This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2035). The ribs appended to the dorsal vertebras may be called the true ribs; these enter into the composition of the thorax, and materially assist in strengthening that region. Each rib, as in the Crocodile, presents a dorsal and a sternal portion, connected together by a joint: the former are attached to the vertebras by a double articulation, their spinal extremity being furcate; while the latter are articulated to the sides of the sternum. A thorax is thus formed, possessing sufficient mobility to perform the movements connected with respiration, but still affording a strong basis to support muscular action; and in order to give the greatest possible strength, from the posterior margin of each dorsal rib a broad flat process is prolonged backwards and upwards to overlap the rib next behind, so as in this manner to bind the whole together into one strong framework.
(2036). The sternum itself is developed in proportion to the enormous size of the three pectoral muscles which constitute the great agents in flight: it is principally composed of the central azygos element before noticed in the Tortoise, which is here remarkably dilated, and in birds of flight prolonged inferiorly into a deep keel-like process, so as to increase materially the extent of surface from which the muscles of the breast take their origin; but in the cursorial genera, such as the Ostrich, the Emeu, etc, where the wings are not available for flying, the keel is entirely wanting, and the sternum forms merely a kind of osseous shield, covering comparatively a very small portion of the breast.
(2037). Whoever considers the position of the hip-joint in the feathered tribes, and reflects how far it is necessarily removed behind the centre of gravity when the bird walks, carrying its body in a horizontal position, will at once perceive that the pelvic portion of the spine, having to sustain the whole weight of the trunk under the most unfavourable circumstances, and at the same time to give origin to the strong and massive muscles wielding the thigh, must be consolidated and strengthened in every possible manner, and that even the slight degree of movement permitted in the dorsal region would here be inadmissible. The lumbar and the sacral vertebrae, and the entire pelvis, are therefore at an early period solidly united together by anchylosis into one bone, and the number of the vertebrae composing this part of the skeleton is only distinguishable from the situation of the intervertebral foramina through which the spinal nerves are given off. In very young birds the pelvis is evidently formed by the three elements that usually enter into its composition; and the ilium, the ischium, and the pubis, as well as the ischiadic notch and obturator foramen, will all be at once recognized by the anatomist, occupying their usual relative positions, although he will not fail to notice one remarkable circumstance, namely that, except in one instance (the Ostrich), the ossa pubis do not meet in front, so that there is no pubic arch or symphysis.
Fig. 354. Skeleton of Eagle.
Fig. 355. Wing of Bird.
(2038). The anterior extremity of a bird, although an instrument of flight, is found, when stripped of those feathers and long quills that form the extensive surface presented by this member during life, still closely to adhere to the general type in accordance with which this part of the skeleton is invariably constructed. The framework of the shoulder exhibits the scapula (fig. 355,b), the clavicle (d), and the cora-coid element (c), notwithstanding that these bones, forming, as they do, the basis of a limb so vigorous, and wielded by such powerful muscles, are necessarily modified in their form and general arrangement, so as to constitute strong buttresses adapted to keep the shoulder-joint firm and steady during flight. The scapula (b) is a long and slender bone placed upon the ribs, and lying parallel to the spine along the dorsal region of the thorax, imbedded in the muscles to which it gives attachment, while at its fixed extremity it assists in forming the cavity of the shoulder-joint. The coracoid bone (c) is the great support of the shoulder; for while at one extremity it sustains the wing, at the opposite it is firmly and securely united to the sternum by a broad articulation. But the most peculiar element of this apparatus is the furculum, or forked bone (d), composed of the conjoined clavicles, which, being anchylosed together in the mesial line, and also strongly connected with the shoulder-joint, materially add to the stability of the whole.
(2039). In the wing itself, the humerus (f) is at once recognized, as also the ulna (g) and the radius (h.) But in some birds, as in the Penguin, the student might be at a loss to identify one or two small bones (p), forming a kind of patella to the elbow-joint; these appear to be the representatives of the olecranon process detached from the ulna. The carpus (i) consists of only two small bones. The metacarpus is formed of two pieces (k, I), anchylosed together at their two extremities; and these, with two, or in some cases three rudimental fingers, complete the wing. The largest finger consists of two, or sometimes three phalanges (m, o); a second (n) offers but a single joint; and the third, which is a mere rudiment when present, is an appendage to the radial side of the carpus.
(2040). In the pelvic extremity (fig. 354) the femur is a short and strong bone; to this succeeds the tibia, upon the outer side of which is fixed a rudimental fibula. The tarsus can scarcely be said to exist, being at a very early age confused with the metatarsus, - the whole forming a single tarso-metatarsal bone, which, in the Wading Birds especially, is of very great length: at its distal extremity are three articular surfaces that support the three anterior toes, while a fourth toe, the hallux, directed backwards, is attached to it posteriorly by the intervention of a small accessory piece; and in Gallinaceous Birds an osseous spur, consolidated with the posterior face of the tarso-metatarsal bone, is generally considered as a fifth toe.