CHAP. HI.] * SIGHT. 1205 



farther towards the centre, but these also eventually disappear, all 

 that is left of them and of the outer molecular layer being the 

 thin layer of neuroglia mentioned above. At the same time the 

 rods, relatively numerous at the peripheral parts of the macula, 

 gradually grow scanty and finally disappear. In this way nothing 

 is left in the fovea itself but the cones and the cone fibres. 



Over the retina generally the several elements seem, as seen 

 in a vertical section, to be disposed vertically the one over the 

 other. During the thinning out and disappearance just men- 

 tioned, this vertical disposition of the several elements gives 

 way, except so far as the cones themselves are concerned, to an 

 oblique disposition, which becomes the more marked as the 

 centre is approached. It is as if the parts of the inner nuclear 

 layer, of the inner molecular layer and of the ganglionic layer 

 belonging to the cones and cone fibres of the central region of 

 the macula were dragged on one side towards the periphery. We 

 may imagine that each (rod or) cone with its (rod or) cone fibre 

 is connected in some way or other with one or more of the cells 

 of the inner nuclear layer, and so with one of the ganglionic 

 cells; over the retina generally these are placed in the same 

 vertical line, and before a ray of light can act on the rod or 

 cone it must pass through these several .other structures. In the 

 fovea these other structures are pulled on one side, so that 

 in the very centre of the fovea the light can gain access to 

 the outer limb of the cone, without having to pass through any 

 other structures than the cone itself and its cone fibre. This 

 oblique disposition is also obvious in the cone fibres, which 

 elsewhere short and vertical are in the fovea much elongated, 

 and radiate obliquely from the centre of the fovea to their more 

 peripherally placed inner nuclei or other structures with which 

 they have to make connections of some kind or another. 



The colour of the macula which is said to be in the living 

 eye brown rather than yellow is most intense in the thickened 

 rim, fading gradually away peripherally and centrally, and being 

 wholly absent from the central fovea. The colour is due to a 

 pigment diffused through the layers lying to the inside of the 

 outer nuclear layer, being absent from this layer as well as from 

 the cones themselves; hence the absence of the colour from the 

 very centre. The pigment seems to be attached at least as much 

 to the neuroglial as to the nervous elements. It thus presents 

 a contrast to the visual purple which is limited to the outer 

 limbs of the rods, not being present elsewhere, not even in the 

 outer limbs of the cones. 



745. The Blood Vessels of the Retina. The central artery 

 and vein of the retina running, as we have seen, with their sheath 

 of connective-tissue in the middle of the optic nerve reach the 

 surface at the centre of the optic disc, which is excavated so that 

 the blood vessels seem, on a surface view of the retina, to arise out 



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