THE CLASS BIKDS. 315 



tremely developed in birds, and they form, with the lumbar 

 and sacral vertebrae, a single osseous mass (Fig. 283). In 

 general this osseous girdle is incomplete anteriorly; the 

 bones of the pubis do not unite with each other in front, 

 whilst the ischiatic portions unite with the sacrum, so that 

 the so-called notch in mammals becomes in birds a foramen 

 or hole. The thigh-bone is short and straight, and the leg 

 is composed, as in mammals, of a tibia, fibula, and rotula ; 

 but the fibula is united to the tibia before reaching the lower 

 part of the leg. A single bone represents the tarsus and 

 metatarsus ; this supports or carries the toes, never more 



Fig. 292. Apteryx of New Zealand. 



than four in number ; sometimes the great toe disappears, 

 and occasionally the one next it, thus leaving three or only 

 two (Fig. 282). The number of the phalanges increases pro- 

 gressively from within outwards, from the great toe to the 

 external or fourth, from two to five. Finally, of these four 

 toes, three in general are directed forward, the thumb or 

 great toe being turned backwards ; but sometimes the 

 external toe is also turned backwards, as especially in 

 climbing birds, the parroquets, tucans, and woodpeckers, &c. 

 (Fig. 291). 



Whilst the bird rests upon its feet on the soil, it is neces- 

 sary that the centre of gravity fall within the base of susten- 

 tation ; hence the utility of the extremely flexed thigh and 

 the obliquity of the tarsus on the leg. When the foot is 

 large and broad, and the neck is so flexible as to carry the 



