CHAP. XVII.] STRUCTURE OF THE IRIS. 



25 



border begins to send off from its anterior surface, or that towards 

 the laminated cornea, a network of elastic fibres, which stretch 

 towards the border, becoming thicker as they advance, until at 

 length the entire thickness of the lamina is expended by being 

 converted into them. These fibres then bend backwards from the 

 whole circumference of the cornea, to the cir- 

 cumference of the front of the iris, and are 

 there implanted, passing in this course across 

 the rim of the anterior chamber, and through 

 the aqueous humor. They are seen more 

 easily in some animals than in others, forming 

 a regular series of pillars around the anterior 

 chamber. Behind these there is a more diffused 

 union of the tissue of the iris with the sclerotic, 

 by means of the ciliary ligament. The iris is 

 continuous behind, near its border, with the 

 ciliary processes, and is only free in the inner 

 half of its extent, near the pupil, where it is 

 covered with a dense layer of pigment, and 

 marked by converging striae. This posterior 

 surface is termed uvea. In consequence of the 

 extreme proximity of the iris to the lens, the 

 posterior chamber is much less capacious 

 than the anterior, as it is likewise of smaller 

 diameter. 





Network of yellow fibrous 

 tissue at the border of the 

 elastic lamina of the cornea : 

 .. . a. Outer border, where the 



The anterior surface of the iris has a bril- fibres approach the iris. At 



their inner end, b, they are 

 lost on the elastic lamina. 



Magnified 70 diameters. 



liant lustre, and is marked by lines accu- 

 rately described by Dr. Jacob, taking a more 

 or less direct course towards the pupil. These lines are important 

 as being indicative of a fibrous structure. Slender, and very numer- 

 ous in the outer three-fourths of the membrane (the pupil being 

 contracted), and often crossed near the border by wave-like differently 

 coloured lines, they unite at about -^ of an inch from the pupil into 

 a circular series of knotted and much thicker elevations, from which 

 finally proceed a multitude of minute branching and anastomosing 

 filaments, to the extreme verge of the pupil. When the pupil is 

 contracted, these converging fibres are stretched; when it is dilated, 

 they are thrown more or less into zigzags. The pupil is nearly cir- 

 cular, and is situated rather to the inner side of the centre of the 

 iris. By the movements of the iris it is dilated or contracted, so as 

 to admit more or less light to the interior; and its diameter under 

 these circumstances may vary from about -^ to -J- of an inch. 



