26 THE STRUCTURE AND LIFE OF BIRDS cnap, 
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 vertebre 
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 
