EYE 



285 



In the retina of all Vertebrates there is a specially modified 

 region of most acute vision. This is called the yellow-spot (fovea 

 centralis or macula lutea\ and is due to the thinning-out of all the 

 layers except that of the neuro-epithelium, and even the rods 

 disappear, only the cones persisting (Fig. 203). A yellow spot 

 is wanting in most Fishes, in Urodeles, and in certain Mammals 

 (e.g. Insectivores, Rodents), and is only slightly marked in the 



Fibrous ~ 

 basket work. 



Concentric 



supporting-cells 



(nucleated). 



Concentric 

 supporting-cells 

 (non-nucleated). 



Radial fibres. 



Radial fibrous 

 cone. 



Rods and cones. 

 / Menibrana 

 1 limitans. 



Nuclear layer of 

 rods and cones. 



Outer reticular 

 layer. 



| Sub-epithelial 



\ganglion-cell 



Star-shaped 

 ganglion-cell. 



Bipolar \ 

 ganglion-cell. J 

 Multipolar X 

 ganglion-cell. J 



Inner reticular 



layer. 



Centrifugal 

 nerve fibres. 



Multipolar 

 ganglion-cell. 



Layer of 

 nerve fibres. 



FIG. 209. DIAGRAM OF THE ELEMENTS OF THE RETINA. (Supporting elements 

 on the left, and nervous elements on the right. ) (After Ph. Stohr. ) 



Anura and most Sauropsida, but in Birds there may be two 

 " yellow spots." Only in Mammals does the retina possess special 

 blood-vessels, and even in certain of these they may be reduced 

 or wanting. In other Vertebrates (except the Eel) the retina is 

 non-vascular, and is nourished by the vessels of the vitreous body, 

 processus falciformis, or " pecten." 



animals the rods are more numerous than the cones. The cones of many Reptiles, 

 of Birds, and of Marsupials, are distinguished by the presence of brightly 

 coloured oil-globules. 



