THE SENSE OF SIGHT. 



639 



with the distinctness of 

 the images there formed. 

 Hence animals in which 

 the choroid is destitute of 

 pigment, and human Albi- 

 noes, are dazzled by day- 

 light and see best in 

 the twilight. The choroid 

 coat ends in front in what 

 are called the ciliary pro- 

 cesses (fig. 173). 



The retina (fig. 174) is 

 a delicate membrane, 

 concave, with the conca- 

 vity directed forwards 

 and ending in front, near 

 the outer part of the 

 ciliary processes in a 

 finely notched edge, 

 the ora serrata. Semi- 

 transparent when fresh, 

 it soon becomes clouded 

 and opaque, with a 

 pinkish tint from the 

 blood in its minute 



Fig. 173-* 



* Fig. 173. Ciliary processes as seen from behind. \. I, posterior 

 surface of the iris, with the sphincter muscle of the pupil ; 2, anterior 

 part of the choroid coat ; 3, one of the ciliary processes, of which about 

 seventy are represented. 



f Fig. 174. The posterior half of the retina of the left eye viewed 

 from before (after Henle) ; s, the cut edge of the sclerotic coat ; ch, the 

 choroid ; r, the retina ; in the interior at the middle, the macula lutea 

 with the depression of the fovea centralis is represented by a slight oval 

 shade ; towards the left side the light spot indicates the colliculus or 

 eminence at the entrance of the optic nerve, from the centre of which 

 the arteria centralis is seen spreading its branches into the retina, 

 leaving the part occupied by the macula comparatively free. 



