818 



THE OKGANS OF SENSE. 



for their support; in this part of each fibre there is an oval nucleus. In the outer 

 nuclear layer they break up into a network of fibrils which surround the rod and cone 

 granules and fibres, and end externally at the bases of the rods and cones in a delicate 

 membrane, the membrana limitans externa. 



Structure of the macula lutea and fovea centralis. The yellow colour 

 of the macula is due to the presence of pigment in the inner layers of the retina. At the 

 circumference of the macula the nerve-fibre layer is greatly thinned and the rods are few 

 in number; the ganglionic layer, on the other hand, is thickened and may contain 

 from seven to nine strata of cells, while the outer granular layer also is thicker and its 

 granules have an oblique direction. At the fovea centralis the retina is much thinned, 

 since here its nerve-fibre and ganglionic layers are absent and its other strata greatly 

 attenuated. The stratum pigmenti, on the other hand, is thicker and its pigmentation 

 more pronounced. The cone nuclei are situated some distance internal to the outer 

 limiting membrane, and thus the thin inner and outer granular layers are in apposition. 

 There are no rods, and the cones, closely crowded together, are narrower and their outer 

 segments more elongated than elsewhere, so that the line of their bases, indicated 

 by. the membrana limitans externa, presents a convexity directed forwards. The fovea 

 centralis and macula lutea are spoken of by physiologists as the "region of distinct 

 vision." 



Structure of the ora serrata. Here the nervous layers of the retina suddenly 

 cease ; the layer of rods and cones ends a little behind the margin of the ora serrata ; the 

 other nervous strata persist as far as its margin. In front of the ora serrata the retina is 

 prolonged over the ciliary processes in the form of two layers of cells : (a) an inner layer of 

 columnar epithelium, and (6) an outer, consisting of the stratum pigmenti, the two 

 forming the pars ciliaris retinae. The same two layers are prolonged over the back of 

 the iris, where both are pigmented and form the pars iridica retinae. 



Vessels of the retina (Fig. 692). The retina is supplied by the arteria centralis retinae, 

 a branch of the ophthalmic artery, which pierces the sheath of the optic nerve about 

 2 c.m. behind the bulb of the eye, and makes its appearance in the centre of the optic 

 papilla. There it divides into an upper and a lower branch, and each of these again bifur- 

 cates into a medial or nasal, and a lateral or temporal, branch. The resulting four branches 

 ramify towards the periphery of the retina, and are named the superior and inferior temporal 



Superior nasal branch 



Optic disc 



Inferior nasal branch 



Superior .temporal branch 



Superior and inferior macular 

 arteries 



Macula lutea 



Inferior temporal branch 



FIG. 692. BLOOD-VESSELS OF THE RETINA. 



and the superior and inferior nasal arteries. The temporal arteries pass laterally above and 

 below the macula lutea, to which they give small branches ; these do not, however, extend as 

 far as the fovea centralis, which is devoid of blood-vessels. The macula also receives two small 

 arteries (superior and inferior macular) directly from the stem of the arteria centralis. The 

 larger vessels run in the nerve-fibre layer near the membrana limitans interna and form two 

 capillary networks an inner, in the nerve-fibre layer, and an outer, in the inner nuclear layer. 

 The inner network arises directly from the arteries and sends numerous small branches to form 

 the outer network, from which the veins take origin. The vessels do not penetrate deeper than 

 the inner granular layer, nor do the arteries anastomose, except through the capillary plexuses. 

 The veins follow the course of the arteries ; they have no muscular coats, but consist merely 

 of a layer of endothelial cells, outside which is a perivascular lymph sheath, surrounded by 

 delicate retiform tissue. 



