58 THE BIOLOGY OF BIRDS 



dead skeleton to the living skeleton enswathed in living 

 muscle and bound together by living tendons. When in 

 walking we rest for a brief moment with one foot on the 

 ground and swing forward the other leg, we are balancing 

 the body on the smooth head of the thigh-bone fitting into 

 the socket of the hip-girdle, and in this balancing alone 

 over a score of muscles are implicated. Similarly, of course, 

 with the Running Bird. 



One is tempted to regard the pectineal process in front 

 of the acetabulum of birds as a relic of a true pubis, the 

 pubic bar of birds being then interpretable as the post- 

 pubis of some extinct reptiles. But as regards the pectineal 

 process, this theor)' does not work out well, as Lebedinsky 

 (1914) points out clearly. Parker and Baur have shown 

 that the pectineal process of the kiwi is formed by both 

 ilium and pubis ; Lebedinsky has shown that the pectineal 

 process of the African ostrich is formed from the pubis 

 only ; Burge, Mehnert, and I^ebedinsky have shown that 

 the pectineal process of Carinatae is formed from the ilium 

 only. With so much heterogeneity of origin, the pectineal 

 process cannot have any historical (phylogenetic) importance. 

 It is probably a new acquisition within the class of birds ; 

 it serves for the insertion of the ambiens muscle — of some 

 use in perching — when that muscle is developed. 



There are other peculiarities of the hip-girdle besides 

 the length of the ilia on each side of the acetabulum and 

 their thorough fusion to the complex syn-sacrum, but their 

 interpretation is more difficult. The backward-going pubic 

 bar is perhaps homologous with the post-pubis of some 

 reptiles. The ischia are firmly fused to the ilia. The 

 whole hip-girdle with its six or more component bones is 

 one piece. There is no ischiac symphysis except in the 

 American Ostrich (Rhea) and no pubic symphysis except 

 in the African Ostrich (Struthio). It may be that the 

 suppression of the true pubes (on one view) or the back- 

 ward turning of the true pubes (on another view) is adaptive 

 to the fact that birds lay relatively large hard-shelled eggs 

 which cannot be safely subjected to pressure. 



