LENS. 



365 



nerve. Thus the optic nerve differs from the peripheral nerves and 

 resembles a cerebral commissure. 



At the posterior surface of the eye the dura blends with the sclera. 

 Continuous with both is the dense elastic lamina cribrosa which is per- 

 forated by the optic nerve fibers. The chorioid and the pia are also in 

 relation with the lamina (Fig. 416). As the optic nerve passes the lamina, 

 its fibers lose their myelin and radiate into the nerve fiber layer of the retina. 

 The central artery and vein of the retina enter the optic nerve in its distal 

 half, and appear at the fundus of the eye in the center of the optic papilla. 

 Their branches spread in the inner layers of the retina, outside of the 

 membrana limitans interna. 



LENS. 



The lens is a biconvex structure having an anterior and a posterior 

 pole, and a vertical equatorial plane. It is enclosed in a thick transparent 

 elastic capsule which is 6.5-25 t a thick in front, and 2-7 fj. thick behind. 

 Within the capsule the anterior surface of the lens is formed by the lens 

 epithelium, a single layer of cells 2.5 n thick at the pole but becoming taller 



..., , 



FIG. 417. LENS FIBERS OP A NEW-BORN 



INFANT. 

 A, Isolated lens fibers, three with smooth, 



one with dentate borders. X 240. 



B, Human lens fibers cut transversely; 



c, section through club shaped ends. 



X 560. 



FIG. 418. CAPSULE AND EPITHELIUM OF A LENS OF 

 ADULT MAN. 



C, Inner aspect. D. Lateral aspect, from a meridional 

 section through the equator of the lens; i, cap- 

 sule; 2, epithelium; 3, lens fibers. X 240. 



at the equator. There they are continuous with the elongated lens fibers 

 of the posterior layer, which collectively are called the substantia lentis. 

 New fibers are formed by the mitosis of cells at the periphery of the pos- 

 terior layer. The lens fibers are generally six-sided prisms, somewhat 

 enlarged at one or both ends. The central fibers have lost their nuclei; 

 their boundaries are wavy or notched. These, which were the first to 



