ANATOMY OF BIRDS 



when either or neither of these connections is made. Usually the 

 last rib, though bearing a perfect hsemapophysis, does not reach the 

 sternum ; in the loon, for example, the last rib floats at loth ends, 

 having connection neither with vertebra nor sternum ; and the two 

 next ribs float at their sternal ends. The perfected ribs are few, — 

 five or six is a usual number, though nine are hsemapophysis-bearing 

 in the loon. The last rib at least is usually sacrarial, as it belongs 

 to a dorsal vertebra which is ankylosed with the sacrarial mass ; 

 and two, or even, as in the loon, three ribs may likewise issue out 

 from under cover of the ilia. These "sacral ribs" or sacrocostals are 

 furthermore distinguished by being devoid of the epipleural or un- 

 cinate processes (Lat. uncm, a hook ; Fig. 56, u) with which other true 

 ribs are furnished, forming a series of splint-bones proceeding 

 obliquely from one rib to shingle over the next succeeding one, 

 and thus increase the stability of the thoracic side-walls. Such 

 splints may be either articulated or ankylosed with their respective 

 ribs; they have independent ossific centres. The upper (pleura- 

 pophysial) part of a rib, or " vertebral rib," when perfected, articu- 

 lates with the side of the body of a vertebra by its head or cwpit- 

 ulum (Lat. dimin. of caput, head), and also with the lateral process 

 of the same vertebra by its shoulder or tuberculum (Lat. dimin. of 

 tuher, a swelling). In well-marked cases the head and shoulder are 

 quite far apart, the rib seeming prolonged above ; either of these 

 vertebral connections may be disestablished, the other remaining, or 

 both may be lost. The lower (hsemapophysial) part of a rib, or 

 " sternal rib," articulates with the side of the sternum by a simple 

 enlargement ; the ends of those sternal ribs which thus join the, 

 sternum tend to cluster closely together at a part of the breast-bone 

 called its costal process (Fig. 58) ; those which do not make the 

 sternal connection are simply bundled together. Commonly five or 

 six, sometimes four, rarely only three ribs reach the sternum. The 

 ribs are ordinarily as slender and strict as those shown in Fig. 56 ;. 

 but in Apteryx, for example, their pleurapophysial parts are expan- 

 sive and plate-like. They lengthen rapidly from before backward,, 

 both in their vertebral and their sternal moieties ; these parts meet 

 at angles of decreasing acuteness from before backward ; but these 

 angles, as those of the ribs both with vertebrae and sternum, inces- 

 santly increase and diminish in the respiratory movements of the 

 chest ; all being in expiration more acute, and more obtuse in 

 inspiration. 



The Avian Sternum (G-r. <nipvov, sternon, the breast; Fig. 56, S) 

 is highly specialised ; its extensive development is peculiar to the 

 class of birds, and its modifications are of more importance in 

 classification than those of any other single bone. Thereupon it 

 becomes an interesting object. Theoretically it is a collection of 



