384 Neuroanatomy: Draw It to Know It
The Eye (Cont.)
In addition to being an anchor for the zonule, the ciliary
bodies also produce aqueous humor, which is a low-
protein, aqueous (ie, watery) fl uid. Show that the ciliary
epithelia actively secrete aqueous humor into the poste-
rior chamber (the space between the lens and iris) and
show that the aqueous humor then fl ows through the
pupil into the anterior chamber (the space between the
iris and cornea), and fi nally show that the aqueous humor
is then reabsorbed at the iridocorneal fi ltration angle
through the trabecular meshwork into the canal of
Schlemm. Th e trabecular meshwork and canal of Schlemm
(along with the scleral spur) lie within the internal scleral
sulcus, which sits at the inner surface of the sclera–corneal
junction, called the limbus. Th e trabecular meshwork
gates the reabsorption of aqueous humor through the
canal of Schlemm, which drains directly into the venous
system. Note that an alternative reabsorption pathway
also exists in which aqueous humor is directly reabsorbed
through the ciliary body into the uveal vessels.
Patholog y in the aqueous system produces increased
intraocular pressure, called glaucoma. Although there
are many causes of glaucoma, glaucoma is commonly
divided into two forms: open-angle (i.e., wide-angle)
and angle-closure (i.e., narrow-angle) glaucoma. In open-
angle glaucoma, the more common form, there is failure
of adequate aqueous reabsorption through the canal of
Schlemm, most commonly due to aberrancy in the tra-
becular meshwork, itself. In angle-closure glaucoma, the
rarer form, there is apposition of key anterior structures,
which trap the fl ow of aqueous humor. For instance,
there is abutment of the iris and cornea or lens and iris.
Common pharmacologic therapies for glaucoma either
reduce aqueous humor production or promote aqueous
humor reabsorption.
Next, draw the vitreous chamber, which contains vit-
reous humor. Like aqueous humor, vitreous humor is
primarily water, but the presence of glycosaminoglycans
and collagen within this substance gives it its gel-like
composition, which helps maintain the eye’s shape.^1 – 3
Now, draw the retina internal to the choroid. We will
parse the retina into its specifi c layers later, but fi rst let’s
draw the optic nerve. Indicate that the retina transitions
into optic nerve where it exits the eye, posteriorly, at the
lamina cribrosa, which is an opening in the sclera. Next,
indicate that in the center of the optic nerve lie the cen-
tral retinal artery and vein. Now, label the optic nerve
head and draw a coronal view of it. Indicate that in the
center of the optic nerve head sits the optic cup, a white-
appearing hole through which the central retinal vessels
emanate. Next, label the optic disc, the pink-colored
ring of nerve tissue that surrounds the optic cup. Th en,
back in our main diagram show that the subarachnoid
space and dura mater, which is an extension of the
sclera, extend along the optic nerve. Th e presence of the
subarachnoid space, here, allows increased intracranial
pressure to translate along the optic nerve and impair
its axoplasmic transport. Impaired axoplasmic transport
at the lamina cribrosa results in optic disc swelling ,
which is called disc edema or, rather, papilledema when
it occurs in the setting of increased intracranial pressure.
Papilledema causes disc margin blurring (i.e., blurring of
the nerve layer around the disc), venous congestion, and
optic disc hyperemia from capillary dilatation secondary
to reduced venous return.