THE FIBROUS TUNIC. 



339 



Epithelium 



similarly covered with endothelial plates, and forms part of the lining of 

 Tenon's lymph-space. Anterior to the muscle-insertions it is covered with 

 a loose-meshed connective tissue, the episcleral tissue, richly supplied with 

 blood-vessels, nerves and lymph-vessels, and continuous with the subcon- 

 junctival tissue of the conjunctiva sclerce. 



The blood-vessels of the sclera arise from the arteries which perforate it 

 to supply the vascular coat of the eye. They form a wide-meshed network 

 on the surface of the sclera, which sends anastomosing vessels to a deeper 

 lying set in the scleral substance. In the neighborhood of the optic nerve 

 entrance, the branches of the short posterior ciliary arteries form an arterial 

 circle, the circulus Zinni, which sends branches to the optic nerve and cho- 

 roid, and is, therefore, of great importance in establishing an anastomosis 

 between the choroidal circulation and the arteria centralis retina which sup- 

 plies the retina. The veins of the sclera empty into the anterior and posterior 

 ciliary veins, and into the 

 venae vorticosae. At the 

 junction of the cornea and 

 sclera is an important cir- 

 cular venous channel, the 

 canal of Schlemm, which 

 will be described later. 



The Cornea. The 

 cornea forms the anterior 

 one-fifth of the fibrous 

 tunic of the eyeball, and, 

 although composed, like 

 the sclera, of bundles of 

 connective tissue, is trans- 

 parent and allows rays of 

 light to enter the eyeball. 



The cornea is com- 

 posed of five distinct layers, 

 which from without in are: 



1 i ) the anterior epithelium, 



(2) the anterior limiting 

 membrane, (3) the substan- 

 tia propria, (4) the poste- 

 rior limiting membrane, 

 and (5) the endothelium. 



The anterior epithe- 

 lium of the cornea is con- 

 tinuous with that covering 

 the surface of the adjacent scleral conjunctiva. 



Corneal cell 



^^~*=^~^~1 Posterior 

 ~~^- -Tr,-.^.-'- limiting 

 sy^^g^^^*! membrane 

 *^g-t*J >n i Endothelium 



FIG. 385. Section of human cornea. X 85. 



It is of the stratified 



squamous variety, usually five cells deep in man, and measures 45 /* in thick- 

 ness at the centre, and 80 jj. at the periphery. The deepest cells are columnar 

 in form, with broad bases resting upon the anterior limiting membrane, to 

 which they are firmly attached by means of minute projections that roughen 

 the anterior surface of the latter. The outer parts of the basal cells contain 

 the nucleus and fit into corresponding depressions in the cells of the super- 

 imposed layers. The middle layers are composed of irregular polyhedral 

 cells, possessed of fine protoplasmic intercellular processes. The superficial 

 layers consist of flattened cells which lie parallel to the free surface and 

 contain nuclei. 



