OF FARRIERY. 



h.5 



but not equally thick all over, the thickest 

 part being its posterior ; this part receiving 

 more pressure against the orbit, when under 

 the influence of the retractor muscle, nature 

 required it to be thicker, to repel the pressure 

 it might receive. Anteriorly, there is a groove 

 receiving the posterior edge of the transpareiil 

 cornea, called the ciliary processes (the v* nite 

 folds at the margin of the %ivea in th*^ eye, 

 covered with black matter) ; there is -^ pas- 

 sage through the sclerotic coat for the optic 

 nerve, which does not pierce it at its c ntre, 

 but pierces the sclerotic and choroid coatG at 

 the inferior part. 



The choroid coat is on the inner side of l.'ie i 

 sclerotic, and is very vascular, possessing j 

 many blood-vessels, nerves, and absorbents. \ 

 The appearance in this coat varies in different ; 

 animals, forming a difference of covering, 

 •which it most commonly possesses. It is most 

 commonly in white ferrets, and also in the 

 human subject, with those classes of persons 

 commonly called " albinos'^ with long white 

 hair, &c. In all these the colouring matter is 

 wanting, and the arteries of the coat are alcne 

 seen ; the consequence is, liie bottom of the 

 eye looks red. In consequence of this forma- 

 tion, there is an incapability of seeing in a 

 strong light, seeing best in the dark, which is 

 most properly called, a diminution of light; 

 there being no such thing as positive darkness : 

 and I may remark, that in sympathy with 

 such, the eye-lashes are always white, and 

 the hair of the head is white also ; and fre- 

 quently on smaller animals all the hair is white : 

 the same occurs to white mice and white rats. 

 In man, generally, both surfaces of the choroid 

 coat, the one in contact with the sclerotic 

 coat, the other with the retina, are covered 

 with a black pigment, which, in the dead 



animal, becomes a kind of mucus. But there 

 is a difference according to circumstances : 

 the negroes in the tropics have it much 

 blacker than those in temperate regions, bv 

 which they have a power to absorb superflu- 

 ous rays of light, so that vision is not painful. 

 From these facts, we may conclude, this pig- 

 ment is a kind of regulator, absorbing superflu- 

 ous rays ; also shewing, that such eyes cannot 

 see well in a weak light, as the rays are all 

 absorbed without affecting the retina ; for 

 nature has never been able to make one piece 

 of mechanism to perform opposite effects wek. 

 This coat, in Horses, is of great impoitance 

 from its having a different coloured pigment 

 from man, which alone has occasioned many 

 mistakes, all the pigment or colouring matter 

 below the optic nerve, being about one-third 

 of the whole, is black, but above it is ffreen 

 and blue, and a distinct line separates them : 

 the compound of this, in the living eye, gives 

 it the appearance of sky blue, which, seen 

 through the humours, is of a greyish blue. 

 Horses frequently, and by very eminent men, 

 have been pronounced unsound from having 

 this grey cast or shade in the eye. The optic 

 nerve having pierced these two coats within, 

 the black covering is lined here with a little 

 pigment ; it is then distributed over the whole 

 of the surface, as far as the junction of the 

 opaque cornea with the ciliary processes. Now 

 the cause of this variegated coat, and why it 

 should occupy the superior part in particular, 

 is, because the rays of light pass through the 

 nerve going beyond the retina. In man, you 

 know, from the colour of the pigment, the 

 rays become absorbed, and thus ended ; but 

 in a pigment that is not black, they are not 

 ended, but reflected back, striking the nerve a 

 second time, and the effect must then be an 

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