76 AVES. 



Cassowary, and which directed backward rests upon the rib imme- 

 diately behind that from which it springs. These processes are 

 rarely wanting entirely, or so slightly developed, as in the Emeu of 

 New Holland, that they can scarcely be perceived. The last, also 

 the second, as in the Ostrich, but more rarely the three last ribs, as 

 in Rhea, do not reach the sternum, and correspond thus with the 

 false ribs in the human subject. The true ribs are united to the 

 sternum by peculiar long bones corresponding to the cartilages of the 

 ribs. These, called sterno-costal bones, are moveably articulated 

 both with the ribs and the lateral borders of the sternum, and are 

 united at both ends by capsular ligaments. 



The Sternum is, of all the bones in Birds, that which exhibits the 

 most numerous and striking variations, being formed upon a very 

 special and definite type, which departs widely from that of the rest 

 of the Vertebraia. It is in general of considerable breadth, and of an 

 elongated quadrangular form, and upon its anterior surface there is 

 found arising from the median line a very strongly developed crest 

 or keel, which is wanting only in the Brevipennes or Struthious 

 birds, and which serves for the attachment of the great pectoral 

 muscle. The development of this crest generally stands in direct 

 relation with the powers of flight of the Bird, and it is accordingly 

 most large and projecting in the Humming-birds, which though the 

 smallest in size, are the most rapid flyers. High-flying Birds, such 

 as the typical Birds of Prey, have a broader sternum, the pos- 

 terior margin of which is uninterrupted by notches, as in birds of 

 feeble flight. Anteriorly, the sternum is united with the coracoid 

 bones, upon either side by a pair of very elongated articulating 

 surfaces. Between them is usually situated a single median and 

 often small process, while a similar one projects upon either side from 

 the outer edge of the coracoid articulation. The greatest number 

 of varieties are met with in the posterior border of the sternum, and 

 which arc frequently employed as characteristics of whole families 

 and genera. There occur here one or several notches (excisura 

 obturatoriae] closed only by tendinous membrane, which are either 

 bounded by long narrow abdominal processes externally, or consti- 

 tute true openings. As regards the diversities which the sternum 

 presents in the several orders, it is short and broad, almost entirely 

 flat or slightly concave in the Brevipennes, with two short abdominal 

 processes in the Ostrich and in the Apteryx, in which last it is per- 

 forated in the middle by two openings ; in the Cassowary it is 

 somewhat roof-shaped. Next to the Humming-birds, the keel is 



