The pectoral girdle in almost all birds is strongly constructed and firmly united to the sternum. It consists of three bones, a dorsal element, the scapula, a posterior ventral element, the coracoid, and an anterior ventral element, the clavicle.
The scapula forms a long curved flattened bone expanded at its anterior end, where it meets the coracoid, and lying across the ribs at its tapering posterior end. It helps to form the imperfect glenoid cavity, with which the humerus articulates. The coracoid, a shorter but stouter bone than the scapula, has its upper end or head thickened and bears on its posterior border an irregular surface, with part of which the scapula articulates, while the rest forms part of the glenoid cavity. The inner border of the coracoid adjoining the articular facet for the scapula is produced into a strong process which helps to complete the foramen triosseum, a space lying between the adjoining ends of the scapula and coracoid, through which the tendon of the second pectoral muscle passes. The lower part of the coracoid, which is much flattened and expanded, and abruptly truncated posteriorly, articulates with the coracoid groove of the sternum. The clavicle is a thickened curved membrane bone, which is fused with its fellow in the middle line below, the two forming the furcula or merrythought. Its dorsal end is drawn out into a process which articulates with the coracoid.
The Anterior Limb or Wing.
This consists of three parts, a proximal part, the upper arm or brachium, a middle part, the fore-arm or antibrachium, and a distal part, the manus. When extended for flight the parts lie almost in the same straight line, but when at rest they are folded on one another in the form of a Z, the brachium and manus pointing backwards, and the antibrachium forwards. When extended for flight the surfaces and borders of the wing correspond in position with those of the primitive vertebrate limb[104], the pre-axial border being directed forwards and the postaxial backwards, while the dorsal and ventral surfaces look respectively upwards and downwards. But when the wing is at rest, the humerus as it extends backwards becomes slightly rotated, so that its dorsal surface looks more inwards than upwards, while the dorsal surface of the antibrachium looks partially outwards and upwards, and that of the manus mainly outwards.
The brachium or upper arm contains only a single bone, the humerus (fig. 57, 1). This is a large nearly straight bone expanded at both ends. The proximal end is specially expanded, forming two tuberosities, and a large convex head articulating with the glenoid cavity. The pre-axial tuberosity is the smaller of the two, but is continued by a prominent deltoid ridge, which extends for a very short distance down the shaft. The postaxial tuberosity is the larger, and below it there is a very deep pit, the pneumatic foramen, which leads into an air cavity in the shaft of the bone. The shaft is long and straight, and at the distal end of the bone is the trochlea with two convex surfaces, one pre-axial with which the radius articulates, the other postaxial for the ulna.
The fore-arm or antibrachium consists of two bones, the radius and ulna. These are of nearly equal length, and are separated from one another by a considerable space except at their terminations.
The radius (fig. 57, 2), the pre-axial and smaller bone, is straight and fairly stout; its proximal end articulates with the humerus by a slightly cupped surface, while its distal end, which articulates with the carpus, is convex and somewhat expanded.
The ulna (fig. 57, 3) is longer, stouter, and slightly curved. Its proximal end is expanded, forming two surfaces which articulate with the trochlea of the humerus; behind them it is drawn out into a short blunt olecranon process. Its distal end is less expanded, and articulates with the carpus and also with the radius.
The Manus. This includes the carpus or wrist, and the hand.
The Carpus. While in the embryo the carpus consists of five distinct elements arranged in a proximal row of two and a distal row of three, in the adult only the proximal bones can be clearly distinguished, the distal ones having become completely ankylosed with the metacarpals to form the carpo-metacarpus.