26 THE STRUCTURE AND LIFE OF BIRDS chap. 



side, and in all there is some flexibility upwards and 

 downwards. But the amount varies much in different 

 species. The tail vertebrae are very different, and, in 

 the freedom with which they move upon one another, 

 approximate to those of the neck. Were it not so, 

 birds could not do what they may easily be seen to do 

 while flying — move their tails for purposes of steering 

 or to check themselves suddenly. The Pygostyle, 

 the large bone which supports the tail, consists of a 

 number of vertebrae fused together (PY, fig. 2). 



The Pelvis. 



The bird's Pelvis, at its anterior end, roofs over 

 the backbone. It is formed of three bones, which 

 in different classes of animals assume forms so 

 different that they are often difficult to recognise. 

 The difficulty, however, will be got over, if we bear in 

 mind what I have already explained, that bones, 

 however much they may change their form, yet keep 

 the same position relatively to each other. One of 

 these bones, the Ilium (IL, fig. 8) attaches to the back- 

 bone, and by that it may be recognised. Its peculiar- 

 ity in the bird is that it unites with so many vertebrae 

 both before and behind the hip-joint, fusing with 

 them and making this part of the backbone absolutely 

 rigid. The two remaining bones assist the Ilium to 

 form the socket of the hip-joint, and they must be 

 distinguished by their positions relatively to it. The 

 Pubis forms the lower front of the socket (PB), the 

 Ischium (IS) the hinder part. The former projects a 



