SKELETON. 



57 



of the ribs, eventually connecting them together (fig. 55). With continued growth 

 these bars of the two sides meet and fuse in the median line, forming a median plate, 

 the sternum. Later this separates from the ribs, and with the appearance of bone, 

 becomes a series of separate elements, the sternebrae (fig. 57), alternating with the 

 ribs; by fusion of sternebrae the parts in man arise. 



In the amphibia the short ribs never extend to the sternum, but skeletal parts 

 occur near the mid-ventral line in a few forms, which may be ventral ribs as they 

 participate in the formation of the sternum. Nothing is known of a true sternum 

 in the stegocephals. In the urodeles it is a short cartilaginous plate, lying mostly 

 behind the girdle, with its sides grooved to receive the medial ends of the coracoids. 



FIG. 55. Development of sternum in 30 mm. human embryo, after Ruge. cl, lower end 

 of clavicle; r, ribs; s, two halves of sternum; ss t suprasternalia. 



In the toads and their allies (arcifera) it has hardly passed beyond the urodele 

 condition, but the hinder angles are produced into long processes. In the frogs 

 (firmisternia) it consists of a slender thread between the medial ends of the girdles 

 (epicoracoids), but in front it expands into an omosternum, ossified behind; while 

 behind the girdle it forms a broad xiphisternum, the anterior part of which is bone. 



In the lizards the sternum is a large rhomboid plate, largely cartilag- 

 inous, sometimes perforated with two foramina and joined by a vary- 

 ing number of ribs (fig. 56). In the crocodilia there is an anterior 

 rhombic plate, joined by two pairs of ribs and followed by a second, 

 so-called abdominal sternum, connected with from five to seven pairs 

 of ribs. Ichthyosaurs, plesiosaurs and snakes have no sternum, 

 while it was imperfectly ossified in theriomorphs and dinosaurs. 



In the birds (fig. 53) the sternum is ossified and at most is con- 

 nected with eight pairs of ribs. Behind it may be rounded, perforated, 



