THE ANATOMY OF BIRDS. — OSTEOLOGY. 



145 



pleurostea, produced in angular ccjstal processes. This border is also thickened, and presents 

 on each side a well-marked, sniootli-faced groove, in which th(; expanded feet of tlie coracoid 

 bones are instepped and firmly articulated. These deej) grooves commonly meet ia the middle; 

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

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

 of articular facets for the ribs, which giv(w this border a fluted 

 or serrate profile. Generally the fore lialf, or rather less, nf the 

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

 costiferous (rib-bearing) extent of sternum which corresfiouds to the 

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

 The singular cariuate sternum of Notornis, and the ratite Ixme of 

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

 coracoids far apart, at tlie sumuiits 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 eommouly longer than 

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

 forked, prominent autero-lateral ctirners, pinched-in sides (bulg- 

 ing in tiuamou) and indeterminate hind border. The keel 

 usually drcjps down lowest in front, slopiug or curving gently up to 

 the general level behind, with a concave (rarely prntuberant) 

 vertical border, and pronounced apex, to which the clavicles may 

 or may not be anchylosed, as they are in a ])elican for instance. In 

 Opistliocomus. the clavicles anchylose with tla; manubrium of 

 the sternum. The external surface, both of body and keel, is 

 ridged in places, iuilicatiug lines of attacliment of the different pec- 

 toral muscles. In a few birds, notably swans and cranes, the keel 

 is ex]ianded and hollowed out to receive folds of the windpipe in its 

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

 configuration belong to systematic ornithology, not to rudimentary anatomy. 



Fig. 58. — Typical passerine 

 sternum, pectoral arches, and 

 sternal ends of ribs ; from tlie 

 robin, Turdu.^ mUjratorluf^, nat. 

 size; Dr. E.W. Sliufeldt, U.S.A. 

 SteiDura single -notched, with 

 prominent costal processes and 

 forked manubriam; five ribs 

 reaching sternum, one rib "float- 

 ing." 



3. THE PliCTdHAL AUCIJ. 



The Pectoral Arch (Lat. pectus, the breast; figs. 1, 2, 50, ,58, 59) is that bony structure 

 by which 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 frcjin the back-bone and united with the breast- 

 bone, whereas the reverse arrtingement obtains iu tlie pelvic, which is fused with tlie sacral 

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

 and coracoid, forming the shoulder-girdle pro).ier, <.ir scapular arch ; and the accessory clavicles, 

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

 an iusignificant sesamoid ossicle, caWbA scapula accessoria or os Immero-scapulcvre (fig. 56, ohs); 

 and in many a rudiment of a bone called procorucoid, wliicli occurs in reptiles, but in birds is 

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

 its fellow, the clavicle, converges to meet each of the two otlier 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 tears the 

 shoulder firmly away from the breast; the scapula steadies the shoulder against the ribs ; the 

 clavicles keep the shoulders apart from each other. Tlie scapular arch is always present and 

 complete ; the clavicular is sometimes defective or u-anting. There are two leading styles of 



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