252 BUREAU OF ANIMAL INDUSTRY. 



fingers the e3'e of an ox which has just been killed, when the clear 

 transparent cornea w^ill suddenly become clouded over with a whitish 

 blue opacit}', and this will remain until the compression is interrupted. 

 The interior of the e3^e contains three transparent media for the refrac- 

 tion of the rays of light, on their waj' from the cornea to the visual 

 nerve. Of these media the anterior one (aqueous humor) is liqtiid, 

 the posterior (vitreous humor) is semisolid, and {he internrediate one 

 (crj'stalline lens) is solid. The space occupied by the aqueous humor 

 corresponds nearly to the portion of the eye covered by the trans- 

 parent cornea. It is, however, divided into two chambers, anterior 

 and posterior, hj the iris, a contractile curtain with a hole in the cen- 

 ter (the pupil), and which maj' be looked on as in some sense a projec- 

 tion 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 raj^s 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. 

 They may, like all other parts, become the seat of disease, but so long 

 as the}^ and the iris retain their clear, dark aspect, without any tints 

 of brown or j^ellow, they may be held to be healthy. 



The vitreous or semisolid refi'acting 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 Q^g^ and a power of refraction of the light rays correspondingh^ 

 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 corre- 

 sponds 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 



