Cornea: Anatomy, Transparency

The cornea is a transparent, avascular, watch-glass
like structure. It forms anterior one-sixth of the outer
fibrous coat of the eyeball.
Dimensions
The anterior surface of cornea is elliptical with
an average horizontal diameter of 11.7 mm and
vertical diameter of 11 mm.
The posterior surface of cornea is circular with
an average diameter of 11.5 mm.
Thickness of cornea in the centre is about 0.52
mm while at the periphery it is 0.7 mm.
Radius of curvature. The central 5 mm area of the
cornea forms the powerful refracting surface of
the eye. The anterior and posterior radii of
curvature of this central part of cornea are 7.8 mm
and 6.5 mm, respectively.
Refractive power of the cornea is about 45
dioptres, which is roughly three-fourth of the
total refractive power of the eye (60 dioptres).
Histology
Histologically, the cornea consists of five distinct
layers. From anterior to posterior these are: epithelium,
Bowman’s membrane, substantia propria (corneal
stroma), Descemet’s membrane and endothelium
(Fig. 5.1).
1. Epithelium. It is of stratified squamous type and
becomes continuous with the epithelium of bulbar
conjunctiva at the limbus. It consists of 5-6 layers of
cells. The deepest (basal) layer is made up of columnar
cells, next 2-3 layers of wing or umbrella cells and the
most superficial two layers are of flattened cells.
2. Bowman's membrane. This layer consists of
acellular mass of condensed collagen fibrils. It is
about 12μm in thickness and binds the corneal stroma
anteriorly with basement membrane of the epithelium.
It is not a true elastic membrane but simply a
condensed superficial part of the stroma. It shows

considerable resistance to infection. But once
destroyed, it does not regenerate.
3. Stroma (substantia propria). This layer is about
0.5 mm in thickness and constitutes most of the
cornea (90% of total thickness). It consists of collagen
fibrils (lamellae) embedded in hydrated matrix of
proteoglycans. The lamellae are arranged in many
layers. In each layer they are not only parallel to each
other but also to the corneal plane and become
continuous with scleral lamellae at the limbus. The
alternating layers of lamellae are at right angle to each
other. Among the lamellae are present keratocytes,
wandering macrophages, histiocytes and a few
leucocytes.
4. Descemet's membrane (posterior elastic lamina).
The Descemet's membrane is a strong homogenous
layer which bounds the stroma posteriorly. It is very
resistant to chemical agents, trauma and pathological
processes. Therefore, 'Descemetocele' can maintain
the integrity of eyeball for long. Descemet's membrane
consists of collagen and glycoproteins. Unlike
Bowman's membrane it can regenerate. Normally it
remains in a state of tension and when torn it curls
inwards on itself. In the periphery it appears to end at
the anterior limit of trabecular meshwork as
Schwalbe's line (ring).
5. Endothelium. It consists of a single layer of flat
polygonal (mainly hexagonal) cells which on slit lamp
biomicroscopy appear as a mosaic. The cell density
of endothelium is around 3000 cells/mm2 in young
adults, which decreases with the advancing age.
There is a considerable functional reserve for the
endothelium. Therefore, corneal decompensation
occurs only after more than 75 percent of the cells are
lost. The endothelial cells contain 'active-pump'
mechanism.
Blood supply
Cornea is an avascular structure. Small loops derived
from the anterior ciliary vessels invade its periphery
for about 1 mm. Actually these loops are not in the
cornea but in the subconjunctival tissue which
overlaps the cornea.
Nerve supply
Cornea is supplied by anterior ciliary nerves which
are branches of ophthalmic division of the 5th cranial
nerve. After going about 2 mm in cornea the nerves
lose their myelin sheath and divide dichotomously
and form three plexuses — the stromal, subepithelial
and intraepithelial.
APPLIED PHYSIOLOGY
The two primary physiological functions of the
cornea are (i) to act as a major refracting medium; and
(ii) to protect the intraocular contents. Cornea fulfills
these duties by maintaining its transparency and
replacement of its tissues.
Corneal transparency
The transparency is the result of :
Peculiar arrangement of corneal lamellae (lattice
theory of Maurice),
Avascularity, and
Relative state of dehydration, which is maintained
by barrier effects of epithelium and endothelium
and the active bicarbonate pump of the
endothelium.
For these processes, cornea needs some energy.