THE LENS AND ZONULE 21 



tion of the capsule is thicker than the posterior in most animals. This 

 is, however, open to interpretation as a positive adaptation rather than 

 a mere accident of difference in secretory capacity, for as will be seen 

 later, accommodation is facilitated by a thick anterior capsule but would 

 be indifferent toward an equally thick posterior capsule. 



The lens epithelium is the source of all of the myriads of lens fibers 

 excepting a very small ball of them at the center of the lens, which are 

 formed directly from the posterior wall of the embryonic lens vesicle — 

 a bubble of tissue which forms as a pit in the skin of the head, from 

 which it closes off and pinches free to sink down into the optic cup. If 

 the lens epithelium could be isolated intact, it would be like a thin, 

 shallow bowl composed of tiny tiles. We must imagine this bowl to be 

 growing constantly by the multiplication of its tiles, with those at the 

 edge of the bowl elongating into rods which, as they get longer and 

 longer, each slide one end along the inner surface of the bowl toward 

 its center, the other end growing in the opposite direction and curving 

 toward a point in space above the center of the bowl. It is in this fashion 

 that each new layer of lens fibers is added over the preceding one (Figs. 

 40 and 41), by the conversion of the epithelial cells at the equator of 

 the lens into long, curved threads which are hexagonal in cross-section 

 so as to fit against one another without intervening space, just as do the 

 cells of the epithelium itself. Any given cell in the epithelium of a grow- 

 ing lens is thus moved steadily toward the lens equator by the mitotic 

 expansion of the epithelium, and, upon finding itself eventually at the 

 rim of the epithelial bowl, proceeds to convert itself into a lens fiber. 



The lens is very prone to opacify, thus giving rise to 'cataract', in re- 

 sponse to any of a number of causes; but it is normally optically empty — 

 that is, completely transparent and with no obvious signs of its elab- 

 orate internal structure. With special lighting arrangements, as with the 

 ophthalmologist's slit-lamp, it is possible to see several concentric sur- 

 faces within the lens, analogous to growth-rings in a tree trunk. These 

 mark periods in life — the same in all of us — at which the optical density 

 of the new-forming lens fibers is changed abruptly to a lower value than 

 that of the previously formed layers of fibers. Thus the optical density of 

 the lens — its effectiveness in slowing the speed of light and hence its 

 focusing power — decreases in several distinct steps from center to sur- 

 face. In a given region, however, the density of the fibers and of '"he 

 scant fluid between them is so nearly identical that the surfaces of the 



