Regulation and Integration of the Body
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
(between the cornea and the iris) and the
(between the iris and the lens)
anterior segment is ﬁlled with
, 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.
Te capillaries of the ciliary processes form the aqueous
humor by ﬁltering plasma into the posterior chamber.
A portion of the ﬂuid freely diﬀuses through the vitreous
humor in the posterior segment while the remainder ﬂows
into the anterior chamber.
AFer ﬂowing through the pupil into the anterior chamber,
aqueous humor drains into the venous blood via the
, 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.
Te pattern of vascularization of the retina makes it susceptible
. 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).
is ﬁlled with a clear gel called
glassy) that binds tremendous amounts of water.
Supports the posterior surface of the lens and holds the neu-
ral layer of the retina ﬁrmly against the pigmented layer
Aqueous humor forms
by ﬁltration from the
capillaries in the ciliary
Aqueous humor ﬂows
from the posterior chamber
through the pupil into the
anterior chamber. Some also
ﬂows through the vitreous
humor (not shown).
Aqueous humor is
reabsorbed into the venous
blood by the scleral venous
Circulation of aqueous humor.
The arrows indicate the circulation pathway.