THE 



831 



the aqueous humor behind the cornea, and in front of the lens, being perforated a 

 little to the nasal side of its centre by a circular aperture, the pupil, for the trans- 

 mission of light. By its circumference it is continuous with the ciliary body, and 

 is also connected with the posterior elastic lamina of the cornea by means of the 

 pectinate ligament ; its inner or free edge forms the margin of the pupil ; its sur- 

 faces are flattened, and look forward and backward, the anterior toward the cornea, 

 the posterior toward the ciliary processes and lens. The anterior surface of the 

 iris is variously colored in different individuals, and marked by lines which con- 

 verge toward the pupil. The posterior surface is of a deep purple tint, from being 

 covered by two layers of pigmented, columnar epithelium, which are continuous 

 posteriorly with the pars ciliaris retinse. This pigmented epithelium is termed the 

 l>nrs indica refiner, though it is sometimes named uvea, from its resemblance in 

 color to a ripe grape. 



Circ. fibres 

 of sclerotic. 



Circ. fibres / 

 of Ciliary muscle 



Radiating 



fibres of 



Oil tin i/ muscle. 



__ f Pars ciliaris 

 \ retinx. 



FIG. 444. Section of the eye, showing the relations of the cornea, sclerotic, and iris, together with the 

 Ciliary muscle and the cavernous spaces near the angle of the anterior chamber. (Waldeyer.) 



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-membrane. This layer is continuous with the epithelial layer covering 

 the membrane of Descemet, and in men with dark-colored irides the cells contain 

 } > i gin ent-granules. 



2. Stroma. The stroma consists of fibres and cells. The former are made of 

 fine, delicate bundles of fibrous tissue, of which some few fibres have a circular 

 direction at the circumference of the iris, but the chief mass consists of fibres 

 radiating toward the pupil. They form, by their interlacement, a delicate mesh, 

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

 of connective tissue are numerous branched cells with fine processes. Many of 

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

 of albinos they are unpigmented. 



3. The muscular fibre is involuntary, and consists of circular and radiating 

 fibres. The circular fibres (sphincter papillae) 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 in width, those near the free margin being closely aggre- 



