456 HISTOLOGY 



prolongations of the stromatic network, the polygonal meshes of which 

 are radially elongated. The dilatator pupilla is a peculiar membrane 

 of smooth muscle fibers on the posterior surface of the vascular 

 layer, stretching from the connective tissue between the muscle 

 bundles of the sphincter, to that between the ciliary muscles. Its fibers 

 consist of an anterior contractile portion, and a posterior nucleated and 

 pigmented portion. The anterior parts form a continuous layer, readily 

 seen in radial sections as "Henle's spindle cell layer," which is a clear 

 non-nucleated stripe, 2-5 p wide (Fig. 463). The nucleated portions of 

 the fibers appear to blend with the pigmented retinal layer of the iris, 

 from which they are derived. These muscles are therefore ectodermal. 



The two layers of the optic cup are intimately blended in the thin 

 stratum which forms the posterior layer of the iris. Except in albinos, 

 this pars iridica retinae is deeply pigmented. Posteriorly it is covered 

 by a continuation of the hyaline membrane of the pars ciliaris. 



TUNICA FIBROSA. 



The sclera, toward the chorioid, is bounded by the pigmented lamina 

 fusca. This is a loose tissue containing branched pigment cells and 

 flattened connective tissue cells. Except for this boundary layer, the 

 sclera consists of densely interwoven bundles of connective tissue, chiefly 

 meridional and longitudinal. Elastic fibers accompany the bundles, and 

 are 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, and anteriorly the cornea and sclera are continuous with one 

 another. The transition, however, is quite abrupt and the boundary is 

 oblique, so that the rim of the cornea is bevelled at the expense of its an- 

 terior surface. 



The cornea (Fig. 464) 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 0.03 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 o.oi mm. thick. 

 Superficially it connects with the epithelial cells by spines and ridges. 

 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. 



The substantia propria consists of fine straight fibrils of connective 



