THE IRIS. 421 



continually in size, according ip the degree of light to which the 

 eye is exposed.* 



The situation of the iris in the cavity of the eye is^such, 

 that, at its circumference, it is nearly in contact with the cir- 

 cumference of the cornea ; it is actually in contact with the 

 anterior edge of the ciliary ligament, and with the anterior 

 surface of that part of the tunica choroides, which is reflected 

 inwards, and forms the ciliary processes. 



As the iris is flat or plane, and the cornea is the segment of 

 a sphere, there must be a considerable vacuity between them. 

 This vacuity constitutes the anterior chamber of the eye. 



The iris, therefore, is a septum, passing across the eye at its 

 anterior part, and separating that portion of the cavity, which 

 is bounded exteriorly by the cornea, from the larger portion 

 formed by the sclerotica. 



The anterior, or external surface of this membrane, is very 

 remarkable for its color; in persons of a light complexion it 

 is generally of a light blue, intermixed with white; or of a 

 gray, or light hazel, &c. In those whose complexions are 

 dark, it is almost invariably dark also. These colors are so 

 arranged, that frequently there is an appearance of lines, some- 

 times nearly radiated, more frequently curved in various direc- 

 tions, but tending from the circumference towards the pupil or 

 centre; and when the general color of the iris is blue, these 

 lines are often whitish. 



When this surface is examined in water with a magnifying 

 glass, it appears to be covered with very fine villi, which proba- 

 bly have the effect of increasing its lustre. 



From a supposed resemblance of these colors to those of the 

 rainbow, the membrane has been called Iris, and is generally 

 known by that name ; but, unfortunately, another appellation 

 has been given to its internal and posterior surface, which is 



* The iris is rather narrower next the nose, than on the side of the temple, 

 by which reason, the pupil is not exactly in the centre, as first stated by 

 Winslow in 1711. P. 



VOL. ii. 36 



