ease, but so long as they and the iris
retain their clear, dark, aspect, without any tints of brown or yellow,
they may be held to be healthy.

The vitreous or semisolid refracting medium occupies the posterior part
of the eye--the part corresponding to the sclerotic, choroid, and
retina--and has a consistency corresponding to that of the white of an
egg, and a power of refraction of the light rays correspondingly greater
than the aqueous humor.

The third or solid refracting medium is a biconvex lens, with its
convexity greatest on its posterior surface, which is lodged in a
depression in the vitreous humor, while its anterior surface corresponds
to the opening of the pupil. It is inclosed in a membranous covering
(capsule) and is maintained in position by a membrane (suspensory
ligament) which extends from the margin of the lens outward to the
sclerotic at the point of junction of the choroid and iris. This
ligament is, in its turn, furnished with radiating, muscular fibers,
which change the form or position of the lens so as to adapt it to see
with equal clearness objects at a distance or close by.

Another point which strikes the observer of the horse's eye is that in
the darkness a bright, bluish tinge is reflected from the widely dilated
pupil. This is owing to a comparative absence of pigment in the choroid
coat inside the upper part of the eyeball, and enables the animal to see
and advance with security in darkness where the human eye would be of
little use. The lower part of the cavity of the horse's eye, into which
the dazzling rays fall from the sky, is furnished with an intensely
black lining, by which the rays penetrating the inner nervous layer are
instantly absorbed.

MUSCLES OF THE EYE.

These consist of four straight muscles, two oblique, and one retractor.
The straight muscles pass from the depth of the orbit forward on the
inner, outer, upper, and lower sides of the eyeball, and are fixed to
the anterior portion of the fibrous (sclerotic) coat, so that in
contracting singly they respectively turn the eye inward, outward,
upward, and downward. When all act together they draw the eyeball deeply
into its socket. The retractor muscle also consists of four muscular
slips, repeating the straight muscles on a smaller scale, but as they
are only attached on the back part of the eyeball they are less adapted
to roll the eye than to draw it down into its socket. The two oblique
muscles rotate the eye on