176 ELEMENTS OF OENITHOLOGT. 



The first one joins the sternum by the intervention of its 

 corresponding sternal rib. All such ribs are called true ribs. 

 Vertebral ribs which do not reach the sternum are termed false 

 or floating ribs. Ordinarily the last sternal rib joins the 

 sternal rib in front of it instead of the sternum. Very 

 rarely, as in the Diver, the last rib floats at either end, being 

 connected with the vertebral column by membrane only. 



There are usually five " true " ribs, though there may, as in 

 the Ehea, be but tbree. Eibs are usually elongated narrow 

 structures, having considerable spaces between them, though in 

 the Apteryx they are exceedingly broad. 



Each vertebral rib joins its sternal rib at a marked angle 

 open forwards, and a synovial membrane is interposed between 

 the ends of each conjoined vertebral and sternal rib. This facili- 

 tates motion, and enables the angle just referred to to be made 

 more or less acute, according as the breast-bone (to which the 

 sternal ribs are fixed at their lower ends) is drawn up towards 

 the back-bone, or the reverse. These movements are most im- 

 portant to a bird, as it empties its lungs by drawing up the 

 breast-bone, and so contracting the body-cavity in which they 

 lie — and fills them by depressing it, and so causing air to rush 

 in and fill the vacuum which thus tends to be formed. To 

 the hinder margin of all the vertebral ribs except the last or 

 the last two or three, a bony process is almost always annexed, 

 called the uncinate process, which projects upwards and back- 

 wards. These processes may be anchylosed to or moveably 

 articulated with the rib, or they may be absent, as in the 

 Horned Screamer, Palamedea cornuta (fig. 46), and in its alUes 

 the Chaja Screamers {Ghauna). 



Each vertebral rib ends superiorly by dividing into two branches, 

 the upper branch of which is called the " tubercle " of the rib, 

 or tuherculum, and articulates with a diapophysis. The lower 

 branch is called the " head and neck " of the rib or capitulum, 

 aud articulates with a parapophysis or parapgphysiai surface. 

 It is the fact of these articulations which has caused the dia- 

 pophysis and the parapophysis to be respectively called the 

 " tubercular " and " capitular " transverse processes as before 

 stated*. The vertebral ribs increase in length from before 

 backwards. 



The sternal ribs are sfiorter than the vertebral ones. They 



* See ante, p. 170. 



