644 



MANUAL OF HISTOLOGY. 



the rods. The cone styles sink at the same time, perhaps, to 0'0009 and 

 O'OOl mm. in diameter, while the cone fibres, on the other hand, pre- 

 serve their original thickness. 



Over the fovea centralis the pigment cells are higher and darker, and 

 enclose the cone styles in long pigmentary sheaths. 



Another point deserves notice here, namely, that as we approach the 

 yellow spot the retina increases in thickness, the distance between mem- 



brana limitans externa and internal 

 granular layer becoming greater. And 

 while the thicker cones increase, the 

 slender rods diminish in number, giv- 

 ing rise to a change also in the external 

 granular layer. In the first place, 

 there are absolutely fewer corpuscles 

 necessary here, while, on the other 

 hand, owing to the large number of 

 cones, the corpuscles of the latter no 

 longer find room in one plain to lie 

 side by side. Thus, the rod and cone 

 fibres present themselves as a purely 

 fibrillated stratum beneath the granules, 

 the latter no longer lying upon the 

 intergranular layer as in other parts of 

 the retina. In this deeper portion of 

 the layer in question, which is destitute 

 of granules, and which has received 

 from Herile the name of external fibre 

 layer, the bacillary and cone fibres 

 commence to abandon their perpendi- 

 cular arrangement more and more as 

 we approach the inner part of the fovea 

 centralis. Here they may be observed 

 to run obliquely downwards and out- 

 wards (Bergmann, H. Miiller), and in 

 the centre of the fovea almost hori- 

 zontally (fig. 597, d), so that it is only 

 after a long course that the cone fibre 

 reaches at last the intergranular layer. 

 At the same time the fibrillated half 

 of the external granular layer increases 

 in thickness about the periphery of the yellow spot, but becomes rapidly 

 thinner in centre of the fovea itself (Schultze). 



According to the latter's observations, farther, it is not only the internal 

 surface of the fovea turned towards the vitreous humour that is concave, 

 but also the opposite side facing the membrana limitans externa. Owing to 

 this the choroidal ends of the fovea cones are inclined towards each other, 

 and consequently more approximated than would be possible were they 

 still arranged perpendicularly, a circumstance which bears upon the 

 delicate sensitiveness of this region. ' The adjacent portion of the retina 

 also takes part in this peculiar curvilinear arrangement of the light- 

 perceiving elements. 



Let us now turn to the ciliary edge of the retina. 



Anteriorly towards the ora serrata the thickness of the latter decreases 



Fig. 598. Cones from the human macula 

 lutea and fovea centralis. a, with half 

 decomposed external member; b, with 

 resolution of the latter into disks (after 

 Schultze). 



