Recent studies, which will be discussed in detail in the chapters on pathology (chapter V), fluorescence angiography (chapter VII) and pathogenesis (chapter VIII), have considerably increased our insight into the development of retinal vein occlusion.
CHAPTER III ANATOMICAL AND PHYSIOLOGICAL ASPECTS OF THE RETINAL CIRCULATIONMany factors can disturb the blood circulation in the eye. Morphologic changes of the vessel wall play an important role in retinal arteriosclerosis, hypertensive retinopathy, occlusion of the retinal veins and senile degeneration of the macula. In order to understand these processes it is necessary to have some knowledge of the anatomy and physiology of the retinal circulation.The metabolism of the retina is served by two vascular systems, i.e. the retinal and the choroidal system which, with the exception of capillary connections at the optic disc, are separate .The retinal vessels supply the inner layers of the retina from the internal limiting membrane to the outer plexiform layer. The choroidal network, which, by diffusion via Bruch's membrane, feeds the outer layers of the retina, i.e. the pigment epithelium, the layer of rods and cones, and the outer nuclear layer, arises from the short posterior ciliary arteries.Both vascular systems arise from the ophthalmic artery. After passing the lamina cribrosa the central retinal artery splits into 2 branches, i.e. the superior and inferior artery, which each split into a nasal and a temporal branch at or outside the margin of the optic disc. A cilio-retinal artery may exist, which arises directly from the choroidal circulation and feeds the papillo-macular area. Figures for the frequency of occurrence of this artery, which in one eye is seldom multiple, are variable. Hayreh (1963) found it in 25% of the eyes he examined, Seitz (1968) in 5-10% and Gass (1968) in 15-20%.The large retinal vessels lie between the nerve fibre layer and the inner plexiform layer of the retina. Each of the arterial branches divides into smaller vessels without anastomoses which finally end in a capillary bed. The retinal capillary bed is a complex network consisting of two layers throughout the greater part of the retina, viz. a superficial and a deep network (Michaelson, 1954). The superficial network lies between the nerve fibre layer and the inner plexiform layer, while the deeper network lies in the outer plexiform layer.In the peripapillary area, where the retina is thickest, three capillary networks are present; in the macular area, at the fovea where there are only three retinal layers (pigment epithelium, layer of rods and cones and outer nuclear layer) there is only one layer of capillaries, the deeper network;
