THE SENSE OF SIGHT 695 



the former. The radius of curvature of the anterior surface at that central 

 portion ordinarily used in vision is 7.829 mm.; that of the posterior surface 

 about 6 mm. 



The substance of the cornea is made up of thin layers of delicate trans- 

 parent fibrils of connective tissue continuous with those found in the sclera. 

 Lymph-spaces are present throughout the cornea, in which are to be found 

 lymph-corpuscles. The anterior surface of the cornea is covered with several 

 layers of nucleated epithelium supported by a structureless membrane, the 

 anterior elastic lamina. The posterior surface also is covered by a layer of 

 epithelium supported by a similar membrane, the posterior elastic lamina or 

 the membrane of Descemet, which at its periphery becomes continuous 



Visual axis Optic axis 



Equatorial \ diameter 



Optic nerve. -'- 



FIG. 283. THE RIGHT EYE VIEWED IN HORIZONTAL SECTION. (After Toldt.) 



with the iris. At the junction of the cornea and sclera there is a circular 

 groove, known as the canal of Schlemm. 



The posterior elastic lamina, near the margin of the cornea, breaks up 

 into fibers to form a network structure, the intervals between the fibers of 

 which are known as the spaces of Fontana. These spaces are in communi- 

 cation with the canal of Schlemm. 



The Chorioid, Iris, Ciliary Muscle, and Ciliary Processes. The 

 Chorioid. The chorioid is the dark brown membrane which extends forward 

 nearly to the cornea, where it terminates in a series of folds, the ciliary 

 processes. Posteriorly, it is pierced by the optic nerve. It is composed 

 largely of blood-vessels, arteries, capillaries, and veins, supported by con- 

 nective tissue. Externally it is loosely connected to the sclera; internally it 

 is lined by a layer of hexagonal cells containing black pigment which, 



