THE SENSORIAL FUNCTION. 133 



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. 



Cataract in the human being may, to a very considerable extent, be remedied. 

 The opaque lens may be extracted, or it may be forced into the vitreous humours, 

 and there existing as a foreign body, it will soon be absorbed and disappear. 

 These operations are impossible in the horse, for, in the first place, there is a 

 muscleof which we have already spoken, and to be presently more particularly de- 

 scribed, that is peculiar to quadrupeds, and of such power as generally to draw 

 back the eye too far into its socket for the surgeon to be enabled to make his inci- 

 sion ; or could the incision be made, the action of this muscle would force out the 

 greater part of the contents of the eye, and this organ would speedily waste away. 

 If, however, the opaque lens could be withdrawn or depressed, and the mechan- 

 ism of the eye were not otherwise injured, the operation would be totally useless, 

 for we could not make the horse wear those convex glasses whose converging 

 power might compensate for the loss of the lens. 



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

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

 the eye, to be of the consistence of a jelly, and of beautiful transparency ; 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 mass of 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, 

 or net-like membrane. It is an expansion of the substance, g, 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 little 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 delicate, that it will tear with the slightest touch, and break even with its 

 own weight. The membrane and the pulp are perfectly transparent in the 

 living animal. The pupil appears to be black, beeause in the daytime it imper- 

 fectly 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 exist- 

 ence and presence. 



It may, however, so happen that from the too great or too little convexity of 

 the eye or a portion of it, the place of most distinct vision may not be imme- 

 diately on the retina, but a little before or behind it. In proportion as this is 

 the case, the sight will be indistinct and imperfect ; nor shall we be able to offer 

 any remedy for this defect of sight. There is a shying, often the result of 

 cowardice or playfulness, or want of work, but at othertimes proving, beyond 

 contradiction, a defect of sight even more dangerous than blindness. A blind 

 horse will resign himself to the guidance 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 intel- 

 ligent person will be slow in attributing it to the right cause— a decay in the 

 organ of vision,— a loss of convexity in the eye, lessening the convergency of 



