THE SENSORIAL FUNCTION. 131 



the cornea is preserved in its protuberant and rounded form. It extends to the 

 crystalline lens q, and therefore a portion of it, although a very small one, is be- 

 hind the iris (m. p. 127). Floating in this fluid is a membrane, with an oblong 

 aperture, called the Iris. It is that which gives colour to the eye. The human 

 eye is said to be black, or hazel, or blue, according to the colour of this mem- 

 brane or curtain ; and it is denominated the iris, or rainbow, from its beautiful, 

 intermingling hues. The colour varies little in the horse, except that it always 

 bears some analogy to that of the skin. We rarely see it lighter than a hazel, 

 or darker than a brown. Horses perfectly white, or cream-coloured, have the 

 iris white and the pupil red. When horses of other colours, and that are 

 usually pied, have a white iris and a black pupil, they are said to be wall-eyed. 

 Vulgar opinion has decided that a wall-eyed horse is never subject to blindness, 

 but this is altogether erroneous. There is no difference of structure that can 

 produce this exemption; but the wall-eyed horse, from this singular and un- 

 pleasant appearance, and his frequent want of breeding, may not be so much 

 used and exposed to many of the usual causes of inflammation. 



The aperture in the iris is termed the pupil, and through it light passes to the 

 inner chamber of the eye. The pupil is oblong, and variable in size. It differs 

 with the intensity or degree of light that falls upon the eye. In a dark stable 

 the pupil is expanded to admit a great proportion of the light that falls upon the 

 cornea ; but when the horse is brought towards the door of the stable and more 

 light is thrown upon the eye, the pupil contracts in order to keep out that extra 

 quantity which would be painful to the animal, and injurious to vision. When 

 opposed directly to the sun, the aperture will almost close. 



This alteration of form in the pupil is effected by the muscular fibres that 

 enter into the composition of the iris. When these fibres are relaxed, the pupil 

 must proportionably diminish. The motions of the iris are not at all under the 

 control of the will, nor is the animal sensible of them. They are produced by 

 sympathy with the state of the retina. When, however, a deficient portion of 

 light reaches the retina, and vision is indistinct, we are conscious of an apparent 

 effort to bring the object more clearly into view, and the fibres then contract, 

 and the aperture enlarges, and more light is admitted. 



This dilatation or contraction of the pupil gives a useful method of ascertain- 

 ing the existence of blindness in one eye or in both. The cornea and crystalline 

 lens remain perfectly transparent, but the retina is palsied, and is not affected 

 by light ; and many persons have been deceived when blindness of this description 

 has been confined to one eye. A horse blind in both eyes will usually have his 

 ears in constant and rapid motion, directing them in quick succession to every 

 quarter. He will likewise hang back in his halter in a peculiar way, and will 

 lift his feet high as if he were stepping over some obstacle, when there is actually 

 nothing to obstruct his passage, and there will be an evident uncertainty in the 

 putting down of his feet. In blindness of one eye little or nothing of this 

 characteristic gait and manner can be perceived. Although a one-eyed horse 

 may not be absolutely condemned for the common business of the carriage or 

 the road, he is generally deteriorated as a hunter, for he cannot measure his dis- 

 tances, and will run into his leaps *. Many a sportsman, puzzled and angry 



* Mr. W. Percivall, however, in his excel- to show this. All I can say on this point is, 



lent Lectures on the Veterinary Art, vol. iii. p. that the best hunter I ever possessed, a horse 



201, says, " The loss of one eye does not en- gifted with extraordinary powers for leaping, 



feeble sight, because the other acquires greater was a one-eyed horse, and this animal carried 



energy, though it much contracts the field of me through a hunting season, without, to my 



vision. It is said to render tho conception recollection, making one single blunder in 



erring, and the case of misjudgment of dis- leaping." 

 lances is the one commonly brought forward 



