THE LENS. 



THE LENS. 



The lens (lens crystallina) is a transparent solid body, of a doubly convex shape, 

 with the circumference rounded off. It is completely enclosed by a transparent, 

 highly elastic membrane known as the capsule of the lens. The anterior surface is in 



Fig. 72. 1, FRONT VIEW; 2, HIND VIEW; 3, LATERAL VIEW OF THE FIBROUS STRUCTURE OF THE 



ADULT LENS (after Arnold), f 



, anterior ; p, posterior pole. The direction of the superficial fibres is indicated by the curved 

 lines. 



contact with the iris towards the pupil, receding from it slightly at the circumference ; 

 the posterior is embedded in the vitreous humour. Around the circumference is the 

 zonula. The capsule is strongest anteriorly (anterior capsule) and thinnest over the 



Fig. 73. DIAGRAM TO ILLUSTRATE THE COURSE OF THE FIBRES IN THE 

 POSTAL CRYSTALLINE LENS. (Allen Thomson.) 



a, anterior ; p, posterior pole. 



posterior surface of the lens (posterior capsule). Chemically 



the lens-capsule yields neither elastiu nor gelatin, but appears 



similar in composition to the sarcolemma of muscle and the 



membranae propriae of glands. An indistinct fibrous and 



lamellar structure has been described in it. The convexity 



of the lens is not alike on the two surfaces, being greater behind ; 



moreover, the curvature is less at the centre than towards the 



margin. When in its natural position it measures about 8mm. 



to 9mm. across, and about 4mm. from before backwards. The radius of curvature 



during life of the anterior surface varies with the condition of accommodation from 



about 10mm. when the eye is accommodated for distant vision, to Gmm. when 



accommodated to the near point of distinct vision. That of the posterior surface is 



about Gmm. in distant vision and a little less in near vision. In a fresh Jens, 



divested of its capsule, the outer portion is soft and easily detached ; the succeeding 



layers are of a firmer consistence ; and in the centre the substance becomes much 



harder, constituting the so-called " nucleus." On the anterior and posterior surfaces 



are faint white lines directed from the poles towards the circumference ; these in the 



