THE ANATOMY OF BIRDS. — OSTEOLOGY. 



151 



plcurostea, produced in angular costal processes. This border is also thickened, and presents 

 on each side a well-marked, smooth-faced groove, in which the expanded feet of the coracoid 

 bones are instepped and firmly articulated. These deep grooves commonly meet in the middle; 

 are occasionally continuous from one side to the other ; sometimes each crosses to the other 

 sidfe a little way. The costal processes on each side also have thickened edges, with a series 

 of articular facets for the ribs, which gives this border a fluted 

 or serrate profile. Generally the fore half, or rather less, of the 

 side border of the sternum is thus articular ; and it is only such 

 costiferous (rib-bearing) extent of sternum which cori'esponds to the 

 whole body of the bone in a mammal, all the rest being " xiphoid." 

 Tlie 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 each 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, prominent antero-lateral corners, pinched-in sides (bulg- 

 ing 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 pronounced apex, to which the clavicles may 

 or may not be anchylosed, as they are in a pelican for instance. In 

 Opisthocomus, the clavicles anchylose Math the manubrium of size; Dr. R.w. Shufelilt, U.S.A. 



the sternmn.' The external surface, both of body and keel, is Sternum single -notched. ^vitl> 



' •' ' prominent costal processes and 



ridged in places, indicating lines of attachment of the diff'erent pec- forked manubrium; five ribs 

 toral muscles. In a few birds, notably swans and cranes, the keel reacMng sternum, one rib "float- 

 is expanded and hollowed out to receive folds of the windpipe in its 



interior (see figs. 99, 100). — But the numberless modifications of the sternum in details of 

 configuration belong to systematic ornithtdogy, not to rudimentary anatomy. 



Fig. 58. —Typical passerine 

 sternum, jiectoral arches, and 

 sternal ends of ribs ; from the 

 robin, Tnrdiis migi-ntorius, nat. 



3. THE PECTORAL ARCH. 



The Pectoral Arch (Lat. i^ectus, the breast; figs. 1, 2, 56, 58, 59) is that bony structure 

 liy wliicli the wings are l)()rue upon the axial skeleton. It is to the fore limb what the pelvic 

 arcli is to the hind liml) ; but is disconnected from the back-bone and united with the breast- 

 bone, whereas the rever.se arrangement obtains in the pelvic, which is fused with the sacral 

 region of the spine. Each pectoral arcli of birds consists (chiefly) of three bones : the scapula 

 and coracoid, forming the shoulder-girdle proper, or scapular arch ; and tlie accessory clavicles, 

 or right and left half of tiie clavicular arch. There is also at the shoulder-joint of most birds 

 an insignificant sesamoid ossicle, called scapula accessoria or as humero-scapulare (fig. 56, ohs) ; 

 and in many a rudiment of a bone called procoracoid, which occurs in reptiles, but in birds is 

 united witli the clavicle. From the ribs, the scapula; fnim the sternum, the coracoid ; from 

 its felldw, tlie clavicle, converges to meet each of the two other bones at the point of the 

 shoulder. Tiie 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 tiie body ; by 

 whicli uninn nf the clavicle,^; the whole pectoral arch is coaptated. The coracoid bears the 

 slioulih'r firmly away fnmi the Itreast : the .scapula steadies the shoulder against the ribs ; the 

 clavicles iceep the .slioulders apart from each other. The scapular arch is always present and 

 complete ; the clavicular is sometimes defective or wanting. There are two leading styles of 



