774 



THE EYE AND VISION 



[CII. LV1. 



connected internally with a rod fibre, very fine, but here and there 

 varicose; in the middle of the fibre is a rod granule, really the 

 nucleus of the rod, striped broadly transversely, and situated about 

 the middle of the external nuclear layer; the internal end of the 

 rod fibre terminates in branchings in the outer molecular layer. 

 Each cone (fig. 573), like the rods, is made up of two limbs, 



outer and inner. The outer 

 limb is tapering and not 

 cylindrical like the corre- 

 sponding part of the 



Fio. 574. The posterior half 01 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. 575. 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.) 



and about one-third only 

 of its length. There is, 

 moreover, no visual purple 

 . found in the cones. The 

 inner limb of the cone is 

 broader in the centre. It is protoplasmic, and under the influence 

 of light has been seen to execute movements. In birds, reptiles 

 and amphibia, there is often a coloured oil globule present here. 

 Each cone is in connection by its internal end with a cone fibre, 

 which has much the same structure as the rod fibre, but is much 

 stouter and has its nucleus (cone granule) quite near to the ex- 

 ternal 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 



