The fovea (fovea centralis) appears as a small (1.5 mm in diameter), shallow depression located at the posterior pole of the optical axis of the eye. Its central region, known as the foveola, is about 200 m in diameter (see Fig.24.14). Except for the photoreceptor layer, most of the layers of the retina are markedly reduced or absent in this region (see Fig. 24.6). Here the photoreceptor is composed entirely of cones (approximately 4,000) that are longer and more slender and rodlike than they are elsewhere. In this area, the retina is specialized for discrimination of details and color vision. The ratio between cones and ganglion cells is close to 1:1. Retinal vessels are absent in the fovea, allowing light to pass unobstructed into the cone’s outer segment. The adjacent pigment epithelial cells and choriocapillaris are also thickened in this region.
The macula lutea is the area surrounding the fovea, approximately 5.5 mm in diameter. It is yellowish because of the presence of yellow pigment (xanthophyll). The macula lutea contains approximately 17,000 cones and gains rods at its periphery. Retinal vessels are also absent in this region. Here the retinal cells and their processes, especially the ganglion cells, are heaped up on the sides of the fovea so that light may pass unimpeded to this most sensitive area of the retina.
Vessels of the Retina
The central retinal artery and central retinal vein, the vessels that can be seen and assessed with an ophthalmoscope, pass through the center of the optic nerve to enter the bulb of the eye at the optic disc (see Fig. 24.2 and page 898,
FIGURE 24.14 • Normal view of the fundus in ophthalmoscopic examination of the right eye. The site where the axons converge to form the optic nerve is called the optic disc. Because the optic disc is devoid of photoreceptor cells, it is a blind spot in the visual field. From the center of the optic nerve (clinically called the optic cup), central retinal vessels emerge. The artery divides into upper and lower branches, each of which further divides into nasal and temporal branches (note the nasal and temporal directions on the image). Veins have a similar pattern of tributaries. Approximately 17 degrees or 2.5 times optic disc diameters lateral to the disc, the slightly oval-shaped, blood-vessel-free, and pigmented area represents the macula lutea. The fovea centralis, a shallow depression in the center of the macula lutea, is also visible. (Courtesy of Dr. Renzo A. Zaldivar.) the section on the development of the eye). The central retinal artery provides nutrients to the inner retinal layers. The artery branches immediately into upper and lower branches, each of which divides again into nasal and temporal branches (see Fig. 24.14). Veins undergo a similar pattern of branching. The vessels initially lie between the vitreous body and inner limiting membrane. As the vessels pass laterally, they also move deeper within the inner retinal layers. Branches from these vessels form a capillary plexus that reaches the inner nuclear layer and therefore provides nutrients to the inner retinal layers (layers 6–10; see pages 907–908). Nutrients to the remaining layers (layers 1–5) are provided by diffusion from the vascular choriocapillary layer of the choroid. The branches of the central retinal artery do not anastomose and therefore are classifed as anatomic end arteries. Evaluation of the retinal vessels and appearance of the optic disc during the ophthalmoscopy not only provides important information on the state of the eye but also provides early clinical signs of a number of conditions, including elevated intracranial pressure, hypertension, glaucoma, and diabetes.