256 



in the main, of iibers witli lymj)ii interspaces, and it is to the condi- 

 tion of these and their condensation and compression that the trans- 

 lucency is largely dne. This may be shown by comjiressing with the 

 fingers the eye of an ox which has just been killed, Avlien 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 refrac- 

 tion 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 x^osterior (vitreous humor) is semi-solid, and the intermediate one 

 (crj^stalline lens) is solid. The space occupied bj^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 i3osterior, 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 inauner of a ring. 

 The action of the two sets is necessarily antagonistic, the radiating 

 fibers dilating the iiupil and exx^osing the interior of the eye to view, 

 while the circular fibers contract this oj)ening 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 ui^x^er border is fringed b}^ several 

 minute black bodies (corjjora nigra) projecting forward and serving 

 to some extent the i)urj)ose of eyebrows in arresting and absorbing the 

 excess of rays of light which fall ui^on the eye from above. These 

 jjigmentary projections in front of the uj)i3er border of the pujiil are 

 often mistaken for the x)roducts of disease or injury, in i)lace 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 they and the iris retain their clear, dark aspect, without any 

 tints of brown or yellow, they may be held to be health}". 



The vitreous or semi-solid refracting medium occupies the posterior 

 l^art of the eye — the jaart corresx^onding to the sclerotic, choroid, and 

 retina — and has a consistency corresponding to that of the white of 

 an egg, and a x^o^ver of refraction of the light-rays corresx^ondingly 

 greater than the aqueous humor. 



The third or solid refracting medium is a biconvex lens, with its 

 convexity greatest on its posterior surface, Avhich is lodged in a 

 dex)resssion in tlie vitreous humor, while its anterior surface corre- 

 sponds to the ox)ening of the pupil. It is inclosed in a membranous 

 covering (capsule), and is maintained in position by a membrane 



