DISEASES OF THE EYE. 275 
and it is to the condition of these and their condensation and com- 
pression 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 abun- 
dance 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 disease, 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 g greatest on its posterior surface, which is lodged in a de- 
pression in the vitreous humor, while its anterior surface corresponds 
to the opening of the pupil. It is inclosed in a membranous covering 
