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CHAPTER XI. 



DISEASES OF THE EYE AND ITS APPENDAGES. 



THE eye of the dog is more spherical than that of the horse; it more nearly resembles in 

 shape the human eye. It is situated in what is called the orbital cavity, which in the dog is 

 destitute of an orbital arch, but has a ligament instead; and this is the reason that the eye 

 of the dog is more easily displaced than that of man. 



The eyelids, which are meant for the protection of the eye, we need not describe ; but 

 there is one membrane in the dog's eye not found in man's which does deserve to be noticed. 

 It is called in medical parlance the membrana nictitans ; in common language, the inner, 

 under, or winking eyelid, and also the haw. It is situated on the inner portion of the eyeball, 

 and the outer margin of it is usually of the same colour as the iris. This also is meant for 

 the protection of the eye-ball, which it keeps clear from dust, and all extraneous matters. To 

 do this, it shuts from within upwards and outwards. Its movement is a purely mechanical 

 one, being caused by the retraction of the eyeball into the socket, which retraction simply 

 pushes the membrane up. It is wholly or partially raised when the dog is asleep. 



If we imagine a needle to be entered at the pupil, and carried right through, until the 

 point emerged at the back part of the eyeball, what should we pierce ? 



Taking things in regular rotation, and not being too precise, we should first puncture a 

 portion of the conjunctiva transparent here and merely epidermal; but this conjunctiva is a 

 mucous membrane which, joining the skin at the eyelids, lines the eyelids, and forms the external 

 covering to every portion of the eye you can see in the living animal. Next to be pierced would 

 be the cornea a transparent membrane composed of three layers, and forming the covering to the 

 front of the eye, joining which is the sclerotic coat, or the principal sheath of the eyeball. 



Still going inwards, the needle would traverse the chambers, divided into the anterior and 

 posterior, and which contain what is called the aqueous humour of the eye ; the use of this being 

 to retain the convexity of the cornea, the aqueous humour and the cornea combining to form a 

 lens which refracts the light passing through it to the crystalline lens and retina. Now, situated 

 between the anterior and posterior chambers is a diaphragm, just like that which is used in the tube 

 of a photographic apparatus. This is called the iris, and the hole in the centre is termed the pupil. 

 The iris in the human being may be blue, grey, or hazel ; in the dog it is usually either a beautiful 

 golden yellow, light or dark brown, or hazel, or nearly black. The iris, in fact, is simply a curtain 

 with a hole in the centre, and the widening or narrowing of this opening is what is termed 

 dilatation or contraction of the pupil. Its use is to regulate the amount of light required for 

 correct vision. In the dog the pupil is circular, as in the human being. 



Behind this the needle finds its way into the crystalline lens, held in position by the ciliary 

 ligaments, which are simply a continuation of the choroid coat. The crystalline lens is a transparent 

 body, and is enveloped in a transparent capsule, and its use is to bring the rays of light to a 

 focus upon the retina. Now, once through the crystalline lens, the needle finds itself in the 

 vitreous or .glass-like body, or the vitreous humour which occupies the largest chamber of the 



