THE ANATOMY OF BIRDS. — OSTEOLOGY. 



145 



pleurostea, produced iu angular costal processes. This border is also thickened, and presents 

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

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

 are occasionally continuous from one side to the other; 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, wliich gives this border a tiuted 

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

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

 costiferoits (rib-bearing) extent of sternum which corresponds to the 

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

 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 each direction, 

 bellying downward; somewhat rectangular, it may be long and 

 nan'ow, 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) p^^ 58. — Typical passerine 

 vertical border, and pronounced apex, to which the clavicles may sternum, iiectoral arches, and 

 ,1 11J j.\. • ^• £ ■ J. T sternal ends of ribs ; from the 



or may not be anchylosed, as they are m a pelican tor instance, '" 



In 



Opistliocomus. the clavicles anchylose with the manubrium of 

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

 ridged in places, indicating lines of attachment of the different pec- 

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

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

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

 configuration belong to systematic ornithology, not to rudimentary anatomy. 



robin, Turdu.i migratorius, nat. 

 size; Dr. R.W. Shufeldt, U.S.A. 

 Sternum single - notched, witli 

 prominent costal processes and 

 forked manubrium; five ribs 

 reaching sternum, one rib "float- 

 ing." 



3. THE PECTORAL ARCH. 



The Pectoral Arch (Lat. x>ectm, the breast; figs. 1, 2, 56, 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 from the back-bone and united with the breast- 

 bone, whereas the reverse arrangement obtains in tlie pelvic, which is fused \Adth the sacral 

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

 and coracoid, forming the shoulder-girdle proper, or scajiular 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 sesauKjid ossicle, called scapula accessoria or as Immero- scapular e (fig. 56, ohs) ; 

 and iu many a rudiment of a bone called procoracoid, 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 

 clavicles keep the shoulders 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 



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