IRIS. 369 



of a network of stellate cells in part pigmented. It is followed by a vas- 

 cular layer of fine loose connective tissue with few elastic fibers. Its 

 stellate cells, which in blue eyes are not pigmented, form elongated polyg- 

 onal meshes. The vessels are radial, and have a thick connective tissue 

 externa but a very weak circular musculature. Among the vessels near 

 the free border of the iris, there are smooth muscle fibers which form a 

 band i mm. wide encircling the pupil. This is the sphincter muscle of 

 the pupil. A few radial muscle fibers also occur among the vessels. 

 The dilator muscle of the pupil is behind the vascular layer. It is a con- 

 tinuous layer of radially arranged smooth muscle fibers, beginning near the 

 pupil and extending to the ciliary body. The contractile portion of the 

 spindle shaped muscle cells forms a membrane-like layer resting against 

 the pars iridica retinae, with which the pigmented nucleated portion of 

 the cells seems to unite. These muscle cells have been thought to arise 

 from the outer layer of the retinal cup. Except in albinos both layers 

 of the retina are here heavily pigmented, and apart from their embryo- 

 logical development they would be regarded as a single layer. 



TUNICA FIBROSA. 



The sclera consists of interwoven bundles of connective tissue, chiefly 

 meridional and longitudinal. Elastic tissue accompanies the bundles and 

 is especially abundant at the insertions of the ocular muscles. The flat 

 irregular cells of the connective tissue are surrounded by tissue spaces as 

 in the cornea. Next to the chorioid, the sclera forms a pigmented lamina 

 fusca which has already been described. The sclera becomes thinner 

 anteriorly where it is absolutely continuous with the transparent cornea. 

 The corneal boundary is oblique, being bevelled at the expense of its 

 anterior surface. 



The cornea (Fig. 422) consists of an outer epithelium, external 

 basal membrane, substantia propria, internal basal membrane, and mesen- 

 chymal epithelium bounding the anterior chamber. The corneal epithe- 

 lium, about .03 mm. thick, is stratified and consists of a basal layer of 

 clearly outlined columnar cells followed by three or four rows of cuboidal 

 cells and several layers of flattened superficial cells. The outer cells 

 retain their nuclei. Peripherally the epithelium is continuous with that 

 of the conjunctiva bulbi. The anterior basal membrane [Bowman's] is 

 an almost homogeneous layer, sometimes as much as .01 mm. thick. 

 Superficially it connects with the epithelial cells by bands and processes. 

 Beneath it blends with the substantia propria, of which it is a modification. 

 Since it is not formed of elastic substance the name "anterior elastic mem- 

 brane" is not justified. 

 24 



