DISEASES OP 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 thie 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 

 soUd. 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 lopked 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 mista;ken 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 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 



