STRUCTURE OF THE RETINA 



225 



many of which are in the form of minute crystals, lie in the inner 

 part of the cell, and after prolonged exposure to the light they are 

 found extending along the cell-processes between the rods (b, c), their 

 function being probably connected with the restoration of the purple 

 colouring matter which has been bleached by the light. 



Fibres of Miiller.The fibres of Miiller (fig. 261) are long stiff fibres 

 which pass through several of the retinal layers. Commencing at the 

 inner surface of the retina by expanded bases which unite with one 

 another to form the so-called internal limiting membrane (fig. 856), the 

 fibres pass through all the layers in succession, until they reach the 

 outer nuclear layer. Here they branch and expand into a sort of reti- 

 cular tissue which serves to support the fibres and nuclei of the rod- and 

 cone-elements. At the bases of the rods and cones, this sustentacular 

 tissue ceases, being here bounded by a distinct margin which has been 

 called the external limiting membrane (ra. e. I.}, but delicate sheaths 

 have been described passing from it round the bases of the rods and 

 cones. Each Miillerian fibre, as it passes through the inner nuclear 

 layer, has a nucleated enlargement (n), indicating the original cell 

 nature of the fibre. 



There are two parts of the retina which call for special description. 



The macula lutea (yellow spot, fig. 263), with its central fovea, lies 

 in the visual axis,*and is the part of the retina which is most immedi- 



FIG. 262. VERTICAL SECTION THROUGH THE MACULA LUTEA AND FOVEA CENTRALIS ; 



DIAGRAMMATIC. 



1, nerve-layer ; 2, ganglionic layer ; 3. inner molecular ; 4, inner nuclear ; and 5, outer molecu- 

 lar layers ; 6, outer nuclear layer, the inner part with only cone-fibres forming the so-called 

 external fibrous layer ; 7, cones and rods. 



ately concerned in direct vision. It is characterised first by its greater 

 thickness (except at the fovea), secondly by the large number of 

 ganglion-cells, which are all distinctly bipolar (2), and thirdly by the 

 large number of cones it contains as compared with the rods. In the 



Q 



