68 THE BIOLOGY OF BIRDS 



which lies above the pygostyle. But the versatiHty is as 

 striking as the relative length. A bird can move its 

 head and neck through i8o degrees, so that the head rests 

 between the shoulders with the tip of the bill pointing 

 more or less backwards. The versatility is of great practical 

 importance in catching rapidly moving booty. The neck- 

 vertebras often show well-developed cervical ribs, usually 

 fused to the transverse processes, and serving for the 

 insertion of numerous muscles. Some posterior cervical 

 ribs may be free, but they never reach the breastbone. 



The two-headed ribs, characteristic of the thoracic 

 vertebras, bear uncinate processes (except in the Pala- 

 mediidae), which overlap the rib behind and are bound to 

 it by little ligaments. These processes are an inheritance 

 from reptilian ancestors ; they are well seen in that living 

 fossil, Sphenodon. Their significance in birds is that they 

 help to form the coherent sterno-costal-vertebral basket — 

 important in connection with both flying and breathing. 

 The upper or vertebral part of the rib, directed backwards, 

 is connected at an angle to a bony (usually cartilaginous in 

 mammals) sternal rib, which is directed forwards and 

 articulates movably with the breastbone. When the angle, 

 open forwards, between the vertebral part of the rib and 

 the sternal part of the rib is lessened, and the distance be- 

 tween the thoracic vertebrae and the breastbone is thus 

 likewise lessened, the air is driven out of the lungs (expira- 

 tion). When the angle is increased and the breastbone 

 sinks to a greater distance from the backbone, air enters 

 passively into the lungs (inspiration). In birds, like divers, 

 with very long ribs the difference in the volume of the body 

 cavity as the angle between the vertebral ribs and sternal 

 ribs increases or decreases is very notable, and has an 

 obvious adaptive significance in relation to the prolonged 

 immersion. 



Behind the complex sacrum there are a number of free 

 caudal vertebrae, terminating except in Running Birds in a 

 pygostyle or ploughshare bone. This terminal fusion of 

 vertebrae is also an adaptation, since it affords a basis for 



