TUNICA FIBROSA OCULI. 



449 



half of the canal-wall, and presents a corresponding circular sulcus, 

 the so-called inner scleral sulcus. 



The blood-vessels of the sclera are derived from the anterior and 

 posterior ciliary vessels. The capillaries enter either into the ciliary 

 veins or into the venae vorticosae. The numerous remaining vessels 

 traverse the sclera, extending to the choroid, iris, or scleral margin. 

 At the corneal margin the capillaries form loops. 



Cornea! 

 epithelium. 



2. THE CORNEA. 



The cornea is made up of the following layers : (i) the ante- 

 rior or corneal epithelium ; (2) the anterior elastic membrane, or 

 Bowman's membrane ; (3) the ground-substance of the cornea, or 

 substantia propria ; (4) Des- 

 cemet's membrane ; (5) the 

 endothelium of Descemet's 

 membrane. 



At the center of the 

 human cornea the epithe- 

 lium consists of from six to 

 eight layers of cells, being 

 somewhat thicker near the 

 corneal margin. Its basilar 

 surface is smooth and there 

 are no connective-tissue pa- 

 pillae. The basal epithelial 

 layer is composed of cylin- 

 dric cells of irregular height ; 

 the following layers contain 

 irregular polygonal cells, 

 while the two or three most 

 superficial layers consist of 



flattened cells. The cells of the corneal epithelium are all provided 

 with short prickles, which are, however, very difficult to demon- 

 strate, and between are found lymph-canalicufi. The lower surfaces 

 of the basal cells also possess short processes which penetrate into 

 the anterior basement membrane. 



In man the anterior elastic or Bowman's membrane is quite 

 thick, measuring from 6 to 8 // in thickness and is apparently homo- 

 geneous, but may be separated into fibrils by means of certain 

 reagents. In structure it belongs neither to the elastic nor to the 

 white fibrous type of connective tissue, and may be regarded as a 

 basement membrane. Numerous nerve-fibers penetrate its pores to 

 enter the epithelium. The thickness of this membrane decreases 

 toward the sclera, and it finally disappears about I mm. from the 

 latter. 



The substantia propria consists of connective-tissue fibrils 

 grouped into bundles and lamellae. Chemically they do not differ 

 29 



Fig. 353. Section through the anterior portion 

 of human cornea ; X 5- 



