THE EYE AND ITS VISUAL STRUCTURE l8l 



the eye, except that portion occupied by the cornea. 

 The choroides, or choroid coat, covered with a black 

 secretion, or black or dark brown paint, called the 

 pigmentum nigrum. 



This delicate membrane extends over the whole 

 internal part of the eye, from the optic-nerve to the 

 cornea. This is intended to absorb the stray rays 

 of light which might dazzle and confuse the animal, 

 and is not found on any portion which may be called 

 the field of vision ; but in its stead a bright green 

 substance is spread, which extends more over the 

 upper than the lower portion, because the objects 

 which are most necessary for him to see are below 

 the level of the head. This in some way, yet un- 

 discovered, enables the animal to see even when it 

 is comparatively dark. In the dusk this beautiful 

 sea-green colour may be seen in the eye of a horse. 



Cream-coloured, or perfectly white horses, have 

 not this dark pigment ; so that the ordinary appear- 

 ance of the pupil is red instead of black. In looking 

 into their eyes we do not see the covering, but the 

 choroid coat itself. 



m, m. The iris, or rainbow-coloured circular membrane, situated 

 under the cornea in front of the eye, and on which 

 the colour of the eye depends. The duplicature 

 behind is the uvea, so called from its colour re- 

 sembling a grape. See also Plate m, fig. 5, *'. The 

 iris acts as a curtain, and floats in the aqueous 

 humour. 



In horses the colour of the iris is subject to little 

 variation, but for the most part has some analogy 

 with that of the hide, varying in different degrees 

 from hazel to dark brown. 



n. The pupil is placed in the centre of the cornea, and 

 through which all the rays of objects pass to the retina 

 or mirror of the eye. See also Plate in, fig. 5, . 



