The cornea with the sclera forms the outer coat or tunic of the eye
ball. The cornea is in front and forms one-sixth of the envelope.
It is a segment of a smaller globe than that of the sclera. It is
about 12 mm. horizontally and 11 mm. in the vertical diameter. Its
thickest part is at its junction with the sclera where it is about
1 mm. This junction is called the limbus. The cornea is inserted
into and rests on the sclera like a watch crystal. The fibers of the
cornea pass continuously into the sclera, however. The normal cornea
is transparent. Most morbid changes of the cornea cause a diminution
in this transparency. In old age a narrow gray line near the corneal
margin makes its appearance. This is known as the =arcus senilis=.
There is a little strip of perfectly clear cornea between the arcus
senilis and the limbus.
The cornea has five layers. These layers should be noted with care, as
in wounds of the eye, foreign bodies in the cornea and ulcerations, the
results depend much upon which layers are affected.
1. The =anterior epithelium= consists of pavement cells of several
layers. This layer of the cornea may be damaged or scratched off in
large patches and still it will heal readily leaving no trace of the
injury.
2. The =anterior elastic lamina= or =Bowman’s membrane= is very thin
and homogeneous; it is just beneath the epithelial layer and forms a
resisting sheath to prevent damage to the next layer.
3. The =stroma= or =substantia propria=. This layer composes about
nine-tenths of the cornea. It is composed of minute connective tissue
fibers between which lie some stroma cells or corneal corpuscles. Some
of these cells are fixed while others are motile. The motile ones are
the white blood-corpuscles which move about in the lymph passages of
the stroma. They increase in any irritation of the cornea.
4. =Descemet’s membrane.= This is a tough homogeneous hyaloid membrane
back of the stroma. When the stroma is diseased and breaks down
Descemet’s membrane may be sufficient to prevent a puncture of the
cornea.
5. The =Endothelial layer= is a single layer of flattened cells which
coat the posterior surface of Descemet’s membrane.
The margin of the cornea is in relation with three membranes, the
conjunctiva, the sclera and the uvea (iris and ciliary body). In a
disease of the cornea, a conjunctivitis, an iritis or a cyclitis is
easily started.
The cornea contains no vessels. It is nourished by imbibition. At
the limbus there is a rich network of marginal loops supplied by the
anterior ciliary vessels. From these loops the blood plasma passes into
the stroma of the cornea.
The nerves of the cornea come from the ciliary nerves and the nerves
of the bulbar conjunctiva. These are from the trigeminus and the
sympathetic. The nerves extend numerously in the stroma passing forward
through Bowman’s membrane into the epithelial layer. This makes the
cornea very sensitive to the touch.