280 THE ANATOMY OF VEETEBRATED ANIMALS. 



tion of the notocliord. As Jiiger* has shewn, it is the 

 homologue of the odontoid ligament in the cranio-spinal 

 articidation ; and of the pulpy central part of the interver- 

 tebral fibro-cartilages in Mammalia. 



All the vertebral ribs in the dorsal region, except, per- 

 haps, the very last free ribs, have widely separated capitula 

 and tubei'cnla. More or fewer have weU ossified uncinate 

 processes attached to their posterior margins, as in the 

 Crocodilia. The vertebral ribs are completely ossified up to 

 their junction with the sternal ribs. The sternum in birds, 

 is a broad plate of cartdage, which is always more or less 

 completely replaced in the adult by membrane bone.f It 

 begins to ossify by, at fewest, two centres, one on each side, 

 as in the Batitoe. In the Carinatce 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 ossi- 

 fication (r. to m. X. in Fig. 81) is tenned the lophosteon, the 

 antero-lateral piece which articulates with the ribs, pleu- 

 rosteon {pi. o.), and the posterolateral bifurcated piece, met- 

 osteon. 



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

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

 the Casuaridce, 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 convexity of its ventral surface. But in other birds, and 

 notably in many Carinatce, the antero-lateral edges, which 

 are grooved to receive the coracoids, form a much more 

 open angle than in the Beptilia, 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 



* " Das Wirbelkorpergelenk sternum, like those further on 



der Yogel." Sitzungsberichte touching the skull, do not apply 



der Wiener Akademie, 18.^8. to Arhccepteryx, in which all these 



f These statements respecting parts are unknown or imperfectly 



the vertebral column, ribs, and known. 



