Oil. LV.] 



THE RETINA. 



753 



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 become 

 rounded, and arranged two or three deep. 



The next figure (fig. 575) represents the structure of the retina 

 as made out by Golgi's method. 



Differences in Structure of different parts. Towards the centre 

 of the inacula lutea all the layers of the retina become greatly 



Pig. 575. Scheme of the retinal elements. A, cones of the fovea centralis ; B, granules 

 (nuclei) of these cones ; C, synapse between the cones and bipolar cells in external 

 molecular layer ; D, synapse between the bipolar and ganglion cells in the internal 

 molecular layer ; a and b, rods and cones in other regions of the retina ; <-, bipolar cell 

 destined for the cones ; rf, bipolar cell destined for the rods ; E, e, ganglion cells ; 

 f, spongioblast ; g, efferent fibre (? trophic), originating from the cell m, in genicnlate 

 body ; ,* optic nerve ; t, terminal arborisations of optic nerve fibres in geniculate 

 body ; j, fibres from the cells of geniculate body on the way to cerebral cortex. 

 (R. y Cajal.) 



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

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

 especially their inner segments, are long 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 gan- 

 glionic layer is especially thickened, the cells being six to eight 

 deep (2, fig. 573). The bipolar inner granules (cone nuclei) are 

 obliquely disposed (figs. 573 and 575) on the course of the cone 

 fibres, and are situated at some distance from the membrana 

 limitans externa, which is cupped towards the fovea (fig. 573) 

 The yellow tint of the macula is due to a diffuse colouring matter 

 in the interstices of the four or five inner layers ; it is absent at 

 the centre of the fovea. 



K.P. 



3 c 



