STRUCT [JRE OF THE LENS. 



G25 



whicli covers the whole anterior surface underneath the capsule. To- 

 wards the edge or equator of the lens the appearance and character 

 of these cells undergo a change : they first gradually take on a columnar 

 form, and then, becoming more and more elongated, present every 

 transition to the nucleated lens-fibres of the superficial layers, into 

 which they are directly continuous. This transition is more gradual 

 find easily traced in the lens of some animals than in man (see fig. 438). 



Fi^. 438. 



438. — Section through the Margin op the Rabbit's Lens, sHo-nrNa the 

 Transition op the Epithelium into the Lens-fibres (Babucliin). 



The capsule of the lens is a transparent, structureless membrane, 

 somewhat brittle and elastic in character, and when ruptured the edges 

 roll outwards. It is very permeable to fluid, and in chemical reactions 

 appears to be intermediate between elastic and white connective tissue, 

 for it is slowly acted on by acids and dissolves by long boiling in water, 

 but the solution does not gelatinize in the cold. The body of the lens 

 itself contains about 60 per cent, water, and 30 per cent, solid matter ; 

 chiefly albuminoids. 



The fore part of the capsule, from about -^ inch from the circumference, 

 ■where the suspensoiy ligament joins it, is much thicker than the back : at the 

 posterior ijole of the lens the capsule is very thin indeed. In the adult it. like 

 the lens itself, is entu'ely non-vascular, but in the foetus there is a network of 

 vessels in the capsule, sujiplied by the tenninal branch of the arteria cen- 

 tralis retinfc, which passes from the optic papiUa throug-h the canal of Stilling' 

 in the vitreous humour to reach the back of the capsule, where it divides 

 into radiating branches. After fonning a fine network, these turn round the 

 margin of the lens and extend forwards to become continuous with the vessels 

 ha the pupillary membrane and ii"is. 



After death a small quantity of fluid (liquor Morgagni) frequently collects 

 between the back of the lens and the capsule : it appears to be derived from 

 the breaking dowTi of the lens fibres. There is no epithelium in this situation, 

 as in front. 



Changes in the lens by age. — In the 



foetus, the lens is nearly spherical (fig. 439, a) : 

 it has a slightly reddish colour, is not perfectly 

 transparent, and is softer, and more readily 

 broken down than at a more advanced age. 



In the adult, the anterior surface of the lens 

 becomes more obviously less convex than the 

 posterior (fig. 439, b) ; and the substance of the 

 lens is firmer, colourless, and transparent. 



In old age, it is more flattened on both 

 ■surfaces (r); it assumes a yellowish or amber 

 tinge, and is apt to lose its transparency as 

 it gradually increases in toughness and specific 

 ffravitv. 



Fig. 4.39. 

 a h O 



Fig. 489. — Side View op' 

 the Lens at Diffkeenj? 

 Ages. 



a, at birth with the 

 deepest convexity ; b, in 

 adult life with medium 

 convexity ; c, in old age 

 with consideraljle flatten- 

 ing of the curvatures. 



s s 



