STEEXUM OF VEETEBEATA. 



443 



Fig. 234. Sternum Fig. 235. Sternum 



of^Buteo vulgaris of Numida melea- 



(seen a little from gris (seen from in 



one side), crs Crista front), 



sterui. / Furcula. sterni. 

 c Coracoid. 



crs Crista 

 c Coracoid. 



six) of ribs connected with it. In the Ratitee it forms a broad piece 

 of bouej which is very convex anteriorly. In the Carinatee, however, 

 it is distinguished by a projecting keel (Fig. 234, cr.s) on the 

 anterior and convex face of 

 the sternum, which serves to 

 inci'ease the surface from 

 which the muscles take their 

 origin. The form of the 

 sternum is therefore correlated 

 with the development of the 

 muscles, while the size of the 

 sternum and of its keel corre- 

 sponds to the power of flight 

 of which its possessor is 

 capable. The hinder end 

 often presents paired fora- 

 mina, which are closed by 

 membrane (Birds of Prey and 

 Water Birds) ; when the peri- 

 phery of these foramina is 

 broken through towards the 

 hinder edge of the sternum, 

 clefts are formed, between _ _ 



which the so-called abdominal processes proiect (Fig. 235). The 

 sternum of Birds has very much the same relations as in Reptiles, 

 owing to its connection with the shoulder-girdle. 



The sternum of Mammals is distinguished from that of the 

 preceding classes by the large 

 number of segments that are 

 separately ossified in it. It is 

 made up of bones, which are 

 placed one behind the other, 

 although they were all laid 

 down in a continuous cartilage. 

 They are not nnfrequently 

 developed from paired centres 

 of ossification, and frequently 

 fuse into one piece. 



The form of the sternum 

 is afiected by its relations to the 

 shoulder-girdle. The anterior 

 piece is connected with the cla- 

 vicles, and is distinguished by 

 its greater breadth ; it forms 

 the manubrium. A ridge-like process (Pig. 236, c ) is developed on the 

 anterior face of this segment in aerial Mammalia ; this is physiologi- 

 cally similar to the keel of Birds. When the clavicles are absent, 

 the anterior end of the sternum is generally much diminished in 

 size. The hinder end is always drawn out into a median piece, which 

 frequently remains cartilaginous (Fig. 237, if) (siphioid process). 



Fig. 236. Sternum 

 of Vesper tilio mu- 

 rina. s Sternum, 

 c' Crest, cl Clavicle, 

 c Ribs. 



Fig. 237. Sternum of 



Cervus capreolus. 



se Costal cartilages. 



X Xiphioid process. 



