36 THE RIDDLE OF MIGRATION 



first pass on their way to the rods and cones. But 

 over a certain very small area of the retina these 

 layers are so thinned as to expose the rods and 

 cones almost direct to the incoming rays. This 

 area is termed the yellow spot or fovea and is the 

 centre of perfect vision. Many birds possess two 

 such foveae, while the best flyers, — terns, swifts, 

 etc. — possess three. 



Before leaving the eye it is of interest to note that 

 the ring of bones so characteristic of the outer wall 

 of the avian eye-ball (particularly well developed in 

 hawks, owls, etc.) is a typical reptilian feature, 

 while apart from birds, only certain reptiles can 

 boast striped muscle fibres in the make-up of 

 the eye. 



The function of the nervous system of birds, like 

 other nervous systems, is to coordinate the activi- 

 ties of the many organs and systems that go to 

 make up an animal's body. Like the Federal 

 Government of the States or Canada, it welds to- 

 gether and unifies a collection of independent and 

 often antagonistic units, converting to the general 

 welfare of the whole the separate interests of the 

 component parts. But in this it is not entirely 

 alone and to the accessory mechanism we shall now 

 have to turn. 



Situated in various parts of the vertebrate body 



