xvi. 6 SACRAL GIRDER 441 



column of other terrestrial vertebrates has been abandoned, and with 

 it the system of braces (back muscles) holding up the weight of the 

 forepart of the body. Instead the whole axis is so shortened that the 

 centre of gravity lies far back, low, and over the feet. This is not 



Fig. 255. Diagram of muscles of the hind leg of a bird. Tendons shown dotted. 



1, Mm. ilio-trochanterici; 2, M. ilio-femoralis; 3, M. obturator; 4, M. ischio-femoralis; 

 5, M. caud-ilio-femoralis; 6, M. pub-ischio-femoralis; 7, M. ilio-tibialis posterior; "ja, 

 M. ilio-tibialis anterior; 8, M. sartorius (ilio-tibialis internus); 9, M. femoro-tibialis medius ; 

 ga, M. femoro-tibialis externus; 11, M. ilio-fibularis; 12, M. ischio-flexorius; 13, M. 

 caud-ilio-flexorius; 14, M. gastrocnemius; 15, M. peroneus superficialis; 16, M. peroneus 

 profundus; 17, M. tibialis anterior; 18, Mm. rlexores digitorum; 19, Mm. extensores 

 digitorum. si. sling for M. ilio-fibularis. (After Stolpe.) 



apparent from Fig. 255, which is not in a normal perched position. 

 Birds whose feet are placed far back for swimming must hold the body 

 nearly upright to achieve a stable position with the centre of gravity 

 over the feet (auks, penguins). The ribs and abdominal muscles trans- 

 fer the weight to the greatly elongated ilia, which are fused to the 

 vertebrae, making a long girder of approximately parabolic form. 

 Though this is composed of bone of almost paper thinness, it is 

 strengthened by longitudinal ridges (Fig. 254). Its strength, like that 

 of the sternum, lies not in its arched shape in the transverse plane, but 

 in the distribution of weight that is achieved by its longitudinal curve 

 and peculiar kinked shape. The whole pelvic girdle is modified to allow 



