68 THE RETINAL BLOOD VESSELS. [BOOK in. 



of a minute crater. Both artery and vein divide into two main 

 trunks, one directed upwards and the other downwards, the division 

 of the vein taking place while it is still imbedded in the nerve ; 

 each trunk divides again into two, and these into several branches 

 which are distributed over the retina as far as the ora serrata, the 

 branches on the nasal side radiating in a more or less straight 

 direction, while those on the temporal side arch round above and 

 below the macula lutea, veins and arteries taking much the same 

 course. 



Both arteries and veins run close to the internal limiting 

 membrane in the layer of optic fibres, being accompanied by 

 sheaths of delicate connective-tissue enclosing perivascular lym- 

 phatic spaces. The capillaries into which the arteries break 

 up, and from which the veins are gathered up, form in the 

 first instance a somewhat close capillary network between and 

 among the optic fibres and ganglionic cells ; but the vessels also 

 extend a certain distance outward and form a second outer also 

 fairly close capillary network in the inner nuclear layer. Some of 

 the loops may reach the outer molecular layer, and in some few 

 animals capillaries may be seen in the outer nuclear layer. In no 

 case do they extend- beyond the external limiting membrane, and 

 as a rule the outer molecular layer may be taken as marking the 

 limit beyond which they do not extend. 



The arteries and veins sweep, as we have said, round the macula 

 lutea, which however is largely supplied by two small arteries and 

 veins coming straight from the optic disc. Capillaries extend into 

 the margin of the macula, reaching as far as the layers of ganglionic 

 cells and the inner nuclear layer ; from the fovea itself, blood vessels 

 are wholly absent. 



746. The Pigment Epithelium. This is a single layer of 

 epithelium cells lying between the rods and cones of the retina 

 proper on the inside and the limiting membrane of the choroid, 

 membrane of Bruch, on the outside. It is wanting where the 

 optic nerve passes forward to join the retina, but is present, and 

 exhibits the same features over the whole of the retina up to the 

 ora serrata, at which line it passes into the more ordinary pigment 

 epithelium of the ciliary processes. . 



It very readily separates from the choroid, and frequently 

 comes away with the retina when the latter is removed from 

 the eyeball. In such cases the layer is, in a surface view, seen 

 to be composed of cells which have a polygonal outline and are 

 loaded with black pigment granules, the nucleus of the cell being 

 more or less obscured by the pigment, and the outlines of the cells 

 being well defined by clear lines of cement material, apparently 

 neurokeratinal in nature, free from pigment ; a similar appearance 

 is presented by the pigment epithelium of the ciliary processes 

 though the outlines are not so well marked. 



A vertical section however taken through the layer in position 



