evaporate flow 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. This layer is a highly vascular,
pigmented tissue that provides nourishment to the
inner structures. Continuous with the choroid is the
ciliary body. The ciliary body is formed by a
thickening of the choroid and fits like a collar into the
area between the retina and iris. Attached to the ciliary
the body are the suspensory ligaments, which blend
with the elastic capsule of the lens and holds it in place.
Iris.The iris is continuous with the ciliary body.
The iris is a circular, pigmented muscular structure that
gives color to the eye. The iris separates the anterior
cavity into anterior and posterior chambers. The
opening in the iris is called the pupil (fig. 1-49). The
amount of light entering the pupil is regulated through
the constriction of radial and circular muscles in the
iris. When strong light is flashed into the eye, the
circular 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 possible. The size and
reaction of the pupils of the eyes are an important
Lens.The lens is a transparent, biconvex
(having two convex surfaces) structure suspended
directly behind the iris. 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 by maintaining intraocular pressure. The lens
separates the eye into anterior and posterior cavities.
INNER LAYER.The inner layer of the eye is
called the retina (fig. 1-48). It contains layers of the
nerve cells, rods, and cones that are the receptors of the
sense of vision. The retina is continuous 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 optic disc
(blind spot) (fig. 1-50).
Rods.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 clearest vision.
Cones.Cones require higher light intensities for
stimulation and are most densely concentrated in the
fovea centralis. The cones are responsible for daytime
The vision process begins with rays of light from
an object passing through the cornea. The image is then
received by the lens, by way of the iris. Leaving the
lens, the image falls on the rods and cones in the retina.
The image then is carried to the brain for interpretation
by the optic nerve (fig. 1-51). Note the image received
by the retina is upside down, but the brain turns it
REFRACTION.Deflection or bending of light
rays results when light passes through substances of
varying densities in the eye (cornea, aqueous humor,
lens, and vitreous humor). The deflection of light in the
eye is referred to as refraction.
ACCOMMODATION.Accommodation is the
process by which the lens increases or decreases its
curvature to refract light rays into focus on the fovea
CONVERGENCE.The movement of the
globes toward the midline, causes a viewed object to
come into focus on corresponding points of the two
RADIALLY ARRANGED SMOOTH
MUSCLE FIBERS OF THE IRIS
CIRCULARLY ARRANGED SMOOTH
MUSCLE FIBERS OF THE IRIS
MOTOR NERVE FIBER
Figure 1-49.Anterior view of the eye.