The convex outer surface of the sternum carries the keel or carina, whence comes the name Carinatae, applied to the vast majority of living birds as opposed to the Ostrich and its allies, known as Ratitae from the raft-like aspect of the keel-less sternum.
The shoulder girdle consists of a scapula, coracoid, and furcula1. The scapula is sword-shaped and thin. There is no separate suprascapula. A small conical process internal to the glenoid facet represents the meso-scapula or acromion. The coracoid is firmly united by ligament to the scapula. A prominent clavicular process rises in front of its glenoid facet, and there is a thin curved subclavicular, or subscapular process ( = praecoracoid), on the internal, or true anterior, border in contact with the acromion. A rough line runs downwards from it to the broad sternal or epi-coracoidal end of the bone and gives attachment to the coraco-clavicular membrane. The coracoid fits into a groove in the sternum. The furcula, a characteristic Avian bone, is formed by the fusion of the ventral ends of the two clavicles. At its upper end each clavicle expands into a disc or epicleidium, which is tied by ligament to the acromion and to the sub-scapular and clavicular processes of the coracoid. There is thus formed a foramen triosseum through which the tendon of the second pectoral muscle, or elevator of the wing, passes to its insertion on the humerus.
The spot where the ventral ends of the clavicles fuse is prolonged into a point, the homologue of the large hypocleidium in the Fowl, and united as it is to the keel of the sternum by a ligament which together with the point represents a portion of the interclavicle.
1 This word is often, but incorrectly, written Furculum. It is written Furcula in Bronn's Klass. und Ordn. des Thierreichs, vi. Abth. 4 by Selenka, without the mention of any other form.
The fore-limb of this specimen is in the position of rest. The humerus lies parallel to the axis of the body, its true ventral surface turned outwards; the fore-arm is flexed on the humerus and the hand is adducted. When the wing is expanded, the hand is abducted: it is incapable of flexion. As to the humerus, its glenoid head is transversely elongated: on its radial or upper margin at the proximal end is a conical process to which the first pectoral, or depressor of the wing, is attached, and dorsally to it is the facet for the insertion of the second pectoral. On the ulnar margin proximally and dorsally is a deep pit, at the bottom of which is a pneumatic foramen. The surface of articulation for the radius is longy,oblique and on the ventral surface, as in Lizards. The radius is rod-like; the ulna stout, somewhat curved, and with a short olecranon. Its outer surface is pitted by the sacs of the secondary wing feathers. There are two carpal bones in the proximal row - a scaphoid (= radiale), and a fused lunar and cuneiform (= intermedium and ulnare). The distal carpalia are fused to the heads of the metacarpalia, forming a carpo-metacarpal bone.
The first metacarpal is a mere process and carries a single phalanx; the second is stout and long and carries two phalanges; while the third is slight, curved and fused distally to the second, and carries but one phalanx. In no Bird are there more than these three digits.
The pelvis has the three bones ileum, ischium, and pubis peculiarly disposed. The first extends backwards and forwards along the whole extent of the sacrum; the ischium lies parallel to the backward extension of the ileum; the pubes to the ischium, and neither of the two latter have a ventral symphysis. All three unite in the acetabulum. The centre of this cavity is membranous in the living animal. Hence in a prepared skeleton it appears to be perforated. A prominent surface - the anti-trochanter - on the posterior-superior margin of the acetabulum, works against the base of the neck which carries the head of the femur. Ileum and ischium fuse distally, and thus inclose an ileo-sciatic foramen. The obturator foramen between the ischium and pubes is long and narrow, and subdivided partially by the obturator process of the ischium. The femur is remarkably short. The head is prominent, and its neck at right angles to the main axis of the bone. The condyles are large and separated by a deep patellar groove. The external one is typically subdivided, and its outer subdivision plays between the tibio-tarsus and fibula. The former of these two bones is the largest in the limb.
It has proximally a cnemial crest on the anterior surface, subdivided into a pro- and ecto-cnemial process; and distally there are two condyles formed from a cartilage in the embryo which represents the proximal tarsalia - astragalus and calcaneum. Anteriorly and above these condyles a narrow bony bar confines the extensor tendons of the toes. The fibula is slender and pointed distally. The third section of the limb is the tarso-metatarsus, a compound bone formed by the union of a bone representing the distal tarsalia to the heads of the second, third, and fourth metatarsalia, of which the third is the longest. Behind the tarsal element lies an ento-calcaneal process, the attachment of the tendo Achillis, pierced and grooved by the flexor tendons of the foot. The first metatarsal is small, incomplete proximally, and united to the second by ligament. There are four digits in all - the first, or hallux, is turned inwards and backwards and carries two phalanges; the three remaining digits carry phalanges increasing successively in number from three to five, the usual succession in Birds. The third is the longest digit; the fourth is so only in a few instances, e. g.
Penguin, Gannet, Pelican, etc.
All the bones in the embryo contain marrow. The degree in which it is replaced by air varies much. Apteryx, Penguin, small Songsters, have air only in the skull: the Hornbill in every bone of the body. A membranous tube - the siphonium of Nitsche - conveys air from the tympanic cavity to the lower jaw, as in the Crocodile. This tube in the Raven, Thrush, etc. becomes bony. The Cretaceous toothed birds had a pneumatic skeleton, as was probably the case in the Dinosaurian Reptile, Coelurus.