VISION. 463 



tween the lens and the cornea is filled with a 

 watery secretion (a), called the Aqueous humour. 

 This space is divided into an anterior and a pos- 

 terior chamber by a flat circular partition (i), 

 termed the Iris. 



The iris has a central perforation (p), called 

 the Pupil^ and it is fixed to the edge of the cho- 

 roid coat, by a white elastic ring (q), called the 

 Ciliary Ligament. The posterior surface of the 

 iris is called the Uvea, and is lined with a dark 

 brown pigment. The structure of the iris is very 

 peculiar, being composed of two layers of con- 

 tractile fibres ; the one, forming concentric cir- 

 cles ; the other, disposed like radii between the 

 outer and inner margin.* When the former act, 

 the pupil is contracted ; when the latter act, the 

 breadth of the iris is diminished, and the pupil 

 is, of course, dilated. By varying the size of the 

 pupil the quantity of light admitted into the 

 interior of the eye is regulated, and accommo- 

 dated to the sensibility of the retina. When the 

 intensity of the light would be injurious to that 

 highly delicate organ, the pupil is instantly con- 

 tracted, so as to exclude the greater portion ; 

 and, on the contrary, when the light is too 

 feeble, it is dilated, in order to admit as large a 

 quantity as possible. The iris also serves to in- 



* See Fig. 47. vol. i, p. 136. 



