COATS OP THE EYE. 



417 



Fig. 208. INTERIOR OF THE EYE. 



c, cornea; s, sclerotic; *', portion of 

 the sclerotic turned back to show 

 the parts beneath; cA, choroid; 

 r, retina; n, optic nerve; ca, an- 

 terior chamber; i, iris; p, pupil; 

 cr, crystalline lens ; pc, ciliary pro- 

 cesses ; v, vitreous humor ; bb, con- 

 junctiva. 



structure c, which is termed the Cornea. The manner in which 

 this cornea is set upon the sclerotic coat, so as to serve as the 

 continuation of it, may be compared to that in which a watch- 

 glass is made to serve as the con- 

 tinuation of the watch-case over 

 the dial. The cornea is rather 

 more convex than the rest of 

 the sphere of the eye ; so that 

 the globe makes a slight addi- 

 tional projection in that part. 

 When the sclerotic coat is 

 removed, we come upon the 

 second coat ch, which is termed 

 the Choroid; this is much more 

 delicate in its structure, con- 

 sisting almost entirely of blood- 

 vessels and nerves ; and it has 

 a deep black hue, owing to its 

 being lined with a thick layer 

 of black pigment, which consists 

 of cells that have the power of 

 secreting a black granular matter in their interior. 1 This coat 

 also changes its character in the front of the eye ; being there 

 continuous with the Iris, or coloured portion, i, which forms 

 a sort of curtain that hangs-down behind the cornea. The 

 surface of the iris is flat, or nearly so ; and there is con- 

 sequently a space between it and the cornea, like that which 

 intervenes between the dial-plate and the glass of a watch ; this 

 space is termed the anterior chamber of the eye. The iris is 

 perforated in its middle by an aperture p, termed the Pupil. 

 This aperture is always round in Man ; but in animals whose 

 range of vision is required to extend widely in a horizontal 

 direction (such as the Euminants, and others which feed 

 upon herbage), it is an ellipse with the long diameter hori- 

 zontal ; whilst in animals which rather seek their food above 

 or below them (such as the Cat and other Carnivora which 

 naturally live among trees and high places), the pupil is an 

 ellipse whose long diameter is vertical. 



1 Similar pigment-cells, having great variety in their form, are to be 

 found composing the black spots on the skin of the Frog, Water 

 Newt, &c. 



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