642 THE EYE 



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 lighter radial and circular markings 

 are due to the blood-vessels. 



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

 iridis) consists of a loose spongy connective tissue of an almost embry- 

 onal type. Its fibers 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 (in- 

 ner) 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, and to the 

 relative transparency of the stroma. In dark blue and black eyes the 

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

 through the more anterior clear layers of the iris. In the brown eye 

 the stroma pigment is dense and opaque. A gray color is produced by 

 a scanty stroma pigment clouded by a rather dense fibrous stroma. In 

 the eyes of albinos the iris lacks pigment altogether; the pink color 

 of the iris is due to the blood in the numerous vessels of the stroma. 



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

 small bundle of non-striated muscle fibers, which are circularly disposed, 

 to form the so-called sphincter muscle of the iris. Its fibers are distrib- 

 uted in a plane parallel to the surface of the iris, and within the inner 

 (posterior) part of its fibrous stroma. They are most abundant near 

 the pupillary margin, and become progressively thinner toward the 

 base of the iris. Internally to the sphincter muscle, and in contact 

 with the basement membrane of the internal epithelium, is an incom- 

 plete layer, more distinct toward the ciliary margin of the iris, which 

 contains radially disposed smooth muscle fibers, the dilator muscle of 

 the iris, apparently of ectodermal origin. Mydriatics (e.g., atropin) pro- 

 duce an enlargement of the pupil, presumably through inhibition of 

 the constrictor and stimulation of the dilator muscles; miotics (e.g., 

 morphin) produce a contrary effect. 



The stroma of the iris is exceedingly vascular, the arteries and veins 

 being meridionally disposed, the capillaries forming an irregular plexus. 

 Near the pupillary margin the vessels form a rich capillary anastomosis, 



