ANA TOMY OF BIRDS 



215 



The singular carinate sternum of Notornis, and the ratite bone of 

 Apteryx, are concave crosswise along the front border, and bear the 

 coracoids far apart, at the summits of antero-lateral projections. 



A sternum is generally concavo-convex in every direction, 

 bellying downward ; somewhat rectangular, it may be long and 

 narrow, or short, broad, and squarish. It is commonly longer than 

 broad, with convex front border, a median beak, which is often 

 forked, jDrominent antero-lateral corners, pinched-in sides (bulging 

 in tinamou) and indeterminate hind border. The keel usually 

 drops down lowest in front, sloping or curving gently up to the 

 general level behind, with a concave (rarely protuberant) vertical 

 border, and j)ronounced apex, to which the clavicles may or may 

 not be ankylosed, as they are in a pelican, for instance. In Opis- 

 thocomus the clavicles ankylose with the manubrium of the sternum. 

 The external surfiice, both of body and keel, is ridged in places, 

 indicating lines of attachment of the different pectoral muscles. 

 In a few birds, notably swans and cranes, the sternal keel is ex- 

 panded and hollowed out to receive folds of the Avindpipe in its 

 interior (see Figs. 99, 100). — But the numberless modifications of 

 the sternum in details of configuration belong to systematic orni- 

 thology, not to rudimentary anatomy. 



3. THE PECTORAL ARCH 



The Pectoral Arch (Lat. pectus, the breast; Figs. 1, 2, 56, 58, 

 59) is that bony structure by Avhich the wings are borne upon the 

 axial skeleton. It is to the fore limb what the pelvic arch is to the 

 hind limb ; but is disconnected from the back-bone and united with 

 the breast-bone, whereas the reverse arrangement obtains in the 

 pelvic, which is fused with the sacral region of the si^ine. Each 

 I>ectoral arch of birds consists (chiefly) of three bones : the scapula 

 and coracokl, forming the shoulcler-ginlle proper, or scapular arch ; and 

 the accessory clavicles, or right and left half of the clavicular arch. 

 There is also at the shoulder-joint of most birds an insignificant 

 sesamoid ossicle, called scapula accessoria or os humero-scapulare (Fig. 

 56, ohs) ; and in many a rudiment of a bone called pirocoracoid, 

 which occurs in reptiles, but in birds is united with the clavicle. 

 From the ribs, the scapula ; from the sternum, the coracoid ; from 

 its fellow, the clavicle, converges to meet each of the two other 

 bones at the point of the shoulder. The lengthwise scapular arches 

 of opposite sides are distinct from each other ; the clavicular arch is 

 crosswise, and nearly always completed on the middle line of the 

 body ; by which union of the clavicles the whole pectoral arch is 

 coaptated. The coracoid bears the shoulder firmly away from the 

 breast ; the scapula steadies the shoulder against the ribs ; the 



