THE ANATOMY OF BIRDS. — OSTEOLOGY. 
145 
plearostea, produced in angular costal processes. This border is also thickened, and presents 
on each side a well-marked, smootli -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 
«ide 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 corresponds to the 
whole body of the bone in 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 
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 
OpistJiocomus, 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 diff'erent 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 
interior (see figs. 99, 100). — But the numberless modifications of the sternum in details of 
configuration belong to systematic ornithology, not to rudimentary anatomy. 
Fig. 58. — Typical passerine 
sternum, pectoral arches, and 
sternal ends of ribs ; from the 
robin, Turdus migratorius, nat. 
size; Dr. R.W. Shufeldt, U.S.A. 
Sternum single -notched, with 
prominent costal processes and 
forked manubrium; five ribs 
reaching sternum, one rib "float- 
ing," 
3. THE PECTORAL ARCH. 
The Pectoral Arch (Lat. pectus, 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 the pelvic, which is fused with the sacral 
region of the spine. Each pectoral arch of birds consists (chiefly) of three bones : the scapula 
and coracoid, forming the slioulder-girdle 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 scapwla accessoria 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 ; 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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