XVI 



ON STRENGTH AND FLEXIBILITY 1011 



on two legs as on a pier, the cantilever being constituted by the 

 pelvic bones, drawn out fore and aft and firmly welded to a long 

 stretch of vertebral column. The centre of gravity is kept in a line 

 passing through the acetabulum^, and the long toes help to preserve 

 an unstable but well-ad j usted equilibrium . One arm of the cantilever 

 carries head, neck and wings, the other, the shorter arm, carries the 

 abdomen; but the whole weight of the viscera hangs in the abdomen 

 as in a bag, and on the other hand head and neck are kept small and 

 light, and their purchase on the fulcrum is under constant modifica- 

 tion and control. A stork or a heron is continually balancing itself; 



Fig. 484. Pelvis oi Apteryx. The line AB is vertical, or nearly so, 

 in the standing posture of the bird. 



as the beak is thrust forward a leg stretches back, as the bird walks 

 along its whole body sways in keeping. No less elegant is the 

 perfect balance of the same birds at rest — the heron standing on 

 one leg, even on a tree top, the flamingo also on one long leg, 

 with its neck close coiled and its head tucked amongst the 

 feathers. 



The approximately parabolic form of the great pelvic cantilever 

 is best seen in the ostrich and other running birds, but more 

 commonly the strength of the cantilever is got in other ways. 

 Usually, as in the fowl, it consists of a thin shell of bone curved 

 over like the bonnet of a motor car and stiffened, or "cambered," by 

 ridges converging on the acetabulum. A doubled sheet of paper, 

 cut roughly to the shape of the pelvis and then pinched up into folds 



