1468 



HUMAN ANATOMY. 



emerges slightly to the nasal side of the centre of the optic disc. Here the artery 

 divides into two main stems (Fig. 1 225), the superior and inferior papillary branches, 

 each of which subdivides at or near the disc-margin into superior and inferior 

 nasal and temporal branches which run respectively mesially and laterally, dividing 

 dichotomously as end arteries, no anastomosis existing. The macular region is 

 supplied by special macular branches, the center of the fovea, however, being free 

 from blood-vessels. The larger branches from the central artery course within the 

 nerve-fibre layer, and send fine twigs peripherally inward to form an inner and an 

 outer plexus, the former on the outer surface of the inner plexiform layer, and the 

 latter within the inner nuclear layer. Beyond the outer plexiform layer the vessels 

 do not penetrate, the visual cells being dependent for their nourishment upon the 

 choriocapillaris of the choroid. At the nerve entrance an indirect communication 

 exists between the arteria centralis and the posterior ciliary arteries, through the 

 medium of the small branches which constitute the circulus arteriosus Zinni. 



FIG. 1225. 



Temporal 



Nasal 



Normal fundus of right eye as seen with ophthalmoscope ; central retinal vessels seen emerging from optic 

 nerve; arteries are lighter, veins darker vessels; fovea centralis shows as light point in macular region, which 

 lies in temporal field and is devoid of large vessels. 



The lymphatics of the retina are represented chiefly by the perivascular lym- 

 phatic spaces which surround all the veins and capillary blood-vessels. These spaces 

 may be injected from the subpial lymph-space of the optic nerve, and by the same 

 method communications may be demonstrated between (i) this space and the 

 interstices between the nerve bundles which converge toward the optic papilla, 

 (2) a space between the membrana limitans interna and the hyaloid membrane of 

 the vitreous, and (3) a narrow cleft between the pigmented cells and the layer of 

 rods and cones. 



Practical Considerations. All pathological conditions of the retina ap- 

 pear as opacities, and thus interfere with sight. The medullary sheaths of the optic 

 nerve-fibres end at the lamina cribrosa. Rarely the sheaths around these may 

 extend some distance into the retina, showing as a white striated margin around 

 the optic disc and continuous with it. Sometimes the blood-vessels of the retina 

 may enter at the margins of the optic disc, instead of at its centre, as usual, which 

 is then free of vessels and very pale. At the entrance of the optic nerve, the 

 transparency of the retina is lessened by the thickening of its fibre-layer 



