CH. LVIII.J 



THE FOVEA 



819 



being six to eight deep (2, fig. 518). The bipolar inner granules 

 (cone nuclei) are obliquely disposed (figs. 518 and 519) 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. 518). 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. 



It is important to notice what is clearly brought out in fig. 519, 

 that at the fovea each cone is connected to a separate chain of 



FIG. 519. 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 ; c, bipolar cell destined for the cones ; d, bipolar cell destined for the 

 rods ; B, e, ganglion cells ; /, spongioblast ; g, efferent fibre (? trophic), originating from the cell m, 

 in geniculate body; h, optic nerve; i, 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.) 



neurons, whereas in other regions the rods and cones are connected 

 in groups to these chains ; this explains the greater sensitiveness of 

 foveal vision. 



At the ora serrata the layers are not perfect, and disappear in 

 this order: nerve-fibres and ganglion cells, then the rods, leaving 

 only the inner limbs of the cones, next these cease, then the outer 

 molecular layer, the inner and outer nuclear layers coalescing, and 

 finally the inner molecular layer also is unrepresented. 



At the pars-ciliaris retince, the retina consists of a layer of 

 columnar cells, which probably represent the Miillerian fibres. These 

 cells externally are in contact with the pigment layer of the retina, 

 which is continued over the ciliary processes and back of the iris. 

 Nervous structures are absent. 



At the exit of the optic nerve the only structures present are 

 nerve-fibres. 



The anterior chamber is the space behind the cornea and in front 

 of the iris. It is filled with aqueous humour (dilute lymph). 



The vitreous humour, which is a jelly-like connective tissue (see 

 p. 36), is situated behind the crystalline lens. It is enclosed in a 

 membrane called membrana hyaloidea, which in front is continuous 



