818 



THE EYE AND VISION 



[CII LV1II. 



as the rod fibre, but is stouter and has its nucleus (cone granule) 

 quite near to the external limiting membrane. Its inner end 

 terminates by branchings in the external molecular layer. 



In the rod and cone layer 

 of birds, the cones usually pre- 

 dominate largely in number, 

 whereas in man the rods are by 

 far the more numerous, except 

 in the fovea centralis, where cones 

 only are present. The number 

 of cones has been estimated at 

 3,000,000. 



10. Pigment-cell layer consists 

 of a single layer of polygonal cells, 

 mostly six-sided, which send down 

 a beard-like fringe to surround the 

 outer ends of the rods. It is this 

 layer which is continuous with the 

 uvea, where, however, the cells be- 

 come rounded, and arranged two 

 or three deep. 



Differences in Structure of differ- 

 ent parts. Towards the centre of 

 the macula lutea all the layers 

 of the retina become greatly 

 thinned out and almost disappear, except the rod and cone layer, 

 and at the fovea centralis the rods disappear, and the cones are long 



Fio. 516. The posterior half of the retina of 

 the left eye, viewed from before; s, The cut 

 edge of the sclerotic coat ; ch, the choroid ; r, 

 the retina ; in the interior at the middle the 

 macula lutea with the depression of the fovea 

 centralis is represented by a slight oval shade ; 

 towards the left side the light spot indicates 

 the colliculus or eminence at the entrance of 

 the optic nerve, from the centre of which the 

 arteria centralis is seen spreading its branches 

 into the retina, leaving the part occupied by 

 the macula comparatively free. (After Henle.) 



FIG. 517. Pigment-cells from the retina. 

 A, Cells still cohering, seen on their 

 surface ; a, nucleus indistinctly seen. 

 In the other cells the nucleus is con- 

 cealed by the pigment granules. B, 

 Two cells seen in profile ; a, the outer 

 or posterior part containing scarcely 

 any pigment, x 370. (Henle.) 



in.l.c. 



FIG. 518. Diagram of a section through half the 

 fovea centralis. 2, Ganglionic layer; 4, inner 

 nuclear; 6, outer nuclear layer, the cone fibres 

 forming the so-called external fibrous layer; 

 7, cones; m.l.e., membrana limitans externa ; 

 m.l.i., membrana limitans interna. (Schafer and 

 Golding Bird.) 



and narrow. At the margin of the fovea the layers increase in 

 thickness, and in the rest of the macula lutea are thicker than 

 elsewhere. The ganglionic layer is especially thickened, the cells 



