THE SENSORIAL FUNCTION. 91 



Cataract in t.ie 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 Avill soon be absorbed and disappear. These 

 operations are impossible in the horse ; for, in the first place, thi^re is a muscle of 

 which we have already spoken, and to be presently more particularly described , that 

 is peculiar to quadrapeds, 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 incision ; 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 opaque lens could be withdrawn or depressed, and the mechanism of the eye were not 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 suflered 

 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, oi 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, 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, con- 

 densed by the lens and the humours, fall, and, producing a certain image correspond- 

 ing 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 convexity 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 proportion as this is the case, the sight will 

 be indistinct and imperfect ; nor shall we be able to otTer any remedy for this defect 

 of sight. There is a skying, 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 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, w on" accustomed to them will 

 deny; and no intelligent 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. lesseninfr the con- 

 vergency of the rays, and throwing the perfect image beyond, and not on, the retina. 

 There is a striking difference in the convexity of the cornea in the colt and the old 

 horse ; and both of them, probably, may shy from opposite causes — tb.e one from a 

 cornea too prominent, and tlie other from one too flat. In the usual examination of 

 the horse previously to purchase, sufficient attention is not always paid to the con- 

 vexity of the cornea. 



The remedy for shyin? will be considered when we speak of the vices of lu^rses. 



There is a provision yet wanting. The horse has a very extended field of view, but 

 man)' persons are not perhaps aware how little of it he can command at a time. 

 There is not one of our readers who can make out a single line of our treatisi^ without 

 changing the direction of the eye. It is curious to follow the motion of the eyes of a 

 rapid reader. Nature has given no less than seven muscles to the horse, in order to 



