EXTERNAL STRUCTURE. 53 



others were constructed, we shall be better able to speak when we have 

 explained the contents of the eye. 



Within the cornea, and occupying- the fore part of the eye, is the aqueous 

 humour, jh 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 (m, p. 89). Floating in this fluid is a mem- 

 brane, with an 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 called the 

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

 varies but httle 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 

 sparkling 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, and 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 difference of structure which can 

 produce this 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 7J//7;//, and through it light passes 

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

 It varies with the 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 

 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 

 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 the aperture in the centre must be 

 greater. When the fibres are relaxed, the pupil must proportionably di- 

 minish. 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 a quantity of light, sufficient to dazzle or 

 give pain, fafls upon the retina, it is exhausted, the fibres of the iris sym- 

 pathize with it, and relax, and the curtain falls. When a deficient portion 

 of lio-ht reaches the retina, and vision is indistinct, we are conscious of an 

 apparent effort to bring the object clearly into view; the fibres then con- 

 tract, and the aperture enlarges, and more light is admitted. 



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

 certaining 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 Hght; and good judges have been deceived when blind- 

 ness 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 wifl likewise hang back in 

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



