THE EYE. 65 



size ; then let the hand be placed over each eye alternately and 

 held there for a little while, and let it be observed whetlier the 

 pupil dilates with the abstraction of light, and equally in each 

 eye. 



In our cut, m gives a duplicature of the iris, or the back sur- 

 face of it. This is called the uvea, and it is covered with a 

 thick coat of black mucus, to arrest the rays of light, and to 

 prevent them from entering the eye in any other way than 

 through the pupil. The color of the iris is, in some unknown 

 way, comiected with this black paint behind. Wall-eyed horses, 

 whose iris is white, have no uvea. 



We now arrive at a body on which all the important uses of 

 the eye mainly depend, the cnjstaUme lens, g, so called from its 

 resemblance to a piece of crystal, or transparent glass. It is of 

 a yielding jelly-like consistence, thicker and firmer towards the 

 centre, and convex on each side, but more convex on the inner 

 than the outer side. It is enclosed in a delicate transparent bag 

 or capsule, and is placed between the aqueous and the vitreous 

 humors, and received into a hollow in the vitreous humor, 

 with which it exactly corresponds. It has, from its density and 

 its double convexity, the chief concern in converging the rays of 

 hght which pass into the pupil. 



The lens it very apt to be affected from long or violent inflam- 

 mation of the conjunctiva, and either its capsule becomes cloudy, 

 and imperfectly transmits the light, or the substance of the lens 

 becomes opaque. The examination of the horse, with a view 

 to detect this, must either be in the shade, or at a stable door, 

 where the light shall fall on the animal from above and in front ; 

 and in conducting this examination, we would once more cau- 

 tion the intended purchaser against a superfluity of white about 

 his neck. Holding the head of the animal a little up, and the 

 light coming in the direction that has been described, the con- 

 dition of the lens vidll at once be evident. The confirmed cata- 

 ract, or the opaque lens of long standing, will exhibit a jjearly 

 appearance, that cannot be mistaken, and will frequently be 

 attended with a change of form — a portion of the lens being 

 forced forwards into the pupil. Although the disease may not 

 have proceeded so far as this, yet if there is the slightest cloudi- 

 ness of the lens, either generally, or in the form of a minute 

 spot in the centre, and with or without lines radiating from that 

 spot, the horse is to be condemned ; for, in ninety-nine cases out 

 of a hundred, the disease will proceed, and cataract, or complete 

 opacity of the lens, and absolute blindness, will be the result. 



Behind the lens, and occupying four fifths of the cavity of the 

 eye, is the vitreous himior (glassy, or resembling glass). It 

 seems, when first taken from the eye, to be of the consistence of 

 5 



