THE EYE AND ITS APPENDAGES. 361 



position, surrounded by connective tissue, which they maintain 

 until their final branching on the papilla. 



The blood-vessels of the retina constitute an independent sys- 

 tem composed of end-arteries ; the only communication between 

 the retinal and ciliary vessels is established within the sclera, close 

 to the optic nerve, by means of minute scleral and choroidal 

 branches. The larger retinal vessels are situated within the 

 inner part of the fibre -layer and supply twigs to the cerebral 

 division alone, the epithelial portion being non-vascular and 

 deriving its nutrition from the adjacent choriocapillaris. 



The capillaries are arranged as two net- works, an inner and 

 an outer. The inner net-work lies within the fibre-layer, is wide- 

 meshed and derived directly from the division of the retinal vessels ; 

 the outer net-work, situated within the inner nuclear layer, is 

 dependent upon the former, since its capillaries are derived from the 

 branches given off from the inner vascular reticulum. The retinal 

 arteries and veins are surrounded by adventitious sheaths, the 

 spaces included between these sheaths and the walls of the vessels 

 constituting perivascular lymph- clefts. 



THE CRYSTALLINE LENS. 



The crystalline lens comprises two genetically distinct portions, 

 the lens-substance and the lens-capsule. 



The lens-substance consists of the epithelium of the lens and 

 the lens -fibres both epithelial structures directly derived from the 

 invaginated ectoderm. 



The epithelium of the lens, the representative of the anterior 

 wall of the primary lens-vesicle, consists of a single layer of low 

 polyhedral cells, about 20 /* in diameter, whose granular proto- 

 plasm contains an oval nucleus, also often vacuoles. These cells lie 

 immediately beneath the anterior capsule and extend backward 

 as far as the equator, at which point the epithelial cells are trans- 

 formed into the lens-fibres. A thin subcapsular stratum of 

 albuminous substance exists as a connecting medium between the 

 epithelium and the capsule, the same substance being continued be- 

 tween the posterior lens-capsule and the lens-fibres behind. Be- 

 neath the epithelium a subepithelial stratum of somewhat simi- 

 lar albuminous substance unites the epithelium and the lens-fibres 

 and occupies the cleft representing the remains of the original 

 cavity of the lens-vesicle ; sometimes a few drops of fluid the 

 liquor Morgagni occupy this subepithelial stratum. 



The lens-fibres are greatly elongated modified epithelial cells, 

 whose ancestors constituted the posterior wall of the lens-sac, but 

 whose more recently formed fellows result from the transforma- 



