630 



THE EYE 



THE CORNEA 



The cornea is a concavoconvex, transparent, colorless disk of approxi- 

 mately equal thickness (1 mm.) throughout all its portions. It is nearly 

 circular in outline, its horizontal exceeding its vertical diameter by only 



FIG. 539. FROM A MERIDIONAL SECTION OF THE HUMAN CORNEA. 



a, anterior corneal epithelium; 6, anterior homogeneous membrane; c, substantia 

 propria; d, posterior homogeneous membrane; e, posterior corneal epithelium. 

 Hematein and eosin. Photo. X 180. 



0.5 mm.; its external surface is convex, its internal surface concave. The 

 cornea forms the anterior one-fourth of the tunica externa, and repre- 

 sents a spheroidal segment whose radius is somewhat shorter than that 

 of the posterior segment of the eyeball. It is inserted into the anterior 

 margin of the sclera much after the manner in which a watch-glass is 



