576 THE EYE 



The iris is suspended in the aqueous humor, its pupillary margin 

 resting gently upon the anterior surface of the lens, its base or cil- 

 iary margin being separated from the lens by an interval, the pos- 

 terior chamber, which is also filled by the aqueous humor. 



The iris may be said to consist of three layers : 1, the e.xternal 

 epithelium; 2, the fibrous stroma; 3, the internal epithelium. 



The external epithelium (endothelium of the iris) is continuous 

 at the margin of the anterior chamber with the posterior epithelial 

 layer of the cornea, which appears to be reflected upon the anterior 

 surface of the iris. At the pupillary border it is also continuous 

 with the internal epithelium of the iris (pars iridis retinae). The 

 cells of the anterior or external epithelium are very much flattened 

 and almost endothelioid in appearance ; at occasional intervals the 

 epithelium is incomplete. These intervals occur either near the 

 pupillary or the ciliary margin, and correspond to recesses which 

 open directly into the fibrous stroma of the iris and become con- 

 tinuous with its lymphatic interstices. 



To the naked eye the anterior surface of the iris presents an 

 uneven appearance, which is apparently due to the presence of 

 slight meridional ridges, with shallow intervals, which extend from 

 the pupillary margin of the iris to its outer border. 



The fibrous stroma of the iris (pars choroidalis iridis, pars 

 uvealis iridis) consists of a loose spongy connective tissue of an 

 almost embryonal type. Its fibres are scanty and are gathered into 

 small bundles, which interlace somewhat, but which are for the 

 most part disposed in a meridional direction. This disposition is 

 especially noticeable near the ciliary margin. 



The fibrous stroma is very rich in connective tissue cells, which 

 are mostly stellate and branch and interlace freely. They contain 

 more or less brownish pigment, which is most abundant near the 

 posterior (inner) surface. The color of the iris, when viewed with 

 the naked eye, is dependent upon the depth of pigmentation in 

 these connective tissue cells, as well as in the cells of the internal 

 epithelial layer. In dark blue and black eyes the stroma pigment 

 is scanty, and the very dark epithelial pigment shows through the 

 more anterior layers of the iris. In the brown eye the stroma pig- 

 ment is dense and opaque. A grey color is produced by a scanty 

 stroma pigment clouded by a rather dense fibrous stroma. 



Embedded in the fibrous stroma, near its pupillary margin, is a 

 small bundle of non-striated muscle fibres, which are circularly dis- 

 posed, to form the so-called sphincter muscle of the iris. Its fibres 



