SENSITIVE SYSTEM, 383 



figure in outline; but, the cornea being concave and the iris flat, 

 an interval is left between them, deepest in the middle, and this 

 is called the anterior chamber, to distinguish it from a corres- 

 ponding space behind the iris, denominated the posterior cham- 

 ber. The iris exhibits a perforation, horizontally elliptical, through 

 its middle, which forms the pupil, or what is vulgarly called the 

 sight of the ei/e : the aperture is rather nearer to the inner than 

 the outer, to the upper than lower side of the globe, and this in 

 course is attended with a correlative variation in the breadth of 

 the iris at these places. The diameter of the pupil will vary — and 

 so indeed will its figure occasionally — according to the condition 

 of the organ and the quantity of light to which it is exposed. 

 The peri|jhery of the pupil, both above and below, is more or less 

 intruded upon by several little, black, pendulous bodies, which 

 are the corpora nigra. 



Smfaces. — We distinguish in the iris two flat surfaces, and 

 two elliptical margins. The anterior surface, the veritable iris, 

 is noted for its brilliancy, and for its colour, which varies in dif- 

 ferent individuals. It is a remarkable fact, that this variety of 

 hue in the iris corresponds with the colour of the hair : bay and 

 chestnut horses have hazel eyes ; brown horses have brownish 

 eyes ; and very dark brown or black horses, eyes of a still darker 

 dusky-brown shade. This curious relation is still more observa- 

 ble in human beings; the diversity of colours and hues in their 

 irides being infinitely greater than any thing we behold among 

 any one species of animals. Cream-coloured and milk-white 

 horses have wall-eyes, and Albinos have red eyes ; in both which 

 instances the iris is said to be destitute of any colouring matter 

 whatever. Both eyes are commonly alike in hue and shade ; 

 though now and then it happens that one is a light, its fellow a 

 dark eye. Upon the anterior surface of the iris are visible two 

 broad elliptical belts, the inner of which may be distinguished 

 from the outer by being a little darker-coloured; and. upon the 

 latter are discernil>le several pliccc, of which two are more elevated 

 and conspicuous than the rest : the inner of these two plicae forms 

 the boundary line of the belts, and casts a shade upon the inner 

 belt, imparting to it the effect of distance when viewed in the 

 living eye. — The posterior surface of the iris is covered with a 

 thick stratum of black mucous matter derived from the pigment, 

 which, from impregnating it with the colour of a grape, has be- 

 got for it the name of uvea ; and it would appear that the colour 

 of the iris (the nature and production of which yet remain a mys- 

 tery) is essentially dependent upon the uvea: for if the pigment 

 be washed off, the iris will be rendered colourless. In wall-eyes, 

 in which the uvea is wanting, the iris is white; so it is in AIbi- 



