toward the inner angle of the eye where they drain
down ducts into the nose.
MIDDLE LAYER. The middle layer of the
eye is called the choroid. It is a highly vascular,
pigmented tissue that provides nourishment to the
inner structures. Continuous with the choroid is
the ciliary body, whose muscular structure at-
taches to the lens by means of suspensory
ligaments and produces changes in the thickness
of the lens. This permits the eye to focus to long-
range or close-up vision.
The iris is continuous with the ciliary body.
It is a circular, pigmented muscular structure that
gives color to the eye. The opening in the iris is
called the pupil (fig 3-46). The amount of light
entering the pupil is regulated through the con-
striction of radial/circular muscles in the iris.
When strong light is flashed into the eye, the cir-
cular muscle fibers of the iris contract, reducing
the size of the pupil. If the light is dim, the pupil
dilates to allow as much of the light in as possi-
ble. The size and reaction of the pupils of the eyes
are an important diagnostic tool.
The lens is a transparent, biconvex structure
suspended directly behind the iris. It separates the
interior eye into anterior and posterior cavities.
The anterior cavity contains a watery solution
called aqueous humor, which helps to give the cor-
nea its curved shape. The optic globe posterior
to the lens is filled with a jellylike substance called
vitreous humor, which helps to maintain the shape
of the eyeball and prevents misshaping by main-
taining intraocular pressure.
INNER LAYER. The inner layer of the eye
is called the retina (fig. 3-47). It contains different
layers of nerve cells, rods, and cones that are the
receptors of the sense of vision. The retina is con-
tinuous with the optic nerve, which enters the back
of the globe and carries visual impulses received
by the rods and cones to the brain. The area where
the optic nerve enters the eyeball contains no rods
and cones and is called the blind spot.
The rods respond to low intensities of light and
are responsible for night vision. They are located
in all areas of the retina, except in the small
depression called the fovea centralis, where light
entering the eye is focused, and which has the
The cones require higher light intensities for
stimulation and are most densely concentrated in
the fovea centralis. The cones are responsible for
Figure 3-46.Eye, anterior view.
Deflection or bending of light rays results
when light passes through substances of varying
densities in the eye (cornea, aqueous humor,
crystalline lens, and vitreous humor) (fig. 3-48).
The deflection is referred to as refraction. Accom-
modation is the process performed by the lens that
increases or decreases its curvature to refract light
rays into focus on the fovea.
The constriction of the pupil by the iris
regulates the amount of light entering the eye. This
process protects the retina from excessive stimula-
tion and prevents a scattering of light rays that
would produce blurred vision.
A movement of the globes toward the midline,
which causes a viewed object to come into focus
on corresponding points of the two retinas, is
called convergence. This gives clear, three-
The end receptors or nerve endings in the rods
and cones that have been stimulated by light con-
duct impulses to the occipital lobes of the
cerebrum, where they are interpreted into vision