THE MIDDLE COAT THE VASCULAR TUNIC 641 



front with the pars iridica retinae. The superficial (innermost) cells 

 present a clear or slightly granular cytoplasm with a centrally situated 

 nucleus. Their cytoplasm is but slightly pigmented, and ofttimes is 

 indistinctly rodded or fibrillated. In shape, these cells are of the low 

 columnar type, but they become progressively flattened toward the iris, 

 where they are continuous with the pars iridica retinse. They represent 

 a continuation of the sustentacular cells of the retina. 



The cells of the deeper (outer or anterior) layer vary in height from 

 a low columnar at the ora serrata to a somewhat flattened cell near the 

 iridal margin, and are continuous with the pigmented cell layer of the 

 retina. This cell layer is deeply pigmented, the entire cytoplasm being 

 filled with the dark brown pigment granules. The nucleus, however, 

 as in the pigmented cells of the choroid, contains no pigment, and 

 therefore, in unstained preparations, appears under the microscope as 

 a clear opening in the dark background of pigmented cytoplasm. 



THE IRIS 



The iris (Figs. 558 and 543) forms an annular curtain which pro- 

 jects from the anterior margin of the ciliary body toward the axis 

 of the eye. It presents a central circular opening, the pupil, which lies 

 in the visual axis. 



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

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

 margin being separated from the lens by an interval, the posterior 

 chamber, which is also filled by the aqueous humor. 



The iris may be said to consist of three layers: 1, the external 

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



The external epithelium (endothelium of the iris) is a mesen- 

 chymal epithelium 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 iridica 

 retinse). 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 continuous with 

 its lymphatic interstices. 



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



