VISION. 



773 



passes forward, and is weakest about two lines from the cornea; the 

 anterior portion again increases in thickness. 



CORNEA. It is joined to the sclerotic coat by direct continuity 

 of tissue. The tissue of the cornea absorbs water readily and becomes 

 opaque after death. The cornea has five layers. The first, or anterior, 

 epithelial layer is composed of several layers of epithelial cells; the 

 deepest are cylindrical, which pass over into the lower polygonal 

 cells, which, on the surface, become flat, nucleated cells. At the edge 

 of the cornea this epithelium becomes continuous with that of the 

 conjunctiva. Second layer: the anterior elastic lamina (of Bowman) 



Fig. 322. Anterior-posterior Section of the Eyeball. (LEVEILLE.) 



1, Optic nerve. 2, Sclerotic. 3, Cornea. 4, Spaces of Fontana. 5, Choroid. 

 6, Ciliary muscle. 7, Ciliary processes. 8, Iris. 9, Retina. 10, Jacobs's mem- 

 brane. 11, Anterior chamber. 12, Posterior chamber. 13, Pupillary area. 14, 

 Aqueous humor. 15, Hyaloid membrane. 16, Canal of Stilling. 17, Canal of 

 Petit. 18, Vitreous humor. 19, Capsule of the lens. 20, Fluid of Morgagni. 

 21, Lens. 



is formed by the superficial part of the proper structure of the cornea, 

 which is denser than the rest of the tissues and free from cor- 

 puscles. This layer is strongly developed in man and is a homogene- 

 ous refractive membrane. Fibrils can he demonstrated in it by 

 means of certain reagents. Third layer: the substantia propria, or 

 the cornea proper, forms the main mass of the cornea. It consists 

 of fibrils of connective tissue bound together in flat lamellae (about 

 60 in number). The fibrils run in various directions, and cross each 

 other at various angles. Between the lamellae are canals and spaces 

 which contain a serous fluid. In these spaces are found the connec- 

 tive-tissue cells, having many processes and large nuclei, around 

 which the serous fluid trickles to carry nutriment to the surround- 



