Sensory Physiology
209
stimulating the receptors of the eye—the
visible spectrum
are between about 400 and 750 nm. Different wavelengths of
light within this band are perceived as different colors.
Overview of Eye Anatomy
The eye is a three-layered, fl uid-fi lled ball, divided into two
chambers (
Figure 7–22
). The
sclera
forms a white capsule
around the eye, except at its anterior surface where it is special-
ized into the clear
cornea.
The tough, fi brous sclera serves as
the insertion point for external muscles that move the eyeballs
within their sockets. The underlying
choroid
layer is darkly
pigmented to absorb light rays at the back of the eyeball, while
in the front the choroid layer is specialized into the
iris
(the
structure associated with eye color), the
ciliary muscle,
and
the
zonular fi
bers.
Circular and radial smooth muscle fi bers
of the iris determine the diameter of the
pupil,
the anterior
opening that allows light into the eye. Activity of the ciliary
muscle and the resulting tension on the zonular fi bers deter-
mines the shape of the crystalline
lens
just behind the iris. The
retina
is an extension of the brain lining the inner, posterior
surface of the eye, containing numerous types of neurons as
well as the eye’s sensory cells, called
photoreceptors.
Features
of the retina that can be viewed through the pupil with an
ophthalmoscope
include: (1) the
fovea centralis,
a region spe-
cialized to deliver the highest visual acuity; (2) the
optic disc,
where neurons carrying information from the photoreceptors
exit the eye as the
optic nerve;
and (3) numerous blood vessels
lying on the inner surface of the retina. The anterior chamber
of the eye, between the iris and the cornea, is fi lled with a clear
fl uid called
aqueous humor.
The posterior chamber of the
eye, between the lens and the retina, is fi lled with a viscous,
jellylike substance known as
vitreous humor.
The Optics of Vision
A ray of light can be represented by a line drawn in the direc-
tion in which the wave is traveling. Light waves diverge in all
directions from every point of a visible object. When a light
wave crosses from air into a denser medium like glass or water,
the wave changes direction at an angle that depends on the
density of the medium and the angle at which it strikes the
surface (
Figure 7–23a
). This bending of light waves, called
refraction,
is the mechanism allowing us to focus an accurate
image of an object onto the retina.
When light waves diverging from a point on an object
pass from air into the curved surfaces of the cornea and lens of
the eye, they are refracted inward, converging back into a point
on the retina (
Figure 7–23b
). The cornea plays a larger quan-
titative role than the lens in focusing light waves because the
waves are refracted more in passing from air into the cornea
than they are when passing into and out of the lens. Objects in
the center of the fi eld of view are focused onto the fovea cen-
tralis, with the image formed upside down and reversed right
to left relative to the original source.
Light waves from objects close to the eye strike the cor-
nea at greater angles and must be refracted more in order to
reconverge on the retina. Although, as previously noted, the
(a)
(b)
Muscle
Ciliary musc
(c)
Optic disc
Fovea centralis
Lens
Sclera
Cornea
Iris
Pupil
Aqueous hum
Vitreous humor
Retina
Blood vessels
Fovea
centralis
Optic nerve
Choroid
Zonular fibers
Blood vessels
Figure 7–22
The human eye. (a) Side-view cross section showing
internal structure, (b) anterior view, and (c) surface of the
retina viewed through the pupil with an ophthalmoscope.
The blood vessels depicted run along the back of the eye
between the retina and vitreous humor, not through the
vitreous humor.
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