THE SENSORIAL FUNCTION. 165 



below, wliile, at the same time, the field of view is perfectly open, so far as 

 it regards the pasture on which the horse is grazing*. 



In oiir cut m gives a duplicature of the iris, or the back surface of it. 

 This is called the uvea, and it is covered with a thick coat of black mucus 

 to arrest the rays o^ light, and to prevent them from entering the eye in 

 any other way than through the pupil. The colour of the iris is, in some 

 unknown way, connected 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 eyo 

 mainly depend, the crystalline lens, g, p. 160, so called from its resemblance 

 to a piece of crystal, or transparent glass. It is of a yielding jelly-like con- 

 sistence, thicker and firmer towards the centre, and convex on each side, 

 but more convex on the posterior than the anterior side. It is enclosed in 

 a dehcate transparent bag or capsule, and is placed between the aqueona 

 and the vitreous humours, and received into a hollow in the vitreous 

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

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

 which pass into the pupil. 



Behind the lens, and occupying four-fifths of the cavity of the eye, is the 

 vitreous humour (glassy, or resembhng glass). It seems, when first taken 

 from the eye, to be of the consistence of a jelly, and of beautiful trans- 

 parency ; but if it is punctured a fluid escapes from it as limpid and as thin 

 as water, and when this has been suffered completely to ooze out, a tissue of 

 thin transparent membraneous bags or cells remains. The vitreous humour 

 consists of a watery fluid contained in these cells ; but the fluid and the 

 cells form a body of considerably greater density than the aqueous fluid 

 in the front of the eye. 



Last of all, between the vitreous humour and the choroid coat, is the 

 retina, o, p. 160, or net-like membrane. It is an expansion of the substance, 

 i, of the optic nerve. When that nerve has reached the back of the eye, and 

 penetrated through the sclerotic and choroid coats, it first enlarges into a 

 httle white prominence, from which radiations or expansions of nervous 

 matter proceed, which spread over the whole of the choroid coat, and form 

 the third investment of the eye. The membrane by which this nervous 

 pulp is supported, is so exceedingly thin and dehcate, that it will tear with 

 the shghtest touch, and break even with its o^v^l weight. The membrane 

 and the pulp are perfectly transparent in the hving animal. The pupil 

 appears to be black, because in the daytime it imperfectly reflects the 

 colour of the choroid coat beneath. In the dusk it is greenish, because, the 

 glare of day being removed, the actual green of the paint appears. 



On this expansion of nervous pulp, the rays of light from surrounding 

 objects, condensed by the lens and the humours, fall, and producing a, 

 certain image corresponding with these objects, the animal is conscious of 

 their existence and presence. 



It may, however, so happen that from the too great or too little con- 

 vexity of the eye or a portion of it, the place of most distinct vision may 

 not be immediately on the retina, but a little before or behind it. In pro- 

 portion as this is the case, the sight will be indistinct and imperfect ; nor 

 shall we be able to ofier any remedy for this defect of sight. There is a 

 shying, often the result of cowardice or playfulness, or want of work, but 

 at other times proving, beyond contradiction, a defect of sight even more 

 dangerous than bhndness. A bhnd horse ^vill resign himself to the guid- 

 ance of his rider or driver ; but against the misconception and starting of 

 a shying horse there is no defence. That horses grow shy as they grow 

 old no one accustomed to them will deny ; and no inteUigent person will 

 be slow in attributing it to the right cause — a decay in the organ of vision, 



