The contour of the crystalline lens is dotted.
]
The =cornea= forms the anterior fifth of the fibrous tunic. It is
transparent, colorless, and non-vascular. Viewed from in front it is
oval in outline, the long axis being transverse and the broad end
internal; it appears more nearly circular when viewed from behind. Its
=anterior surface= (Facies anterior) is convex and is more strongly
curved than the sclera; its central part is termed the =vertex corneæ=.
The =posterior surface= (Facies posterior) is concave; it forms the
anterior boundary of the anterior chamber, and is in contact with the
aqueous humor. The margin (limbus corneæ) joins the sclera; the latter
overlaps the cornea more in front than behind, and more above and below
than at the sides, thus explaining the difference in outline of the two
surfaces. The cornea is thinnest at the vertex.
The cornea consists, from before backward, of the following layers: (1)
The =epithelium corneæ= is continuous with that of the conjunctiva
scleræ, and is of the stratified squamous type. (2) The =lamina limitans
anterior= is merely a condensation of the next layer. (3) The
=substantia propria= forms the bulk of the cornea and is composed of
interlacing bundles of connective tissue, arranged in part in lamellæ
disposed parallel with the surface. In the amorphous cement substance
between the lamellæ are flattened connective-tissue cells, the corneal
corpuscles. These have branching processes which unite with those of
other cells, thus forming a protoplasmic network.[218] (4) The =lamina
elastica posterior=, also termed the membrane of Descemet or Demours, is
a thin and practically homogeneous membrane which is less intimately
attached to the substantia propria than the anterior lamina. It is
clear, glistening, and elastic. At the periphery the lamina divides into
three sets of fibers. The anterior fibers join the sclera, the middle
give attachment to the ciliary muscle, while the posterior pass into the
iris and form the =ligamentum pectinatum iridis=. (5) The =endothelium=
(cameræ anterioris) consists of a layer of flattened polygonal cells,
and is reflected on to the anterior surface of the iris.
The cornea is without =blood-vessels= except at its periphery, where the
terminal twigs of the vessels of the sclera and conjunctiva terminate in
loops. The =nerves= are derived from the ciliary nerves. They form a
plexus around the periphery (Plexus annularis), from which fibers pass
into the substantia propria, become non-medullated, and form the
fundamental or stroma plexus. From this perforating branches pass
through the anterior limiting layer and form a subepithelial plexus,
from which filaments ascend between the epithelial cells. Other branches
from the plexuses in the substantia propria end as fibrils which are in
close relation with the corneal corpuscles.
[Illustration: