176 ELEMENTS OF ORNITHOLOGY. 
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 Rhea, be but three. Ribs 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 toa 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 wncinate 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 allies 
the Chaja Screamers (Chauwna). 
Each vertebral rib ends superiorly by dividing into two branches, 
the upper branch of which is called the “tubercle ” of the rib, 
or tuberculum, and articulates with a diapophysis. The lower 
branch is called the ‘head and neck” ot the rib or capitulum, 
and articulates with a parapophysis or parapophysial 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 shorter than the vertebral ones. They 
* See ante, p. 170. 
