TRANSPARENT MEDIA. 



561 



though the form elements of the original mucous tissue have 

 nearly all disappeared. 



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

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



FIG. 217. 



o 



Diagram of lens at different periods of life. a, At birth ; 6, Adult; c, Old age. (Allen 



Thomson.) 



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 

 but little denser than a stiff jelly. It is inclosed by a firm, elastic 



FIG. 218. 



Showing early stages of the development of the lens. c, Epithelial tissue going to form 

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



capsule, which is drawn tightly over the anterior surface, and 

 influences its shape. The lens is held in its position by a thick- 

 ened part of the soft, elastic hyaloid membrane, called the sus- 

 p&asory ligament, which is attached to the anterior surface of the 



