44° THE BIRDS xvi. 5- 



The last thoracic (rib-bearing) veitebra is united with about five 

 that can be regarded as lumbars, two sacrals and five caudals to make 

 a synsacrum, which is also fused with the ilium. This produces a very 

 thin plate-like structure, whose ridged shape gives it sufficient strength 

 to carry the bird's weight. Finally, there is a short bony tail of about 

 six free caudal vertebrae, carrying four that are fused together to form 

 the upturned pygostyle, supporting the tail feathers. 



The joints of the vertebral column are therefore reduced so as to 

 allow movement only in the cervical region, between the thorax and 



>c±ggs; 



Fig. 254. Diagrams of the pectoral and pelvic girdles of an eagle, to show the methods of 



support in flying and walking. In each case the weight is carried on an arch, the strength 



of which is obtained by the peculiar kinked shape of the thin sheets of bone. 



synsacrum and in the tail. The axial muscles have been correspond- 

 ingly reduced. Those of the neck are large and the hinder cervical and 

 the thoracic vertebrae have special ventral hypapophyses for attach- 

 ment of the flexor muscles of the neck. The other back muscles, except 

 those of the tail, are reduced and the whole back forms a single rigid 

 strut, carrying the weight of the breast and viscera through the ribs 

 and the abdominal muscles either to the pelvic girdle or to the sternum. 

 In flying this weight is suspended on the wings and there is therefore 

 a compression stress throughout the ribs, and this no doubt accounts 

 for the ossification of their ventral parts. The weight of the bird when 

 resting on its wings (Fig. 254) is thus carried by the pectoralis major 

 as a tension member, through the plate-like sternum; the ribs, and 

 especially the coracoid, act as compression members. The last-named 

 bone lies nearly in the plane of the pectoralis major and is very strongly 

 built. 



6. The sacral girder and legs 



In standing, perching, and walking the weight is balanced on two 

 legs. To achieve this posture the type of girder found in the vertebral 



