3. The Breast-bone (Sternum) presents two different styles–according to whether it exhibits on its ventral surface a median ridge or keel (carina), or not. In the former case, which is that of by far the greater number of existing Birds (hence termed Carinatae), the keel is of variable size, being correlated with the power of flight. It is exceedingly deep in the Swifts, Humming Birds, and certain Petrels, but dwindles almost to disappearance in some flightless forms such as the Dodo, the Kakapo (Stringops), the extinct New Zealand Goose (Cnemiornis), and a good many Rails.
Fig. 2.–Skeleton of the trunk of a Falcon. Ca, coracoid, which articulates with the sternum (St) at †; Cr, keel of sternum; Fu (Cl), furcula (clavicles); G, glenoid cavity for humerus; S, scapula; Un, uncinate process; V, vertebral, and Sp, sternal, portion of rib. (From Wiedersheim.)
The absence of a keel is characteristic of the other and smaller group of Birds, made up of the Ostrich, Rhea, Emeu and Cassowary, Moa and Kiwi, which from the resemblance the sternum thus bears to a flat-bottomed boat (ratis) are known as Ratitae. Whether keeled or not, the breast-bone affords a surface of attachment to the principal muscles of the fore-limbs, and its anterior end supports the coracoids, as in Fig. 2. Various processes are in most cases developed on the sides of the sternum itself, behind its junction with the ribs, especially towards the posterior portion, where they often take the form of prolongations, the extremities of which occasionally meet and enclose what are called fenestrae; but these are unimportant when compared with the features presented by the anterior part.
4. The Pectoral Arch, or Shoulder-Girdle, consists of three pairs of bones, the Coracoids, the Scapulae or Shoulder-blades, and the Clavicles or Collar-bones, the last two usually coalescing in the median line into a V-shaped or U-shaped Furcula (the well-known Merry-thought); but in some groups, as certain Parrots, the clavicles are practically absent, while in others, as several Owls, they do not unite. The furcula often ossifies firmly with the anterior portion of the keel, and in Fregata, Didus and the Ratitae, the coracoids and scapulae are fused together.
Fig. 3.–Skeleton of the Limbs and Tail of a Carinate Bird. (The skeleton of the body is indicated by dotted lines.) F, digits; Fi, fibula; HW, carpus; MF, tarsometatarsus; MH, carpometacarpus; OA, humerus; OS, femur; Py, pygostyle; R, coracoid; Rd, ulna; Sch, scapula; St, sternum, with its keel (Cr); T, tibiotarsus; Ul, radius; Z, Z1, digits of foot. (From Wiedersheim.)
5. The Anterior Limbs, or Wings, are composed of the Humerus, or upper arm-bone, the Ulna and Radius (making the fore-arm), the Carpus or wrist, the Metacarpus and Digits, corresponding with the hand and fingers. The first of the three metacarpals bears the Pollex, or thumb, with one or two phalanges (joints); the second the Index, representing man's first finger, with two or three joints; the third a weak digit with only one phalanx, except in Archaeopteryx, where there are four. The Casuarii and Apteryges possess an index only, which in the Sphenisci fuses with the pollex. The basal joint of this is the normal place of attachment of the "bastard wing" (alula spuria). Archaeopteryx had claws on all its fingers, but in recent Birds they occur on the first two only, being functionless in the adult. Wing-spurs arise from the carpal and metacarpal bones.
6. The Pelvic Arch consists of the Ilium, Ischium, and Os pubis, these three paired bones meeting from each side at the cup (acetabulum) that receives the head of the femur, and coalescing early in life; while the incisura ischiadica or notch between the ischium and the ilium becomes an inclosed space (foramen) in all Birds except the Ratitae and Crypturi.
