TRANSPARENT MEDIA. 



559 



roid. The vitreous humor is developed from the young connec- 

 tive tissue of the mesoblast, and we find in the adult that mucus 

 is the most striking chemical substance in its texture, though the 



FIG. 217. 

 a, Z> 



o 



Diagram of lens at different periods of life. a, At birth ; b, Adult ; 

 c, Old age. (Allen Thomson.) 



form elements of the original mucous tissue have nearly all dis- 

 appeared. 



The most important of the dioptric media is the crystalline lens. 

 It is placed between the aqueous and the vitreous humors, just 



FIG. 218. 



Showing early stages of the development of the lens. c. Epithelial tissue 

 going to form lens; o, Optic cup; a, Epidermis. (Cadiat.) 



behind the iris, which lies in contact with its anterior surface. 

 It is like a strong magnifying glass, biconvex in shape, the poste- 

 rior surface being more convex than the anterior. The lens is 

 much harder than the vitreous humor, but its outer layers are 



