THE NERVOUS TUNIC. 



sheet, the membrana limitans internet. As the fibres traverse the retinal 

 layers, they give off delicate lateral offshoots, which break up into a fine 

 supporting reticulum. Within the inner nuclear layer each fibre presents a 

 broad expansion containing the oval nucleus of the sustentacular cell. After 



Cerebral layer 



Sustentacular cell 

 Fibre of Midler 



Neuroepithelial layer 



Pigmented layer 



Inner plxiform layer 



* Inner nuclear layer 

 (bipolar iierve-ctlls) 



ter plexiform layer 



Onter nuclear layer 

 (bodies of visual eel Is) 



Rods and cones 



iPigmented layer 



" J ^OHWM^^^^^^^^^'^^V^^HW^^ -^^^ 



Choroid 

 FIG. 400. Diagram illustrating structure of retina and relations of three fundamental layers. (Greeff.) 



traversing the outer nuclear layer their broadened peripheral ends come into 

 contact and form a' continuous sheet, the membrana limitans externa. From 

 the latter delicate offshoots continue outwards and embrace the bases of the 

 individual rods and cones. In addition to the robust fibres of Miiller, neu- 

 roglia cells, in the form of spider cells, are found in the nerve-fibre and 

 ganglion-cell layers. 



The Macula Lutea. The structure of the retina undergoes important 

 modifications in two areas, at the macula lutea and at the ora serrata. In the 



654 32 



FIG. 401. Section of human retina through the fovea centralis. i, fovea; 2, 3, cones and nuclei of 

 visual cells; 4, pigmented layer; 5, outer plexiform layer; 6, bipolar cells; 7, ganglion-cells; 8, inner 

 plexiform layer ; 9, internal limiting membrane. X 80. 



former the ganglion-cells increase rapidly in number as the macula is reached, 

 so that instead of forming a single layer they are distributed in from eight to 

 ten strata. The inner nuclear layer is also increased in thickness. Within 

 the fovea centralis, however, in order to reduce to a minimum the layers 

 traversed by the light-rays, the cerebral layers are almost entirely displaced, 



