OR OSSEOUS SYSTEM. J)l 



take place during respiration. The broad and thick anterior 

 ends of the sternal ribs are received into deep articular 

 cavities on the sides of the sternum, and principally of the 

 hyo-sternal portions of that bone, and they move freely 

 and securely in these sternal cavities. The sternum in birds, 

 as in chelonia, covers the greater part of the anterior surface 

 of the trunk, and presents, excepting in the strutheous 

 birds, an elevated, median, longitudinal, external crest, 

 which greatly extends the surface for the attachment of the 

 pectoral muscles. Its elements are anchylosed together, 

 like those of the cranium and pelvis in the adult ; but at an 

 early period the rudiments of nine elements can be detected 

 in its composition, which are generally disposed as repre- 

 sented in that of the peacock, pavo cristatus, (Fig. 48,) 

 although they vary much in their relative development in 

 different species. The two epi- sternal pieces (/,) are small, 

 compressed, anchylosed portions, which rise FIG 

 upwards between the two coracoid bones 

 (c y ) behind the united clavicles (/*,) and are 

 ossified to the upper edge of the large and 

 long ento-sternal element (o,) which is the 

 largest element of the sternum, and that 

 which has to sustain almost the entire force 

 of the pectoral muscles during flight. The 

 ento-sternal piece (o,) forms the crest of the 

 sternum, which is hollow and open above in 

 many aquatic birds, to admit a turn of the 

 trachea, and is thick and solid in the strong- 

 est rapacious birds ; it admits air into its interior by aper- 

 tures on its inner and upper part, and it receives the arti- 

 cular surfaces of the two coracoid bones (c y ) at its upper 

 edge. The two lateral portions (m,) which give attachment 

 to the sternal ribs (s, s,) are the hyo-sternal elements, which 

 are very large in the ostrich. Extending downwards and 

 backwards from the posterior margin of the hyo-sternal 

 element (m,) is a long narrow bone, generally bifurcated in 

 the gallinaceous birds (n,) which is analogous to the hypo- 

 sternal portion of this bone in the chelonian reptiles ; it is 

 more extensively developed in the water birds, and most of 

 all in the raptorial species, where it forms a continuous 

 piece with the lower end of the ento-sternal. The small 



