The eyeball consists of concentrically arranged coats and of refracting
media inclosed in them. The coats are three in number, namely, (1) an
external protective tunic made up of the sclerotic and cornea; (2) a middle
vascular and pigmentary tunic, the choroid; (3) an internal nervous layer,
the retina. The sclerotic is the white, opaque part of the outer tunic, of
which it forms about the posterior five-sixths, being coextensive with the
larger sphere already mentioned. The cornea forms the remaining one-sixth
of the outer tunic, being coextensive with the segment of the smaller
sphere. It is distinguished from the sclerotic by being colorless and
transparent. The choroid coat will be recognized as the black layer lying
subjacent to the sclerotic. It does not line the cornea, but terminates
behind the line of junction of that coat with the sclerotic by a thickened
edge--the ciliary processes. At the line of junction of the sclerotic and
cornea the iris passes across the interior of the eye. This (which may be
viewed as a dependency of the choroid) is a muscular curtain perforated by
an aperture termed the pupil. The retina will be recognized as a delicate,
glassy layer, lining the greater part of the choroid.

The refracting media of the eye are three in number, namely (1) the aqueous
humor, a watery fluid inclosed in a chamber behind the cornea; (2) the
crystalline lens and its capsule, a transparent, soft solid of a biconvex
form, and placed behind the iris; (3) the vitreous humor, a transparent
material with a consistence like thin jelly, and occupying as much of the
interior of the eye as is subjacent to the choroid.

The sclerotic is a strong, opaque, fibrous membrane, which, in a great
measure, maintains the form of the eyeball and protects the more delicate
structures within it. Its interior portion, which is covered by the ocular
conjunctiva, is commonly known as the "white of the eye." In form it is
bell-shaped, and the optic nerve pierces it behind like a handle, the
perforation being a little to its inner side. In front, the rim of the bell
becomes continuous with the cornea. The outer surface of the membrane
receives the insertion of the muscles of the eyeball. The coat is thickest
over the posterior part of the eyeball, and is thinnest a little behind its
junction with the cornea.

The cornea is the anterior transparent portion of the outer coat of the
eyeball. It may be viewed as a part of the sclerotic