The postcapillary venules join to become retinal veins that accompany the retinal arteries and exit through the lamina cribrosa and drain into the superior ophthalmic vein. The blood vessel walls are normally transparent. The retinal arteries and veins are interconnected via an extensive capillary network that extends outward to the external border of the inner nuclear layer. The retinal arterial circulation is ordinarily an end-artery system that does not communicate with the blood vessels of the choroid or ciliary body. Occasionally a large cilioretinal artery may supply virtually the entire macula. In approximately 20% of patients Justice and Lehmann found that a variable portion of the papillomacular area was supplied by one or more cilioretinal arteries derived from the ciliary circulation ( Figure 1.02A ). The right-angle branches are referred to as first-order arterioles and venules ( Figure 1.02A ). The branching is predominantly dichotomous as they course peripherally. They give off arteriolar and venular branches that posteriorly occur primarily at right angles to the parent vessel. These major blood vessels lie in the nerve fiber layer close to the internal limiting membrane of the retina. The corresponding retinal venous branches have much the same distribution as the arteries. These trunks divide into two branches, one supplying the nasal and the other the temporal quadrant of the retina. The blood supply to the inner half of the retina is by way of the central retinal artery, which usually divides into a superior and inferior trunk within the optic nerve head. The foveal depression can be visualized with the narrow slit-lamp beam. In younger patients an oval or round halo light reflex at the inner retinal surface may correspond with the foveal margin ( Figure 1.02B ). The margins of the capillary-free zone of the retina that in most patients measures approximately 500 μm in diameter angiographically can only be estimated biomicroscopically because the perifoveolar capillary network is not visible. There are no consistent ophthalmoscopic landmarks to indicate the margins of either the 0.35-mm diameter foveola or the 1.5-mm diameter fovea. The foveal reflex, which is present in most normal eyes, appears to lie just in front of the center of the foveola and therefore overlies the anatomic umbo. The center of the macula appears as a poorly defined, one-fourth to one disc diameter size zone of greater pigmentation that is maximum in the foveolar area. Ophthalmoscopically, the anatomic subdivisions of the macula are ill defined ( Figure 1.02 ). This zone is in turn surrounded by a 1.5-mm zone referred to as the perifoveal area. A 0.5-mm-wide ring zone where the ganglion cell, inner nuclear layer, and outer plexiform layer of Henle are the thickest is called the parafoveal area. A small depression in the center of the foveola is called the umbo. It lies within the capillary-free zone or foveal avascular zone, which measures approximately 0.5 mm (500 μm) in diameter in most patients. It measures approximately 0.35 mm in diameter. The central floor of the fovea is called the foveola. It measures approximately 1.5 mm (1500 μm) or one disc diameter in size. The fovea (fovea centralis) is a depression in the inner retinal surface in the center of the macula. On the basis of microscopic anatomy, the macular area can be further subdivided into several zones. It measures approximately 5.5 mm in diameter and is centered approximately 4 mm temporal to and 0.8 mm inferior to the center of the optic disc ( Figure 1.01 ). Anatomically the macula (macula lutea or central retina) is defined as that portion of the posterior retina that contains xanthophyll and two or more layers of ganglion cells. The retina is a delicate transparent tissue of maximal thickness (approximately 0.55 mm) at the foveal margin and minimal thickness (0.13 mm) at the umbo.
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