1101 



Many of them in dark eyes contain pigment granules, but in blue eyes and the pink eyes of 

 albinos they are unpigmented. The muscle fibres are unstriated and consist of circular and 

 radial fibres. The circular fibres (m. sphincter pupillae) surround the margin of the pupil on the 

 posterior surface of the iris, like a sphincter, forming a narrow band about one-thirtieth of an 

 inch (0.8 mm.) in width, those near the free margin being closely aggregated; those more 

 external somewhat separated, and forming less complete circles. The radial fibres (in. dilator 

 pupillae) converge from the circumference toward the centre, and blend with the circular fibres 

 near the margin of the pupil. These fibres are regarded by some as elastic, not muscular. 



3. Pigmented epithelium (pars iridica retinae). This is a continuation of the pars ciliaris 

 retinae, and consists of two layers of pigraented, columnar epithelial cells. The situation of the 

 pigment differs in different irides. In the various shades of blue eyes the only pigmentation is 

 that of the pigmented epithelium. The color of the eye in these individuals is due to this coloring 

 matter showing more or less through the texture of the iris. In the albino even this pigment is 

 absent. In the gray, brown, and black eye there are, as mentioned above, pigment granules 

 to be found in the connective tissue cells of the stroma and in the endothelial layers on the front of 

 the iris; to these the dark color of the eye is due. 



ANTERIOR CILIARY ARTERIES 



.LONG CILIARY 

 ARTERY 



ANTERIOR CILIARY ARTERIES 



FIG. 809. Iris, front view. (Testut.) 



The arteries of the Choroid and Iris (Figs. 804 and 809). The short posterior ciliary arteries, 

 from six to twelve in number, arise from the ophthalmic, or some of its branches', they pass for- 

 ward around the optic nerve to the posterior part of the eyeball, pierce the sclera around the 

 entrance of the nerve, and supply the choroid coat and ciliary processes. The arteries of the iris 

 are derived from the long yoxtcrior ciUani ami > iliari/ arteries and from the vessels of the 



ciliary processes (see p. 610). The long posterior ciliary arteries (Figs. 801 and 802), two in 

 number, pass through the sclera, one on the inner and one on the outer side of the optic nerve, 

 and pass forward between the sclera and choroid, and, having reached the attached margin 

 of the iris (Figs. 801 and 809), divide into an upper and a lower branch, and, encircling the 

 ins, anastomose with corresponding branches from the opposite side: into this vascular zone 

 (circulus iridis major) (Fig. 820) the anterior ciliary arteries (Fig. 820), from the lacrimal and 

 anterior ciliary from the muscular branches of the ophthalmic, pour their blood. From this 

 zone vessels converge to the free margin of the iris, and these communicate by branches from one 

 to another and thus form a second zone (circulus iridis minor] in this situation. The veins pass 

 toward the ciliary margin and communicate with the veins of the ciliarv processes and of the 

 canal of Schlemm (Figs. 801 and 802). 



The nerves of the Choroid and Iris (Fig. 800) are the short ciliary, the ciliary branches of 

 the lenticular ganglion, and the long ciliary from the nasal branch of the ophthalmic division 

 of the trigeminal. They pierce the sclera around the entrance of the optic nerve, and run for- 



