THE SENSORIAL FUNCTION. 89 



the thickest darkness, it is white ; and the ferret, who is destined to hunt his prey 

 through all its winding retreats, and in what would be to us absolute darkness, has 

 no paint on the choroides. 



Tracing the choroides towards the fore part of the eye, we perceive that it is 

 reflected from the side to the edge of the lens, n, and has the appearance of several 

 plaits or folds. They are actually foldings of the membrane, it is not diminished 

 in size, but it has less space to cover, and there must be duplicatures or plaits. They 

 are usefully employed in the place in which we find them. They prevent tlie passage 

 of any rays of light on the outside of the lens, and which, proceeding forward in 

 various directions, and uncondensed by the power of the lens, would render vision 

 confused or imperfect. These folds of the choroides are called the ciliary processes. 



Within the cornea, and occupying the fore part of the ej'^e, is the aqueous humour, 

 p, so termed from its resemblance to pure water. It is that by which 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 behind the iris (nt, p. 86). 

 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 membrane 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 tva/l- 

 eyed. Vulgar opinion has decided that a wall-eyed horse is never subject to blind- 

 ness, 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 unpleasant 

 appearance, and his frequent want a'i 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 pain- 

 ful to the animal, and injurious to vision. W"hen 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 pro- 

 portionably 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 enlaro-es, and more 

 light is admitted. 



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

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



* Mr. W. Percivall, however, in his excellent Lectures on the Veterinary Art, vol. iii. p> 

 8* M 



