66 THE BIOLOGY OF BIRDS 



different principle from that seen in types like skate and 

 frog (platybasal), where the brain-case extends foi-wards 

 between the eyes. 



(e) The bird's skull has great freedom of movement on 

 its single occipital condyle — a reptilian feature of course. 

 It is usually very light for its size, the roofing bones having 

 a markedly spongy texture. In this case, far from the 

 lungs, the cavities of the bones are filled with air from the 

 nasal and auditory passages. 



We cannot leave the bird's skull without noticing two 

 points of importance. Looked at broadly, as indicated above, 

 it is a congeries of adaptations of a general nature. It may 

 also show special adaptations, such as the broadening out 

 of the bill in ducks or the elongation of the bill in some 

 humming-birds. But these special adaptations concern 

 superficial features, and in many cases they show their 

 relative novelty by not becoming marked till some time 

 after hatching. Quite apart from these superficial diff"erences 

 among birds' skulls are deeper non-utilitarian peculiarities, 

 notably in the precise architecture of the palate. These 

 are sure guides in the discovery of affinities ; it seems that 

 they have remained constant while more conspicuous 

 adaptive features have been tacked on. In fact, there are 

 deep architectural differences in the skulls which afford 

 extraordinarily sure criteria of blood-relationship. 



§ 5. Adaptations in the Backbone 



The typical curvature of the articular surfaces of the 

 bodies or centra of the vertebras of birds is heterocoelous or 

 saddle-shaped. It does not occur elsewhere. Looked at 

 from in front, the centrum is concave from side to side and 

 convex from above downwards. Looked at from behind, 

 it is like a saddle. This typical shape is most marked in 

 the cervical vertebrae, and its significance is in the freedom 

 of movement it allows. 



In rare cases other shapes occur, e.g. opisthocoelous 

 (hollow behind) in the thoracic region of gulls and penguins. 



