Integrative Systems 



205 



Layer of rods and cones 

 Membrana Hmitans 



Stellate ganglion cell 

 Bipolar cells 



Amakrine cells 



Centrifugal nerve fiber 



Multipolar ganglion cell 



Collateral 



Fig. 178. Diagrammatic section showing the nervous layers of the human 

 retina. (Modified from Bremer: "Text-Book of Histology," Philadelphia, The 

 Blakiston Company.) 



Peripheral to the rods and cones is the pigmented epithelium 

 (Fig. 177). When the retina is flooded by strong light, processes of the 

 pigment-cells penetrate between the receptors. In light of low intensity 

 or in darkness, these processes are withdrawn. 



Functions of Rods and Cones 



The rods and cones are functionally unlike. In animals which 

 habitually live in dim light, the retina ordinarily contains only rods. 

 In animals accustomed to relatively strong light and having good 

 discriminative vision, the cones are most numerous in the central 

 region of the retina and fewer or entirely lacking in zones more or less 

 remote from the central region. Within a small spot at the exact visual 

 center, the retinal structure is modified. Its layers are pervaded by a 

 yellowish pigment, the spot being therefore called the 44 macula lutea." 

 At the center of the macula is a shallow concavity of its surface, the 

 fovea centralis (Fig. 175). It is as if local pressure had been applied 

 with the result that the constituent elements of the retina had been 

 squeezed away centrifugally in all directions from the center of the 

 spot and to such an extent that, at the deepest region of the fovea, 

 nothing is left but the layer of receptors — and these are all cones. 



Retinal zones remote from the optic center produce mental pic- 

 tures, which, according to the degree of remoteness, are more or less 



