Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/738

720 yirdle and Sternum, plate 22, figs. 19, 20). J The ver tebral ribs are completely ossified up to their junction with the sternal ribs.

.

The sternum in Birds is a broad plate of cartilage which is always more or less completely replaced in the adult by membrane bone. It begins to ossify by, at fewest, two centres, one on each side, as in the Ratitw. In the Cari- natce it usually begins to ossify by five centres, of which one is median for the keel, and two are in pairs for the lateral parts of the sternum. Thus the sternum of a Chicken is at one time separable into five distinct bones, of which the central keel-bearing ossification (fig. 30) is termed the lopkosteon, the antero-lateral piece which articulates with the ribs, pie urosteon, and the postero-lateral bifurcated piece, metosteon. Fio. 30. -Sternum of a Chick (Callus domesticus) three days old, lower view, X three diameters. The cartilage is shaded and dotted, and the bony centres are light and stiinted. The front external processes are the &quot;costals,&quot; and are ossified eacli by a pleurofteon ; the median front process is the rostrum, and on each side of it are seen the corncoid grooves. * The fore-part of the middle, most of which is carinute, is ossified already by the lophosteon ; the forked xiphoids on each side are each largely occupied by a metosteon ; on the light bide the sternal ribs are shown. [In Turnix there are two more centres, mesiad of the pleurostea, these are the coracostea ; in Dicholophus the median part suddenly dilates, behind, into a heart-shaped flap of cartilage, which has an endosteal patch, the urosteonJ] Though the sternum, in most Birds, seems to differ very much in form from that of the Iteptilia, it is rhomboidal in the Casuariidce, where it differs from the Reptilian sternum chiefly in the greater proportional length of its posterior sides, the absence of median backward prolongations, and the convexity of its ventral surface. But in other Birds, and notably in many Carinatce, the antero-lateral edges, which are grooved to receive the coracoids, form a much more open angle than in the Reptilia, while the postero-lateral edges become parallel or diverge; and a wide, straight, or convex transverse edge takes the place of the posterior angle. Two, or four, membranous fontanelles may remain in the posterior moiety of the sternum when ossification takes place, and give rise to as many holes, or deep notches, separating slender pro cesses in the dry skeleton. All these correspond with so many divisions of the xiphoid process of the sternum in Mammalia, and hence are called middle, internal, and external xiphoid processes. Sometimes a median process, rostrum or manubrium (figs. 30, 31), is developed from the anterior angle of the sternum, and its antero-lateral angles are developed into costal processes, which may bear the articular surfaces for more or fewer of the ribs. The two last-named structures are very distinct in the Coracomorpha?, or Passerine Birds. FIG. si. Aside view of the chick s sternum, showing the perforation through the rostrum, and the depth and apiculntion of the keel. The external and internal (posterior) &quot;xiphoid processes&quot; are seen to end in pedate expan sions. In this, as in the lower view, the great size of (ho &quot;notches&quot; is shown. The extent to which the keel of the lophosteon is de veloped in the Carinate birds varies very much. In Strigops it is rudimentary ; in birds of powerful flight, as well as in those which use their wings for swimming, it is exceedingly large. FIG. 32. Shoulder-girdle of adult Fowl, nat. size; oblique side view Inverted, tc., scapulae; co., coracoids;/., clavicles; tip., hypocleidium, o; inter-clavicle; ?. ., glenoidal cavity The pectoral arch presents a long, narrow, and recurved scapula (fig. 32), without any supra-scapula, and a coracoid 