Text-book of ophthalmology . yer too are filled with pigmentgranules. Thus the two layers in conjunction form a uniformly pigmentedstratum, which, as the pars iridica retina? (retinal pigment layer of the iris), DISEASES OF THE RETINA 569 covers the posterior surface of the iris up to the margin of the pupil, aboutwhich the two layers turn and then unite. 512. Retinal Vessels.—The retina has its own system of blood-vessels,which is almost entirely separate from the adjoining system of ciliary ves-sels. It is formed by an expansion of the arteria and vena centralis nervioptici, which break up i
Text-book of ophthalmology . yer too are filled with pigmentgranules. Thus the two layers in conjunction form a uniformly pigmentedstratum, which, as the pars iridica retina? (retinal pigment layer of the iris), DISEASES OF THE RETINA 569 covers the posterior surface of the iris up to the margin of the pupil, aboutwhich the two layers turn and then unite. 512. Retinal Vessels.—The retina has its own system of blood-vessels,which is almost entirely separate from the adjoining system of ciliary ves-sels. It is formed by an expansion of the arteria and vena centralis nervioptici, which break up into branches in the optic papilla. These branchessubdivide in the retina as far as the ora serrata without anastomosingtogether (see Fig. 153, a, ah and b,bl; Fig. 20 represents the branching of ves-sels in the retina as seen with the ophthalmoscope). At the papilla alone,minute communications exist between the retinal and the ciliary vessels (seepage 382). The retinal arteries are hence to be regarded as terminal arteries. Large loops. Non-vascularregion of fovea. [Fig. 246.—Blood-vessels of the Yellow Spot Injected. After Bohm and Von Davidoff. (.Norris and Oliver.)—D.] (Cohnheim). Consequently, disturbances of circulation in the retina, aris-ing from contraction or plugging of a vessel, cannot be compensated for bymeans of a collateral circulation. Within the retina, the vessels lie only in the inner layers, so that theexternal layers of the retina are non-vascular, and are hence in part depend-ent for their nutrition upon the neighboring chorio-capillaris. This is espe-cially true of the fovea centralis, the central part of which contains no vesselsat all, while, on the other hand, the vascular network of the chorio-capil-laris is here particularly dense. 513. Function of the Retina.—The objects of the outer world throwtheir images upon the retina. It is the function of the latter to convert therays of light, of which the images are composed, into nervous
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