THE ANATOMY OF BIBDS. — OSTEOLOGY. 



145 



pleurostea, 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 froui one side to the other; sometimes each crosses to the other 

 aide 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 

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

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

 The singular oarinate 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 lunger than 

 broad, with convex iront 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) fi<j. 58. -Typical passerine 

 vertical border, and pronounced apex, to which the clavicles may sternam, pectoral arches, and 



or may not be anchylosed, as they are in a pelican for instance. In sternal ends of ribs ; from the 



.J^ '"^ ^ robin, Tturclms nagratonua, nat. 



Opisfhocomus, the clavicles anchylose with the manubrium of size; Dr. R.W. shufeldt, U.S.A. 



the sternum. The external surface, both of body and keel, is Sternum single -notched, with 



. .,..,. . promment costal processes and 



ridged m places, mdicatmg Imes of attachment of the different pee- forked manubrium; five ribs 



toral muscles. In a few birds, notably swans and cranes, the keel reaching stemum.one rib "floats 



ing." 

 is expanded and hoUowed 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 ornithology, not to rudimentary anatomy. 



3. TUB PECTORAL ARCS. 



The Pectoral Arcb (hsbt. pectus, the breast; figs. 1, 2, 56, 58, 59) is that bony structure 

 by which the wings are boi'ne 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 spine. Each pectoral arch of birds consists (chiefiy) of three bones : the scapula 

 and coracoid, forming the shoulder-girdle proper, or scapular a/rch ; 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 accesswia or os humero-scapulare (fig. 56, ohs) ; 

 and in 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 ; fi'om 

 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 ooaptated. 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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