180 STRUCTURE AND CLASSIFICATION OF BIRDS 



straighten the posterior margin of the sternum, while 

 that of the abdominals would be to pull it out, perhaps 

 irregularly. Hopping and walking birds might therefore 

 be expected to have a more notched sternum than purely 

 flying birds ; that there is some relation of this kind seems 

 possible when we contrast the sternum of the running 

 gallinaceous bird with that of the essentially aerial eagle. 

 Moreover, since the pull of the abdominal muscles is in two 

 directions, one antero-posterior (recti), the other oblique (the 

 obliqui), we might expect to find what we actually do find, a 

 direction of the xiphoid processes which corresponds with 

 the resultant of these two forces, as is indicated in the 

 annexed diagram. There are a few other modifications in 

 the shape of the sternum which have been made use of for 

 systematic purposes, besides the keel and the notches, or 

 excavations of the posterior border. The rostrum of the 

 bone is sometimes very pronounced, and sometimes practi- 

 cally absent altogether. According to its position, more" 

 dorsally or more ventrally, the process has been called by 

 FuRBBiNGBE Spina externa, or spina interna sterni. In 

 the gallinaceous birds the two are combined in a vertically 

 compressed plate of bone which arises both from the lower 

 and from the upper side of the sternum. In the passerines 

 and in the todies, and a few of the allies of these groups of 

 birds, the anterior process of the sternum is more or less 

 distinctly bifurcate. 



The sternum of birds arises, as does that of other verte- 

 brates, in the first place 'between the ends of the ribs which 

 fuse together. Birds invariably have a few ' floating ' ribs 

 at both ends of the sternum which are no longer connected 

 with it, this connection being often lost ontogenetically. 

 There is, in fact, usually a shortening of the sternum during 

 development. The keel arises from the conjoined edges of 

 the two sets of fused ribs ; it is not preformed separately as 

 a median piece. This seems to settle in the negative an 

 earlier view that the carina sterni was the surviving repre- 

 sentative of the interclavicle of the reptiles, a view which 

 commended itself to more than one anatomist of distinction^ 



