438 THE BIRDS xvi. 5 



and leg bones are pneumatized in this way in very good fliers, such as 

 some birds of prey and the albatross. Fusion of bones has proceeded 

 so far that the skeleton consists of a few hollow girders and large 

 plates of special shape (p. 441). This result is achieved by limiting 

 the joints at which movement occurs and simplifying the muscular 

 system. The long bones ossify from a single diaphysis, there are no 

 epiphyses at the ends. 



The skeleton of the backbone and limb girdles is so modified as to 

 allow the weight of the body to be carried in two quite distinct ways, 



Fig. 252. Metacarpal bone from the wing of a vulture, sectioned to show the 

 arrangement of the struts similar to that known to the engineer as a Warren's truss, 

 such as is often used in aeroplane wings. (After Prochnow and D'Arcy Thompson.) 



on the wings or on the legs. For this purpose there are two plate-like 

 girders, the sternum and the synsacrum, curved in opposite directions. 

 The muscles around the shoulder and hip joints balance the weight 

 on these girders and produce propulsion. The main thrusts come from 

 the pectoralis major in flying and from the leg retractors in walking. 

 Perhaps no other animals are suited so perfectly for locomotion by two 

 distinct means, and of course many birds can swim as well as fly and 

 walk. 



The whole axis of a bird is morphologically shorter than that of any 

 other vertebrate except a frog or a tortoise (Fig. 253): only the neck 

 remains a long and mobile structure. The number of cervical verte- 

 brae varies and is greater in the birds with longer necks; there are 

 fourteen in the pigeon, if we include two that bear ribs not articulating 

 with the sternum. The cervical centra have saddle-shaped surfaces, 

 the concavity running from side to side on the front and up and down 

 behind, allowing great mobility in all directions. 



There are four or five thoracic vertebrae, all except the last united 

 into a single mass. The ribs are large, double-headed, and jointed to 

 the vertebrae. They bear uncinate processes on their vertebral por- 

 tions, hook-like projections overlapping the rib next behind and thus 

 strengthening the whole thoracic cage. There is a well-marked joint 



