552
UNIT 3
Regulation and Integration of the Body
15
Contributes to intraocular pressure, helping to counteract
the pulling force of the extrinsic eye muscles
Vitreous humor forms in the embryo and lasts for a lifetime.
Te iris divides the
anterior segment
into the
anterior
chamber
(between the cornea and the iris) and the
posterior
chamber
(between the iris and the lens)
(Figure 15.8)
. Te
entire
anterior segment is filled with
aqueous humor
, a clear
fluid similar in composition to blood plasma. Unlike the vitre-
ous humor, aqueous humor forms and drains continually and is
in constant motion.
1
Te capillaries of the ciliary processes form the aqueous
humor by filtering plasma into the posterior chamber.
2
A portion of the fluid freely diffuses through the vitreous
humor in the posterior segment while the remainder flows
into the anterior chamber.
3
AFer flowing through the pupil into the anterior chamber,
aqueous humor drains into the venous blood via the
scleral
venous sinus
, an unusual venous channel that encircles the
eye in the angle at the corneoscleral junction.
Normally, aqueous humor forms and drains at the same rate,
maintaining a constant intraocular pressure of about 16 mm Hg,
which helps to support the eyeball internally. Aqueous humor sup-
plies nutrients and oxygen to the lens and cornea and to some cells
of the retina, and it carries away their metabolic wastes.
Homeostatic Imbalance
15.5
Te pattern of vascularization of the retina makes it susceptible
to
retinal detachment
. Tis condition, in which the pigmented
and neural layers separate (detach) and allow the jellylike vitre-
ous humor to seep between them, can cause permanent blind-
ness because it deprives the photoreceptors of nutrients.
Retinal detachment usually happens when the retina is torn
during a traumatic blow to the head or when the head stops
moving suddenly and then jerks in the opposite direction (as
in bungee jumping). Te symptom that victims most oFen de-
scribe is “a curtain being drawn across the eye,” but some people
see sootlike spots or light flashes. If diagnosed early, it is oFen
possible to reattach the retina with a laser before photoreceptors
are permanently damaged.
Internal Chambers and Fluids
As we noted earlier, the lens and its halolike ciliary zonule divide
the eye into two segments, the anterior segment in front of the
lens and the larger posterior segment behind it (see ±igure 15.4a).
Te
posterior segment
is filled with a clear gel called
vitreous
humor
(
vitre
5
glassy) that binds tremendous amounts of water.
Vitreous humor
²ransmits light
Supports the posterior surface of the lens and holds the neu-
ral layer of the retina firmly against the pigmented layer
Sclera
Bulbar
conjunctiva
Scleral venous
sinus
Posterior chamber
Anterior chamber
Anterior
segment
(contains
aqueous
humor)
Corneoscleral
junction
Cornea
Cornea
Corneal epithelium
Corneal endothelium
Aqueous humor
Iris
Lens
Lens epithelium
Lens
Posterior
segment
(contains
vitreous
humor)
Ciliary zonule
(suspensory
ligament)
Ciliary
processes
Ciliary
muscle
Ciliary
body
1
Aqueous humor forms
by filtration from the
capillaries in the ciliary
processes.
1
2
3
2
Aqueous humor flows
from the posterior chamber
through the pupil into the
anterior chamber. Some also
flows through the vitreous
humor (not shown).
3
Aqueous humor is
reabsorbed into the venous
blood by the scleral venous
sinus.
Figure 15.8
Circulation of aqueous humor.
The arrows indicate the circulation pathway.
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