EXTERNAL STRUCTURE. 93 



otliei's were constructed, we shall be better able to speak whcr- we liave 

 explained the contents of" the eye. 



Within the cornea, and occupying the fore part of" the eye, is the aqaeouit 

 humour, p, so termed from its resemblance to pure water. It is tiiat 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 (m, p. 89). Floating in this fluid is a mem- 

 brane, with an oblong aperture, called the iris. It is that whicli givca 

 colour to the eye. The human eye is said to be black, or iiazel, or blue, 

 according to the colour of this membrane or curtain ; and it is called the 

 iris, or rainbow, from its beautiful intermingling hues. The colour varies 

 but little in the horse, and always bears some analogy to that of the skin. 

 We rarely see it lighter than a hazel or darker than a brown. The spark- 

 ling black of the human being is never found. Horses perfectly white, or 

 cream-coloured, have the iris white and the pupil red. When horses of 

 other colours, which 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 we believe to be altogether 

 erroneous. There is no ditrerence of structure which can produce tliis 

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

 appearance, and his frequent want of breeding, may not be 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 varies with tiie intensity or degree of light which falls upon the eye. In 

 a dark stable, the pupil is expanded to admit a great proportion of the light 

 which 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 con- 

 tracts, 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 

 which enter into the composition of the iris. When these fibres contract, 

 it is plain that the pupil must enlarge. The membrane itself must be drawn 

 into less compass, and therefore tlie aperture in the centre must be greater. 

 When the fibres are relaxed, the pupil must proportioiiably diminish. The 

 motions of the iris are not at all under the control of the will, nor is the ani- 

 mal sensible of them: they are produced by sympathy with the state of the 

 retina. When a quantity of light, sufficient to dazzle or give pam, falls 

 upon the retina, it is exhausted ; the fibres of the iris sympathize with it, 

 and relax, and the curtain falls. When a deficient portion of light reaches 

 the retina, and vision is indistinct, we are conscious of an apparent effort to 

 bring the object clearly into view; the fibres then contract, and the aper- 

 ture enlarges, and more liglit is admitted. 



This dilatation or contraction of tiie pupil gives a useful method of 

 ascertaining the existence of blindness in one eye or in both. There is a 

 description of blindness, which a close examination of the eye, even by 

 a person accustomed to horses, will not always detect. The cornea and 

 crystalline lens remain perfectly transparent, but the retina is palsied, and 

 is not affected by light; and good judges have been deceived when ijlind- 

 ness of this description has been confined to one eye. A horse blind in 

 boch eyes will usually have his ears in constant and rapid motion, directing 

 tliom in quick succession to every quarter; he will likewise hang back in 

 lis halter in a peculiar way ; and will lift his feet high, as if he were stepping 



