240 THE ANATOMY OF VERTEBRATED ANIMALS. 



All the vertebral ribs in the dorsal region, except, perhaps, 

 the very last free ribs, have wideh'-separated capitula and 

 tubercula. More or fewer have "well-ossified uncinate processes 

 attached to their posterior margins, as in the Crocodlliu. 

 The vertebral ribs are completely ossified up to their junction 

 with the sternal ribs. The sternum, in birds, is a broad plate 

 of cartilage, which is always more or less completely replaced 

 in the adult by membrane-bone.* It begins to ossify' by, at 

 fewest, two centres, one on each side, as in the Hatiice. In 

 the Garinatm it usually begins to ossify by five centres, of 

 which one is median for the keel, and two are in pairs, for the 

 lateral parts of the sternum. Thus the sternum of a chicken 

 is at one time separable into five distinct bones, of which the 

 central keel-bearing ossification (r. to rn. x. in Fig. 81) is termed 

 the lophosteon, the antero-lateral piece which articulates with 

 the ribs, pleurosteon [pi. o.), and the posterolateral bifurcated 

 piece, metosieon. 



Though the sternum, in most birds, seems to differ very 

 much in form from that of the MeptUia, it is rhomboidal in 

 the Gasicaridce, where it differs from the reptilian sternum 

 chiefly in the greater proportional length of its posterior sides, 

 the absence of median backward prolongations, and the con- 

 vexity of its ventral surface. But in other birds, and notably 

 in many CarinatCB, the antero-lateral edges, which are grooved 

 to receive the coracoids, form a much more open angle than 

 in the Reptilia, while the postero-lateral edges become 

 parallel, or diverge ; and a wide, straight, or convex, transverse 

 edge takes the place of the posterior angle. Two, or four, 

 membranous fontanelles may remain in the posterior moiety 

 of the sternum when ossification takes place, and give rise to 

 as many holes, or deep notches, separating slender processes 

 in the dry skeleton. All these correspond with somanj* divis- 

 ions of the xiphoid process of the sternum in Mammalia^ and 

 hence are called middle, internal, and external xiphoid pro- 

 cesses. Sometimes, a median process, rostrum or manubrium 

 (r., Fig. 81), is developed from the anterior angle of the ster- 

 num, and its antero-lateral angles are frequently produced into 

 costal processes (c p., Fig. 81), which may bear the articular 

 surfaces for more or fewer of the ribs. The two last-named 

 structures are very distinct in the Coracomorplim, or Passerine 

 Birds. 



* These statements respcctincr tlio vertebral column, ribs, and sternum, 

 like those furtlier on toucnino; the skull, do not apply to Ayckaofteryx, in 

 which ail these parts are unknown or imperfectly known. 



