ADAPTATIONS OF THE SKELETON 57 



§ 3. Adaptations to Bipedal Progression 



If a perpendicular be dropped from the hip-joint 

 (acetabulum) where the thigh-bone (femur) articulates with 

 the hip-girdle, it will be seen that in most birds a large 

 portion of the body lies in front of this. In man's case the 

 anterior part of the body is in line with the vertical axis 

 of the leg. In the old man's stoop the muscles that keep 

 the anterior part of the vertebral column vertical are losing 

 their tone. In adaptation to the condition of affairs in 

 birds the ilia are much elongated in front of and behind the 

 acetabulum, and are fused to a long stretch of vertebrae — 

 the syn-sacrum. This gives the hip-girdle a long and strong 

 grip of the backbone, obviously facilitating the balance of 

 the body on the apex of the leg. The risk of toppling 

 forward is obviated. 



The syn-sacrum has a complex make-up. It consists 

 of two or so thoracic vertebrae (bearing ribs), all the lumbar 

 vertebras, the two true sacrals which in the embryo are 

 first connected with the girdle, and, finally, about half of 

 the caudals. Beyond it, posteriorly, there project the free 

 caudals ending in the ploughshare bone in Flying Birds. 

 The formula of the syn-sacrum for the pigeon is : 

 Thoracics, i ; +Lumbars, 5 or 6 ; +Sacrals, 2 ; 

 +Caudals, 5 — a dry detail at first sight, but eloquent when 

 we appreciate the mechanical significance of the complex 

 coalescence. 



It is very instructive to contrast the bird with an animal 

 like the frog, which does not and could not stand on its 

 hind legs. In the frog there is only one sacral vertebra, 

 and the tips of the long ilia are connected to its transverse 

 processes by a loose ligamental union, an arrangement 

 well-suited for swimming and spasmodic leaping. In the 

 bird there are over a dozen " sacral " vertebrae and the 

 ilio-sacral connection is a fusion, an arrangement well- 

 suited for bipedal progression. It need hardly be said that 

 in thinking of such problems one must get beyond the 



