
with that of a vertebrate eye. A
fruit fly, for example, must be closer
than 3 cm to see another fruit fly as
anything but a single spot. However, a
compound eye is especially well suited
to detecting motion, as anyone who
has tried to swat a fly knows.
|Figure 35-31 Compound eye of an insect. A single ommatidium is shown enlarged at right
Eyes of certain annelids, molluscs,
and all vertebrates are built like a
camera—or rather we should say that a
camera is modeled somewhat after vertebrate
eyes. A camera-type eye contains
in the front a light-tight chamber
and lens system, which focuses an
image of the visual field on a lightsensitive
surface (the retina) in the
back (Figure 35-32).
The spherical eyeball is built of
three layers: (1) a tough outer white sclera
for support and protection,
(2) middle choroid coat,
blood vessels for nourishment, and (3)
light-sensitive retina (Figure 35-32).
The cornea is a transparent anterior
modification of the sclera. A circular,
pigmented curtain, the iris
the size of the light opening, the pupil
. Just behind the iris is the lens,
a transparent, elastic oval disc that,
with the aid of ciliary muscles
alter the curvature of the lens and
bend light rays to focus an image on
the retina. In terrestrial vertebrates the
cornea actually does most of the bending
of light rays, whereas the lens
adjusts focus for near and far objects.
Between cornea and lens is an outer
filled with watery aqueous humor
; between lens and retina is a
much larger inner chamber
with viscous vitreous humor.
The retina is composed of several
cell layers (Figure 35-33). The outermost
layer, closest to the sclera, consists
of pigment cells. Adjacent to this
layer are the photoreceptors, rods
Approximately 125 million rods
and 1 million cones are present in each
human eye. Cones are primarily concerned
with color vision in ample light;
rods, with colorless vision in dim light.
Next is a network of intermediate
(bipolar, horizontal, and
amacrine cells) that process and relay
visual information from the photoreceptors
to the ganglion cells whose
axons form the optic nerve. The network
permits much convergence,
especially for rods. Information from
several hundred rods may converge on
a single ganglion cell, an adaptation
that greatly increases the effectiveness
of rods in dim light. Cones show very
little convergence. By coordinating
activities between different ganglion
cells, and adjusting the sensitivities of
bipolar cells, horizontal and amacrine
cells improve overall contrast and
quality of the visual image.
|Figure 35-33 Structure of a primate retina, showing organization of intermediate neurons that connect
photoreceptor cells to ganglion cells of the optic nerve.
The fovea centralis
or fovea, the
region of keenest vision, is located in the
center of the retina (Figure 35-32),
in direct line with the center of the
lens and cornea. It contains only cones, a
vertebrate specialization for diurnal
(daytime) vision. The acuity of an animal’s
eyes depends on the density of
cones in the fovea. The human fovea
and that of a lion contain approximately
150,000 cones per square millimeter.
But many water and field birds have
up to 1 million cones per square millimeter.
Their eyes are as good as our
eyes would be if aided by eight-power
One of several marvels of the vertebrate
eye is its capacity to compress the enormous
range of light intensities presented
to it into a narrow range that can be handled
by optic nerve fibers. Light intensity
between a sunny noon and starlit night
differs by more than 10 billion to 1. Rods
quickly saturate with high light intensity,
but cones do not; they shift their operating
range with changing ambient light
intensity so that a high-contrast image is
perceived over a broad range of light conditions. This shift is made possible by
complex interactions among the network
of nerve cells that lie between the cones
and the ganglion cells that generate the
retinal output to the brain.
|Figure 35-32 Structure of the human eye.
At the peripheral parts of the
retina only rods are found. Rods are
high-sensitivity receptors for dim
light. At night, the cone-filled fovea is
unresponsive to low levels of light
and we become functionally color
blind (“at night all cats are gray”).
Under nocturnal conditions, the position
of greatest visual acuity is not at
the center of the fovea but at its
edge. Thus it is easier to see a dim
star at night by looking slightly to
one side of it.