upwards and backwards, and passes over the rib next in succession behind, where it is bound down by ligament. The first and last dorsal ribs carry no uncinate processes, and in some cases the processes continue throughout life as separate pieces (fig. 299, B). Anteriorly, the ribs articulate with a series of straight bones, which are called the "sternal ribs," but which in reality are to be looked upon as the ossified "costal cartilages." These sternal ribs (fig. 299, B) are in turn movably articulated to the sternum in front, and "they are the centres upon which the respiratory movements hinge" (Owen). In front the thoracic cavity is completed by an enormously-expanded sternum or breast-bone, which in some birds of great powers of flight extends over the abdominal cavity as well, in some cases even reaching the pelvis. The sternum of all birds which fly, is characterised by the presence of a greatlydeveloped median ridge or keel (fig. 299, A), to which are attached the great pectoral muscles which move the wings. As a general rule, the size of this sternal crest allows a very tolerable estimate to be formed of the flying powers of the bird to which it may have belonged; and in the Ostriches and other birds which do not fly, there is no sternal keel. At its anterior angles the sternum exhibits two pits for the attachment of the coracoid bones.

The scapular or pectoral arch consists of the shoulder-blade. or scapula, the collar-bone or clavicle, and the coracoid bone, on each side. The scapula, as a rule (fig. 299, A, s s), is a simple elongated bone, not flattened out into a broad plate, and carrying no transverse ridge, or spinous process. Only a portion of the glenoid cavity for the articulation with the head of the humerus is formed by the scapula, the remainder being formed by the coracoid. The coracoid bones (fig. 299, A, k k) correspond with the coracoid processes of man; but in birds they are distinct bones, and are not anchylosed with the scapula. The coracoid bone on each side is always the strongest of the bones forming the scapular arch. Superiorly it articulates with the clavicle and scapula, and forms part of the glenoid cavity for the humerus. Inferiorly each coracoid bone articulates with the upper angle of the sternum. The position of the coracoids is more or less nearly vertical, so that they form fixed points for the action of the wings in their downward stroke. The clavicles (fig. 299, A, c) are rarely rudimentary or absent, and are in some few cases separate bones. In the great majority, however, of birds, the clavicles are anchylosed together at their anterior extremities, so as to form a single bone, somewhat V-shaped, popularly known as the

"merry-thought," and technically called the "furculum " ("fourchette" of the French). The outer extremities of the furculum

Fig. 299-A, Breast-bone, shoulder-girdle, and fore-limb of Penguin (after Owen): Sternum, with the sternal keel; ss Scapula; kk Coracoid bones; c Furculum or merry-thought, composed of the united clavicles; h Humerus; Ulna; Radius; Thumb; m Metacarpus: pp Phalanges of the fingers. B, Ribs of the Golden Eagle: a a Ribs giving off (6b) uncinate processes; cc Sternal ribs.

articulate with the scapula and coracoid; and the anchylosed angle is commonly united by ligament to the top of the sternum. The function of the clavicular or furcular arch is "to oppose the forces which tend to press the humeri inwards towards the mesial plane, during the downward stroke of the wing" (Owen). Consequently the clavicles are stronger, and their angle of union is more open, in proportion to the powers of flight possessed by each bird. The furculum is rudimentary in the Ostrich, Emeu, and Cassowary, and it is absent in the Apteryx and some Parrots.

We have next to consider the structure of the bones which compose the fore-limb or "wing" of the bird; and as this organ is the one which chiefly conditions the peculiar life of the bird, it is in it that we find some of the most characteristic points of structure in the whole skeleton. Though considerably modified to suit its function as an organ of aerial progression, the wing of the bird is readily seen to be homologous with the arm of a man or the fore-limb of a Mammal (fig. 299, A, and fig. 300). The upper arm (brachium) is supported by

a single bone, the humerus, which is short and strong, and articulates above with the articular cavity formed partly by the scapula and partly by the coracoid (fig. 300, h). The

Fig 300.-Fore-limb of the Jer-falcon. Humerus; r Radius; u Ulna; "Thumb;" "Metacarpals, anchylosed at their extremities; Phalanges of fingers.

humerus is succeeded distally by the fore-arm (antibrachium), constituted by the normal two bones, the radius and ulna (fig. 300, r, u), of which the radius is the smaller and more slender, and the ulna the larger and stronger. The ulna and radius are followed inferiorly by the bones of the wrist or carpus; but these are reduced in number to two small bones, one radial and one ulnar, "so wedged in between the antibrachium and metacarpus as to limit the motions of the hand to those of abduction and adduction necessary for the folding up and expansion of the wing; the hand is thus fixed in a state of pronation; all power of flexion, extension, or of rotation, is removed from the wrist-joint, so that the wing strikes firmly, and with the full force of the contraction of the depressor muscles, upon the resisting air" (Owen). One other bone of the normal carpus (namely, the "os magnum ") is present, but this is anchylosed with one of the metacarpals. There are thus really three carpal bones, though only two

appear to be present. (According to Morse, there is a fourth carpal, which early anchyloses with the base of the metacarpal of the middle finger). The carpus is followed by the metacarpus, the condition of which agrees with that of the carpal bones. The two outermost of the normal five metacarpals are absent, and the remaining three are anchylosedtogether with the os magnum-so as to form a single bone (fig. 300, m). This bone, however, appears externally as if formed of two metacarpals united to one another at their extremities, but free in their median portion. The metacarpal bone which corresponds to the radius is always the larger of the two (as being really composed of two metacarpals), and it carries the digit which has the greatest number of phalanges. This digit corresponds with the "index" finger, and it is composed of two, or sometimes three, phalanges (fig. 300, p). At the proximal end of this metacarpal, at its outer side, there is generally attached a single phalanx, constituting the so-called "thumb" (fig. 300, t), which carries the "bastard-wing," and is sometimes furnished with a claw. The digit which is attached to the ulnar metacarpal corresponds to the middle finger, and never consists of more than a single phalanx (fig. 300). In the Apteryx and the Cassowary there is only one complete digit to the hand.

As regards the structure of the posterior extremity or hindlimb, the pieces which compose the innominate bones (namely, the ilium, ischium, and pubes) are always anchylosed with one another; and the two innominate bones are also always anchylosed, by the medium of the greatly-elongated ilia, with the sacral region of the spine. In no living bird, however, with the single exception of the Ostrich, are the innominate bones united in the middle line in front by a symphysis pubis. The stability of the pelvic arch, necessary in animals which support the weight of the body on the hind-limbs alone, is amply secured in all ordinary cases by the anchylosis of the ilia with the sacrum.

As in the higher Vertebrates, the lower limb (fig. 301, A) consists of a femur, a tibia and fibula, a tarsus, metatarsus, and phalanges; but some of these parts are considerably obscured by anchylosis. The femur or thigh-bone (fig. 301, A, f) is generally very short, comparatively speaking. The chief bone of the leg is the tibia (f), to which a thin and tapering fibula (r) is anchylosed. The upper end of the fibula, however, articulates with the external condyle of the femur. The ankle-joint is placed, as in Reptiles, between the proximal and distal portions of the tarsus. The proximal portion of the tarsus,

consisting of two bones representing the astragalus and calcaneum or the former only, is undistinguishably amalgamated

Fig. 301.-A. Hind-limb of the Loon (Colymbus glacialis)-after Owen: i Innominate bone: Thigh-bone or femur; t Tibia, with the proximal portion of the tarsus anchylosed to its lower end; Fibula; m Tarso-metatarsus, consisting of the distal portion of the tarsus anchylosed with the metatarsus; pp Phalanges of the toes. B, Tail of the Golden Eagle: s Ploughshare-bone, carrying the great tail-feathers.

with the lower end of the tibia. The distal portion of the tarsus is anchylosed with the second, third, and fourth metatarsals to constitute the most characteristic bone in the leg of the Bird-the "tarso-metatarsus" (m). In most of the longlegged birds, such as the Waders, the disproportionate length of the leg is given by an extraordinary elongation of the tarsometatarsus.

The tarso-metatarsus is followed inferiorly by the digits of the foot. In most birds the foot consists of three toes directed forwards and one backwards-four toes in all. In no wild bird are there more than four toes, but often there are only three, and in the Ostrich the number is reduced to two. In all birds which have three anterior and one posterior toe, it is the posterior thumb or hallux (that is to say, the innermost