The only important point to note with regard to the ribs, is the presence on some of them of backwardly directed hooked processes (up, fig. on p. 241), which aid in giving firmness to the thoracic region. The number of ribs is variable, but there is never a large number connected with the dorsal vertebræ, as there are in some Carnivora, in Hyrax, and in the Horse.
STERNUM OF FREGILUPUS VARIUS. (After Murie.)
(cl) Clavicle; (sc) Scapula; (co) Coracoid; (cs) Keel of Sternum.
The fore and hind limbs are connected to the body by a series of bones, which form the breast and hip girdles respectively; with the former series is also connected that large, long bone with its sharply-projecting ridge (is), which is known as the breast-bone, or sternum, and in the depressions on which so much muscle is collected. This sharply-projecting ridge to the sternum, which is known as the carina, or “keel” (cs), is found only in the flying birds, though here and there, as in the Parrot of New Zealand (Strigops), it is very rudimentary. The lower edge of the bone is often imperfect, so that, as in the fowl, there are two deep clefts on each side, or there may be but a single cleft, and this again may be converted into a rounded space; in all cases these clefts or holes are covered, or filled by membrane, during the life of the animal. Projecting in front of the sternum, and often intimately connected with it, are the two clavicles (cl), which unite in the middle line to form the bone of childhood’s delight—the furcula, or “merrythought.” Above, this bone is connected with two bones, one of which, called the coracoid (c), descends on each side to fit into a depression on the upper edge of the sternum, while the other, known as the scapula, or shoulder-blade (sc), is set at an angle to the coracoid. The scapula has a backward and downward direction; while it may be noted that among mammals the coracoid is well developed only in Echidna and Ornithorhynchus. These two last bones form, at their point of junction, a cavity into which is fitted the head of the long bone of the arm (wing). In the Ratitæ, it must be observed, these two bones are not set at an angle to one another, and they become more firmly united together.
PELVIS OF AN ADULT FOWL, SIDE VIEW. (Reduced.)
(After W. K. Parker.)
(il) Ilium; (is) Ischium; (pb) Pubes; (dl) Dorso-lumbar vertebræ; (cd) Caudal vertebræ; (am) Acetabulum.
As in all the vertebrate animals except fishes, the fore-limb may be divided into three parts (fig. on p. 237)—upper arm, in which there is one bone, the humerus (a); fore arm, in which there are two, radius (d) and ulna (c); and hand (E), which can again be divided into three parts, which in man would be called wrist, palm, and fingers. Now, in some animals the wrist-bones may be ten in number, and the palm-bones five, while the number of small bones in the fingers varies a good deal, but the number of fingers is five. In most birds all these numbers are reduced. Just beyond the fore arm, the larger bone of which has often small projections indicating the points at which the secondary feathers have been attached, there are two small bones (f), then comes a longer bone (g h i), as it seems, in which there is an elongated space. Now, this bone consists of three metacarpals and one wrist-bone; the two outer metacarpals are absent, the two innermost ones have completely united with one another, and with the (true) middle metacarpal bone at their upper end; while the second and third metacarpals are also united at the other—or finger—end. The inner digit (k), or that which corresponds to man’s thumb, has two joints (phalanges), and may be clawed; the next has three joints, and may also be clawed; while the third finger, which has never more than two joints, is never known to carry a claw. In the Archæopteryx the metacarpal bones are well developed, and are not, as in recent birds, united together. No idea of a bird’s flying powers can be fairly gathered from the length of the hand, for it is long in Swifts and short in Albatrosses, for example; although it is to be noted that in the former the single bone (humerus) of the arm is short, and in the latter long.
As in the breast-girdle, the bones of the hinder or hip-girdle, by which the hind-limbs are connected with the body, are three in number; of these the upper one is greatly flattened out and projects very far forwards, thus aiding in the formation of the firm back of flying birds; the other two bones are much more slender, and are directed backwards and downwards. It is a curious circumstance that it is in one bird only, in either case, that these bones are directly connected at their lower ends with their fellow on the opposite side; those which are known as the pubes (pb) are so in the African Ostrich, and those which are known as the ischia (is) in the Rhea of South America. These two bones, with the large, flat ilia (il), take part in forming the cavity in which the head of the thigh-bone plays; the outer of the two bones (fi) which are found in the leg is rarely as long as, and is always much more slender than the other (ti), which has a strong ridge on its front face. There is yet another very remarkable point of resemblance between birds and reptiles, in that the “ankle-joint” is in both cases situated between the two rows of bones which make up the “ankle” (tarsus). In birds this arrangement is carried to a still further extent, for the single bone of the upper row is early united with the shin-bone, as may be seen under those unfortunate circumstances in which the poulterer has provided an aged fowl (aged, that is, for eating); in more fortunate cases it will be found possible to separate a small bone from the lower end of the shin-bone of the leg.