The =vitreous body= (Corpus vitreum) is a semifluid transparent
substance situated between the crystalline lens and the retina. In front
it presents a deep cavity, the =fossa hyaloidea=, which receives the
posterior surface of the lens. It consists of a framework of delicate
fibrils, the =stroma vitreum=, the meshes of which are filled by the
fluid =humor vitreus=. The surface is covered by a condensation of the
stroma known as the membrana hyaloidea.
The =crystalline lens= (Lens crystallina) is a biconvex, transparent
body, situated in front of the vitreous body and in partial contact with
the posterior surface of the iris. Its periphery, the =æquator lentis=,
is almost circular and is closely surrounded by the ciliary processes.
The =anterior surface= (Facies anterior) is convex; it is bathed by the
aqueous humor and is in contact with the iris to an extent which varies
with the state of the pupil. The =posterior surface= (Facies posterior)
is much more strongly curved than the anterior. It rests in the fossa of
the vitreous body. The central points of the surfaces are the =anterior=
and =posterior poles= (Polus anterior et posterior lentis), and the line
which connects them is the axis of the lens (Axis lentis).
The transverse diameter of the lens is about 2 cm., the vertical
diameter is slightly smaller, and the axis measures about 13 mm. The
radius of curvature of the anterior surface is 13.5 mm., and of the
posterior surface 9.5 to 10 mm. But the curvatures of its
surfaces—especially that of the anterior—vary during life according as
the eye is accommodated for near or far vision.
The =zonula ciliaris= (of Zinn) or suspensory ligament of the lens (Fig.
552) consists of delicate fibers (Fibræ zonulares) which pass in a
meridional direction from the ciliary processes to the capsule of the
equator of the lens. Many fibers cross each other, and the spaces
between the fibers (Spatia zonularia) are filled with aqueous humor;
they communicate with each other and with the posterior chamber.
The =substance= of the lens (Substantia lentis) is inclosed by a
structureless, highly elastic membrane, the =capsule= of the lens
(Capsula lentis), and consists of a softer =substantia corticalis=, and
a dense central part, the =nucleus lentis=. The capsule is thickest on
the anterior surface, and here it is lined by a layer of flat polygonal
cells, the epithelium of the lens capsule. The lens substance, when
hardened, is seen to consist of concentric laminæ arranged somewhat like
the layers of an onion, and united by an amorphous cement substance. The
laminæ consist of lens fibers, hexagonal in section, and of very
different lengths. Faint lines radiate from the poles and indicate the
edges of layers of cement substance which unite the groups of lens
fibers. These lines, the =radii lentis=, are three in number in the
fœtus and new-born, and form with each other angles of 120 degrees. On
the anterior surface one is directed upward from the pole and the other
two diverge downward; on the posterior surface one is directed downward
and the others diverge upward. The developed lens has neither vessels
nor nerves.
In the fœtus the lens is nearly globular, and is soft and pinkish in
color. During part of fœtal life it is surrounded by a vascular
network, the tunica vasculosa lentis. This is derived chiefly from a
temporary vessel, the hyaloid artery, which is a continuation forward
of the arteria centralis retinæ through the hyaloid canal that
traverses the vitreous body. In old age the lens tends to lose its
elasticity and transparency; it also becomes flatter and the nucleus
especially grows denser.