66 THE MACULA LUTEA. [BOOK in. 



of the impact of light into true nervous impulses, may be looked 

 upon as being carried out by the tangle of nerve fibrils, in the two 

 molecular layers and elsewhere. 



744. The Macula Lutea and Fovea Centralis. On the temporal 

 side of the optic disc, at a distance of about 4 mm. from, and a little 

 below the horizontal level of, its centre an oval area, with its long 

 axis of about 2 mm. placed horizontally, is distinguished from the 

 rest of the retina by its yellowish or brownish tint ; this is the 

 macula lutea or ' yellow spot.' At the edges of the macula the 

 retina is thickened but in the centre becomes very thin. So that 

 the macula presents a central depression, about '3 mm. in 

 diameter, the fovea centralis, surrounded by a raised rim. 



A vertical section through the macula lutea shews that, in 

 contrast to the retina generally, in which the rods are more 

 numerous than the cones, in the very centre of the fovea cones 

 alone are present ; their outer limbs, however, are very elongated, 

 60 /u, long by 2 /z or 3 ^ broad, and indeed are rod-like. Moreover, 

 all the several layers of the retina described above are here absent 

 except the layer of cones, and the outer nuclear layer or layer of 

 cone fibres. Between the pigment epithelium and the vitreous 

 humour are found only these elongated cones, with the nucleated 

 cone fibres belonging to them ; the latter supported by a delicate 

 reticular neuroglia curve away towards the periphery of the fovea ; 

 next to the vitreous humour lies an exceedingly thin layer of 

 neuroglia. About 7000 cones are supposed to be crowded 

 together in the very centre of the fovea. 



A vertical section through the thickened rim of the macula 

 shews all the layers present, the layer of ganglionic cells being 

 exceedingly prominent and consisting not as elsewhere of a single 

 layer or at most of two layers but of several, eight or nine, layers 

 of cells ; indeed the greater thickness of the retina at the rim of 

 the macula is chiefly due to an increase in the layer of ganglionic 

 cells, the inner nuclear layer being somewhat increased, but not to 

 any great extent, and the others hardly at all. In the layer of rods 

 and cones, the cones are more numerous than outside the macula, 

 and their outer limbs are somewhat elongated ; but otherwise 

 the features of the several layers are the same as in the retina 

 generally ; we may add, however, that the ganglionic cells appear 

 bipolar rather than multipolar, one process being continued as 

 elsewhere into an optic fibre and the other directed obliquely 

 towards the central parts of the macula ; the latter process, 

 however, sooner or later divides into fine branches. 



If in a vertical section taken across the whole macula we 

 examine the features of the section from the periphery towards 

 the centre, we find the layer of optic fibres rapidly thinning out 

 and soon disappearing, and the layer of ganglionic cells, after its 

 temporary thickening, also thinning out and disappearing. The 

 inner molecular layer and the inner nuclear layer extend a little 



