DISEASES OF THE EYE. 167 



whole front part of the eye and the inside of the eyelids. It 

 is perfectly transparent when in health, but becomes blood- 

 shot or filmy when diseased. We shall see, as we proceed 

 with this chapter, that it is extremely liable to become in- 

 flamed. 



Another noticeable part which remains to be described is 

 the iris, (literally, the rainbow,) so called from the beautifully 

 intermingling hues which it displays. The cut, being a se.c- 

 tional view, does not show the iris any further than to im- 

 perfectly indicate its location at m m. It is a most elegant 

 and delicate curtain, outstretched in the space between the 

 cornea and crystalline lens. Its oflice is to moderate the light 

 entering the eye, according to the intensity of the rays. This 

 is the membrane that gives color to the eye, so variable in 

 different animals and in the human species. In the horse its 

 color is generally brown, or verging upon it. 



But this curtain is not a continuous one : if it were, the 

 inner chamber of the eye would be in a state of almost entire 

 darkness. There is an aperture. in the center, forming the 

 'pu'pil, which is the last thing we now have to describe in 

 connection with the eye. The substance of the iris is com- 

 posed of contractile tissue. When the glare of a noon-day 

 sun, or other bright light, falls on the retina, the iris con- 

 tracts, and the pupil is made smaller; but when the light 

 which falls upon the retina is feeble — as it is at twilight, for 

 instance — the iris relaxes and the pupil is enlarged, in order 

 that all the rays possible may be admitted. The difference 

 in the size of the pupil may be plainly seen by examining 

 the eye of a sound horse, first in the stable, and then in the 

 bright sunlight. One's own eyes (if a looking-glass be used), 

 or those of a friend, will show similar changes, in this re- 

 spect, when exposed to different intensities of light. 



CAUSES OF DISEASED EYES. 



The eyes of the horse are very subject to disease; for their 

 affections, although few in number, are lamentably prevalent. 

 Large numbers of horses are ruined yearly, to the value of 



