xvi. 29 RETINA 485 



packed receptors. In birds that live on the sea, in the desert, or other 

 open spaces the area often has the form of an elongated horizontal band 

 (Fig. 295), whereas in tree-living birds it is circular. Some birds have 

 two areae, a central one in the optic axis and a second placed on the 

 temporal surface of the eye, so that the image of objects in front of the 

 head falls on the temporal areae of both eyes. This arrangement is 

 common in birds that follow moving prey (shrike, Lanius) or for 



Fish (Serrjnus) 



fovea 

 (dFcer Kahmjnn) 



Reptile [Chameleon) 



Fovea 

 (aFter Detwiler) 



uila \ X Aquila 



central fa ea \ temporal Fovea 



(after Polijak) \ (after Pol yak) 



Alcedo 

 central fovea 

 (after Kolmerj "<~ 



(after Detwiler) 



Fig. 296. Forms of the fovea in various vertebrates, showing 

 the development from moderately to sharply convexiclivate types 

 in foveas adapted for detection of movement, but flattening of 

 the fovea where there is binocular vision. (From Pumphrey.) 



some other reason require accurate perception of distance (swallows, 

 humming-birds; the latter feed their young on insects caught on the 

 wing). The density of the cones is so high in diurnal birds, even outside 

 the areae, that they probably obtain a good detailed picture in all 

 directions. They do not, therefore, scan the world with the central 

 area of the retina as we do; indeed, the eyes move relatively little. 

 Instead the bird is able to detect very small movements anywhere in 

 its surroundings. The bird's-eye view usually lacks stereoscopic solidity 

 and it is possible that in compensation for this the animals appreciate 

 distance by movements of the intrinsic eye-muscles. The familiar 

 cocking of the head of a bird before pecking may be its means of 

 judging distance. 



As in man there is often within the central area of the eye a fovea 

 or pit, and in many birds the sides of this pit are steeply curved (Fig. 

 296). Walls has suggested that since the vitreous humour and retina 

 differ in refractive index this curvature serves to magnify the image 



