614 AVES [CH. XXIV 



In Reptiles only two vertebrae are joined to the ilium, but in 

 their case the weight of the body is supported on all four limbs, 

 whereas in a Bird the whole vertebral column has to be balanced 

 about two points of support, and hence the ilium must be quite 

 immovably strapped to the vertebral .column. The result of this 

 has been atrophy of some of the hinder ribs, and the ventral 

 halves of some of these form the xiphoid processes of the sternum. 

 The ischium is directed backwards parallel to the hinder part of 

 the ilium, and often fused with it so as to surround a space 

 called the ilio-ischiatic foramen. The pubis is a very slender 

 bone which is also directed backwards. It is in fact a postpubis 

 corresponding to the lateral process on the pubis of the Lizard (see 

 p. 577). Except in the Ostrich the two pubes never unite with 

 one another ventrally to the cloaca, as they do in Reptiles and 

 Mammals, the absence of a pubic symphysis facilitating the laying 

 of the egg, which is very large relatively to the size of the animal. 

 The thigh is bent sharply forwards and the shank backwards, and 

 the ankle is raised to a considerable height above the ground by 

 the great length and upward direction of the bones of the sole or 

 metatarsals (Fig. 307). Thus a Bird walks on its toes and like 

 Reptiles possesses an intertarsal ankle-joint. In Birds however, in 

 order to give firmness to the leg, the metatarsals are closely united 

 together and the small bones of the tarsus have entirely disap- 

 peared, the proximal row having been incorporated with the tibia, 

 while the distal bones have fused with the metatarsals. Thus in 

 an adult Bird the ankle-joint is a simple hinge between two 

 compact bones, the upper being a tibio-tarsus, the lower a tarso- 

 metatarsus. There are usually four toes, but the first, corre- 

 sponding to the human great toe, is sometimes absent, while 

 its metatarsal remains distinct from the other three. This toe, 

 except in Steganopoda, is directed backwards. In the Parrot the 

 fourth toe is also directed backwards. In the Cuckoo the fourth 

 toe is directed backwards but can be turned forwards at will. In 

 swimming and diving birds the second, third and fourth toes are 

 generally connected by a web of skin. Only in Steganopoda is 

 the hallux included in this web and in these birds this toe is 

 turned forwards like the rest. In other swimming and diving birds 

 the hallux is either absent or when present is free from the web and 

 turned backwards. The raised sole of the foot really constitutes 

 the visible " leg " of most birds, the thigh being altogether, and the 

 shank mostly, buried in the feathers. In many birds the sole is 



