EYE 



451 



dense connective tissue with many elastic fibers. The outer connective 

 tissue bundles tend to be longitudinal and the inner circular. Internally 

 the outer sheath is connected with the arachnoid layer by a few dense 

 strands of tissue, and the arachnoid joins the pial sheath by many branched 

 trabeculae. The pia surrounds the entire nerve and sends anastomosing 

 septa among the bundles of nerve fibers. The latter are slender and 

 medullated, but without a neurolemma; they are supported by long- 

 rayed neuroglia cells, which are found between the individual fibers, but 

 are most numerous at the periphery of the bundles and around the entire 

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

 resembles a cerebral commissure. 



At the posterior surface of the eye-ball (or bulbus oculi}, the dura 

 blends with the sclera. Continuous with both is the dense elastic lamina 

 cribrosa which is perforated by the optic nerve fibers. The chorioid and 

 the pia are also in relation with this lamina (Fig. 458). As the optic 



Central artery. 

 Fibers of the lamina cribrosa. | Central vein. 



Hyaloid membrane / 

 loosened. 



Bundles of the optic nerve 



Pial sheath. 



Arachnoidal sheath 



Dural sheath 



W m 



FIG. 458. LONGITUDINAL SECTION OF THE OPTIC ENTRANCE OF A HUMAN EYE. X is. 



Above the lamina cribrosa is seen the narrowing of the optic nerve, due to its loss of myelin. The central 

 artery and vein have been for the most part cut longitudinally, but above at several points trans- 

 versely. 



nerve penetrates 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, which are covered by the membrana limitans 

 interna (Fig. 453). 



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 



