i6 THE ARTISTIC ANATOMY OF ANIMALS 



the example of man, for that which follows, will be more 

 striking, on account of the mobility of his upper limbs) the 

 sternum gives origin to the pectoral muscles, and that these 

 muscles are inserted into other parts of the thoracic limbs, 

 designed by their contraction to draw the arms downwards, 

 forwards, and inwards — that is, when these are in a state of 

 abduction and in a horizontal direction, they draw them 

 towards the anterior surface of the thorax and downwards. 

 Now, this movement is similar to that made by birds 

 during flight. It is necessary to add that, in the latter 

 case, the more the displacement of the upper limbs has 

 of force and extent, the more the pectoral muscles are 

 developed. 



For these reasons, birds, in which, during flight, the move- 

 ments of the thoracic limbs — the wings — are necessarily 

 energetic, present a great development of the pectoral 

 muscles ; having consequently, because an extent of surface 

 for the origin of the muscles commensurate with their de- 

 velopment is necessary, a very large and peculiarly shaped 

 sternum (Figs. i8, 6 ; and 21, 6). Indeed, not only is the 

 sternum large, but, further, in order to form a deeper surface, 

 proportionately adapted to the muscles which arise from and 

 cover it, its anterior surface presents, in the median line, a 

 prominent crest known as the keel. This prominence forms 

 two lateral fossae. We cite as examples, the sternum of the 

 eagle, the vulture, the falcon, and the hawk. 



All birds are not, however, equally adapted to flight, for 

 in the domestic cock, which flies but a short distance, and 

 badly, the sternum is less developed (Fig. 7) ; it is also 

 diminished by slots, which diminish its surface. These slots, 

 two on each side, are called from their position the internal 

 and external slots. They are bounded by narrow, elongated, 

 bony processes, an internal and an external ; the expanded 

 lower extremity of the latter overlaps the last inferior ribs 

 (see p. 19). The part of the external border which surmounts 

 this external process is hollowed out into grooves, which 

 receive the inferior ribs, and terminates superiorly in an 

 osseous projection known as the costal prominence. 



In the ostrich, the cassowary, and the apteryx, which run, 



