THE ETE.]| 



THE HOESE, AND 



[the etb. 



at all. There is considerable sympathy be- 

 tween the iris and the retina, as in cases of 

 gutta Serena, or glass eyes, as it is termed ; or 

 when the optic nerve and the iris have lost 

 their sensibility. 



In considering the outer coats of the eye, 

 we will commence with the sclerotic, which is 

 composed of tendinous material, very dense, 

 but not equally thick all over, the thickest 

 part being its posterior. This part, as it re- 

 ceives more pressure against the orbit, wheu 

 under the influence of the retractor muscle, 

 nature has proportioned the means to the end, 

 and made it thicker, in order that it might be 

 capable of repelling the pressure it was destined 

 to receive. Anteriorly, there is a groove re- 

 ceiving the posterior edge of the transparent 

 cornea, called the ciliary processes — the white 

 folds at the margin of the uvea in the eye, 

 covered with black matter ; there is a passage 

 through the sclerotic coat for the optic nerve, 

 which does not pierce it at its centre, but 

 pierces the sclerotic and choroid coats at the 

 inferior part. 



The choroid coat is on the inner side of the 

 sclerotic, and is very vascular, possessing many 

 blood-vessels, nerves, and absorbents. Its ap- 

 pearance varies in different animals. In white 

 ferrets, and also in those classes of persons 

 commonly called " albinos," with long white 

 hair, the colouring matter is wanting, and the 

 arteries of the coat are alone seen. The con- 

 sequence is, the bottom of the eyes look red, 

 "Where this formation exists, there is an inca- 

 pability of seeing in a strong light, objects 

 being best seen in the dark, which is most 

 properly called a diminution of light ; and in 

 sympathy with such, the eye-lashes are always 

 white, and the hair of the head white also, 

 Frequently in smaller animals all the hair is 

 white, as we find in white mice and white rats. 

 In man, generally, both surfaces of the choroid 

 coat, the one in contact with the sclerotic coat, 

 the other witli the retina, are covered witli a 

 black pigment, which, in the dead animal, be- 

 comes a kind of mucus. But here there is a 

 difference according to circumstances. Negroes 

 have it much blacker than the inhabitants of 

 temperate regions, and have a power of ab- 

 sorbing superfluous rays of light, to an extent 

 that renders vision even painful. Thia coat, 

 in borses, is of great importance. All the 

 202 



pigment or colouring matter below the optic 

 nerve, being about one-third of the whole, is 

 black, but above it is green and blue, which are 

 separated from the other by a distinct hue. 

 This compound, in the living eye, imparts to it 

 the appearance of sky-blue, which, seen through 

 the humours, is of a greyish hue. Horses fre- 

 quently, 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, tlie 

 black covering is lined here with a little pig- 

 ment. It is then distributed over the whole 

 of the surface, as far as the junction of the 

 opaque cornea with the ciliary processes. The 

 object of this variegated coat, and why it should 

 occupy the superior part in particular, is, that 

 the rays of light may pass through the nerve 

 going beyond the retina. In man, from the 

 colour of the pigment, the rays become ab- 

 sorbed, and terminate accordingly ; but in a 

 pigment that is not black, they do not termi- 

 nate, but are reflected back, striking the nerve 

 a second time, and the effect is then an in- 

 creased degree of vision. In the horse, the 

 rays strike the nerve twice, but only at the 

 upper part, where the green pigment is situated. 

 The rays thus received are obviously the weak- 

 est, especially when the head of the animal is 

 near the ground, which is his natural position 

 when in the act of grazing ; the black pigment 

 then receives the strong superior rays ; and 

 thus by a skilful combination the light is 

 equalised, and sharp keen sight produced. 



" The reader may see in the dusk, or even 

 when duskiness is fast yielding to utter dark^ 

 ness," says Mr. Youatt, " the beautiful sea-green 

 reflection from the eye of the horse. ... If 

 the reader has not examined this beautiful pig- 

 ment, he should take the earliest opportunity of 

 doing so. He will have a beautiful illustration of 

 the care taken by the Supreme Being that each 

 shall suit his situation. The horse has not the 

 intelligence of man, and may not want, for any 

 [)urpose of pleasure or improvement, the vivid 

 picture of surrounding objects which the retina 

 of the human being presents. A thousand 

 minute but exquisite beauties would be lost 

 upon him. Ir', therefore, his sense of vision 

 may not be so strong during the day, it is made 

 up to him by tlie increased power of vision in 

 the uiirht." 



