I 



THE TUNICS OF THE EYE 



1013 



behind the peripheral part of the iris, and in front of the suspensory ligament 

 of the lens and the ciliary processes. In the adult the two chambers communi- 

 cate through the pupil, but in the fetus up to the seventh month they are separ- 

 ated by the membrana pupillaris, 



Structure. The iris is composed of the following structures: 



1. In front is a layer of flattened endothelial cells placed on a delicate hyaline basement mem- 

 brane. This layer is continuous with the endothelium covering the posterior elastic lamina of 

 the cornea, and in individuals with dark-colored irides the cells contain pigment granules. 



2. The stroma (stroina iridis) of the iris consists of fibers and cells. The former are made up 

 of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular 

 direction; but the majority radiate toward the pupil, forming by their interlacement, delicate 

 meshes, in which the vessels and nerves are contained. Interspersed between the bundles of 

 connective tissue are numerous branched cells with fine processes. In dark eyes many of them 

 contain pigment granules, but in blue eyes and the eyes of albinos they are unpigmented. 



3. The muscular fibers are involuntary, and consist of circular and radiating fibers. The 

 circular fibers form the Sphincter pupillse; they are arranged in a narrow band about 1 mm. in 

 width which surrounds the margin of the pupil toward the posterior surface of the iris; those 

 near the free margin are closely aggregated; those near the periphery of the band are somewhat 

 separated and form incomplete circles.. The radiating fibers form the Dilatator pupilke; they 

 converge from the circumference toward the center, and blend with the circular fibers near the 

 margin of the pupil. 



ANTERIOR CILIARY ARTERIES 



LONG CILIARY 

 ARTERY 



ANTERIOR CILIARY ARTERIES 



FIG. 878. Iris, front view. 



4. The posterior surface of the iris is of a deep purple tint, being covered by two layers of 

 pigmented columnar epithelium, continuous at the periphery of the iris with the pars ciliaris 

 retinae. This pigmented epithelium is named the pars iridica retinae, or, from the resemblance 

 of its color to that of a ripe grape, the uvea. 



The color of the iris is produced by the reflection of light from dark pigment cells underlying 

 a translucent tissue, and is therefore determined by the amount of the pigment and its distribu- 

 tion throughout the texture of the iris. The number and the situation of the pigment cells differ 

 in different irides. In the albino pigment is absent; in the various shades of blue eyes the pigment 

 cells are confined to the posterior surface of the iris, whereas in gray, brown, and black eyes 

 pigment is found also in the cells of the stroma and in those of the endothelium on the front of 

 the iris. 



The iris may be absent, either in part or altogether as a congenital condition, and in some 

 instances the pupillary membrane may remain persistent, though it is rarely complete. Again, 

 the iris may be the seat of a malformation, termed coloboma, which consists in a deficiency or 





