BIRDS 



419 



the central fovea may be situated in a band but this does not inckide 

 the temporal fovea wliich is situated above and separate from the 

 former (the tern, Sterna Mr undo). 



(7) Infular. Some water-birds have a horizontal band only with 

 no macular area and in it may be a linear trough-like fovea : gulls, 

 flamingo. 



Histologically the retina of birds is the most beautiful and 

 elaborate in its arcliitect ure in the animal kingdom ^ ; layers and sub- 

 layers are clearly defined with each cell 

 accurately in place (Fig. 523). As with 

 other Sauropsida the pigmentary epi- 

 thelial cells send slender processes con- 

 taining fuscin granules extending 

 inwards to the inner segments of the 

 visual cells ; their movements with 

 variations of light and shade are rapid 

 and Extensive, possibly making up for 

 the relative inertia of the pupil to light. 

 In the visual retina the ganglion cells lie 

 in 2 or 3 rows. The inner plexiform 

 layer is unusually thick and stratified 

 at the levels at wliich the arborizations 

 of the amacrine cells deploy. The inner 

 nuclear layer is expanded to have three 

 strata — innermost the (integrative) 

 amacrine cells which may even out- 

 number the bijDolars, outermost the 

 (conductive) bipolar elements, and in 

 the middle a single com23act row of 

 Miiller's fibres. This layer as a whole is 

 thus very tliick, and mainly because of 

 the unusual development of this and 



the inner plexiform layer, the retina of Birds is some one-and-a-half 

 times to twice as thick as that of the majority of Vertebrates, being 

 approached in this respect only by a few Teleosteans. 



The visual cells are slender and closely packed (Fig. 524). The 

 retina is duplex in type, containing rods and single and double cones. 

 The rods are slender with a long thin paraboloid and contain 

 rhodopsin but have no oil-droplets, resembling in their general structure 

 those of Chelonians or Crocodilians ; in nocturnal birds they pre- 

 dominate while in diurnal types they may be very few and limited to 



1 H. Miiller (1856-63), Krause (1863-94), Merkel (1870), Dobrowolsky (1871), 

 Schultze (1873), Waelchli (1881-83), Dogiel (1888-95), Cajal and Greeff (1894), Fritsch 

 (1911), Rochon-Duvigneaud (1919-43), Kajikawa (1923), Kolmer (1924-36), Chard 

 (1938), van Eck (1939). O'Day (1940), Walls (1942), Lockie (1952), Yamamoto (1954). 



Fig. 524. — The Visual Cells of 

 Birds. 



From the left, a single cone, a 

 double cone, both from the peri- 

 phery ; a peripheral rod, and a 

 central rod of the English sparrow, 

 Passer domesticus. p, the para- 

 boloid (X 1,000) (Gordon Walls). 



Tern, 

 Sterna 



