346 HORSE, SWINE AND POULTRY DISEASES 



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 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 pro- 

 jections 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 

 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 pos- 

 terior 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 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 

 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 OP THE EYE. 



These consist of four straight muscles, two oblique and one re- 

 tractor. The straight muscles pass from the depth of the orbit for- 

 ward 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. 



