ME. J. W. HULKE ON THE CHAMELEON'S EETINA. 227 



7. Optic Nerve-layer. — The general appearances of the optic nerve have been already 

 described ; its finer distribution does not exhibit any special characters. The bundles 

 are disposed in a plexus, the meshes of which transmit the connective radial fibres. 



The plexus is separated from the membrana limitans interna by a narrow band, which 

 has a finely granular and striated texture. Its minute structure agrees with that of the 

 granular layer. This band is bounded by the membrana limitans interna, from the 

 inner surface of which the connective radial fibres arise by fine converging roots ; their 

 course has been given in the descriptions of the difierent layers. 



Fovea Centralis. 



Owing to the great thickness of the retina immediately around it (fig. 13), the fovea is 

 encircled by a raised margin, from which the surface rapidly declines centrally to the 

 bottom of the fovea, while it slopes gently towards the periphery of the retina. Oppo- 

 site its margin the optic nerve, granular, inner granule, layers, cone-fibre plexus, and 

 the inner limit of the outer granule-layer, begin to bend outwards, and becoming gi-adu- 

 ally thinner, they converge to the bottom of the fovea, where they cease, the cones 

 (with their fibres diverging on all sides towards the outer granule-layer) alone, covered 

 by a thin fibro-granular band, existing at this spot. 



Reviewing the layers at the fovea in order, from the inner surface of retina outwards, 

 we find the optic nerve-fibres in decreasing number from the margin to the bottom of the 

 fovea. 



The ganglion-cells around the bottom lie in a single and interrupted series, which, 

 midway towards the margin, becomes continuous, and double or treble, while at the 

 margin the cells lie four or five deep. The greatest development of the ganglionic 

 layer is not, however, here, but about ^"' from the centre of the fovea. 



The granular layer blends in the bottom of the fovea with the granular band, which 

 in other parts lies between the optic nerve-layer and the membrana limitans interna. 

 This band in the fovea exhibits a very distinct vertical striation. 



The inn£r granule-layer, in sloping towards the bottom of the fovea, closely approaches 

 the outer granule-layer, as the cone-fibre plexus which lies between them becomes 

 thinner. It attains its maximum development slightly nearer the fovea than that of the 

 ganglionic layer. 



The cone-fibre plexus commences near the bottom of the fovea, in the angle between 

 the inner and outer granules, and steadily increases in breadth till it reaches its maxi- 

 mum development, slightly nearer the fovea than that of the inner granules. 



The outer granule-layer, also commencing near the bottom, swells very rapidly, and 

 opposite the margin of the fovea acquires a remarkable thickness. From this point it 

 slowly declines for about f " from the fovea, and then becomes abruptly thin. Its extra- 

 ordinary enlargement immediately around the fovea is due to the presence here, not only 

 of the outer granules which belong to the cones lying immediately at its outer surface, 

 but also of those outer granules that belong to the cones which lie at the centre of the 



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