482 THE BIRDS xvi. 28- 



of the robin (Erithacus) is the agent that releases attacks by other birds 

 (Fig. 292). Similar phenomena are known in fishes and other verte- 

 brates (p. 225), and it remains to be shown whether they can be attri- 

 buted to any single or particular neural basis. 



29. The eyes of birds 



Birds depend more on their eyes than on the other senses; they are 

 perhaps more fully visual than are any other animals. The eyes are 

 extremely large : those of hawks and owls, for instance, may be abso- 

 lutely larger than in man. The shape is not spherical, the lens and 



Fig. 292. The red tuft of feathers is attacked by male robins holding territory, but 

 the complete juvenile bird (withou red) is left alone. (After Lack, from Tinbergen.) 



cornea bulge forwards in front of the posterior chamber, this form 

 being maintained by a ring of bony sclerotic plates (Fig. 293). In most 

 birds the whole eye is thus broader than it is deep, but in those with 

 very acute sight it is longer, and in some eagles and crows becomes 

 almost tubular. The great distance between lens and retina allows 

 broadening of the image, thus improving the fine two-point discrimin- 

 ation that is needed by these diurnal birds. The shape of the back of 

 the eye is such that 'the retina lies almost wholly in the image plane, 

 so that all distant objects within the visual angle are sharply focused 

 on the photosensitive cells, whereas in the human eye this is only 

 true of objects lying close to the optic axis' (Pumphrey; Fig. 294). 

 The lens is usually soft and accommodation is effected by changing 

 its shape, and especially the curvature of its anterior surface, by the 

 pressure upon it of the ciliary muscles behind. These, like the iris 

 muscles, are striated, presumably allowing for the quick accommoda- 

 tion necessary in a rapidly moving bird, though it must not be for- 

 gotten that these muscles are also striated in lizards. The ciliary 

 muscle is characteristically divided into 'anterior' and 'posterior' por- 

 tions, the muscles of Crampton and Briicke (Fig. 293). The latter 

 draws the lens forward into the anterior chamber so that since the 



