e in health a perfect translucency, from which it has derived the
name of transparent cornea. This transparent coat is composed, in the
main, of fibers with lymph interspaces, and it is to the condition of
these and their condensation and compression that the translucency is
largely due. This may be shown by compressing with the fingers the eye
of an ox which has just been killed, when the clear transparent cornea
will suddenly become clouded over with a whitish-blue opacity, and this
will remain until the compression is interrupted. The interior of the
eye contains three transparent media for the refraction of the rays of
light on their way from the cornea to the visual nerve. Of these media
the anterior one (aqueous humor) is liquid, the posterior (vitreous
humor) is semisolid, and the intermediate one (crystalline lens) is
solid. The space occupied by the aqueous humor corresponds nearly to the
portion of the eye covered by the transparent cornea. It is, however,
divided into two chambers, anterior and posterior, by the iris, a
contractile curtain with a hole in the center (the pupil), and which may
be looked on as in some sense a projection inward of the vascular and
pigmentary coat from its anterior margin at the point where the
sclerotic or opaque outer coat becomes continuous with the cornea or
transparent one. This iris, or curtain, besides its abundance of blood
vessels and pigment, possesses two sets of muscular fibers, one set
radiating from the margin of the pupil to the outer border of the
curtain at its attachment to the sclerotic and choroid, and the other
encircling the pupil in the manner of a ring. The action of the two sets
is necessarily antagonistic, the radiating fibers dilating the pupil and
exposing the interior of the eye to view, while the circular fibers
contract this opening and shut out the rays of light. The form of the
pupil in the horse is ovoid, with its longest diameter from side to
side, and its upper border is fringed by several minute, black bodies
(corpora nigra) projecting forward and serving to some extent the
purpose of eyebrows in arresting and absorbing the excess of rays of
light which fall upon the eye from above. These pigmentary projections
in front of the upper border of the pupil are often mistaken for the
products of disease or injury in place of the normal and beneficent
protectors of the nerve of sight which they are. Like all other parts,
they may become the seat of dis