called meta-sternum. The anterior margin of the sternum, between the right and left anterior lateral processes receives in sockets the feet of the coracoids. Between them arises a median crest, which varies much in extent and composition, and is of considerable taxonomic value. It is represented either by a spina interna or by a spina externa, or by both, or they join to form a spina communis which is often very large and sometimes ends in a bifurcation. Eventually, when the right and left feet of the coracoids overlap each other, the anterior sternal spine contains a foramen. The keel, or carina sterni, is formed as a direct cartilaginous outgrowth of the body of the sternum, ossifying from a special centre. This keel is much reduced in the New Zealand parrot, Stringops, less in various flightless rails, in the dodo and solitaire. It is absent in the Ratitae, which from this feature have received their name, but considerable traces of a cartilaginous keel occur in the embryo of the ostrich, showing undeniably that the absence of a keel in the recent bird is not a primitive, fundamental feature. The keel has been lost, and is being lost, at various epochs and by various groups of birds. The swimming Hesperornis (see Odontornithes) was also devoid of such a structure. In many birds the spaces between the meta-sternum and the posterior processes and again the spaces between this and the oblique process are filled up by proceeding ossification and either remain as notches, or as fenestrae, or they are completely abolished so that the breastbone is turned into one solid more or less oblong plate.
|Fig. 11.—Sternum of a Chick (Gallus domesticus) three days|
old, lower view. The cartilage is shaded and dotted, and the
bony centers are light and striated.
Shoulder Girdle.—Scapula, coracoid and clavicle, meet to form the foramen triosseum, through which passes the tendon of the supracoracoideus, or subclavius muscle to the tuberculum superius of the humerus. The coracoid is one of the most characteristic bones of the bird’s skeleton. Its upper end forms the acrocoracoid process, against the inner surface of which leans the proximal portion of the clavicle. From the inner side of the neck of the coracoid arises the precoracoidal process, the remnant of the precoracoid. Only in the ostrich this element is almost typically complete, although soon fused at either end with the coracoid. Near the base of the precoracoidal process is a small foramen for the passage of the nervus supracoracoideus. In most birds the feet of the coracoids do not touch each other; in some groups they meet, in others one overlaps the other, the right lying ventrally upon the left. The scapula is sabre-shaped, and extends backwards over the ribs, lying almost parallel to the vertebral column. This is a peculiar character of all birds. The clavicles, when united, as usual, form the furcula; mostly the distal median portion is drawn out into a hypocleidium of various shape. Often it reaches the keel of the sternum, with subsequent syndosmosis or even synostosis, e.g. in the gannet. In birds of various groups the clavicles are more or less degenerated, the reduction beginning at the distal end. This condition occurs in the Ratitae as well as in the well-flying Platyrcecinae amongst parrots.
|Fig. 12.—Bones of Fowl’s right wing, adult, nat. size.|
r′, u′, Radial and ulnar carpal bones; with the three digits I., II., III.
The fore-limb or wing (fig. 12); highly specialized for flight, which, initiated and made possible mainly by the strong development of quill-feathers, has turned the wing into a unique organ. The humerus with its crests, ridges and processes, presents so many modifications characteristic of the various groups of birds, that its configuration alone is not only of considerable taxonomic value but that almost any genus, excepting, of course, those of Passeres, can be “spotted” by a close examination and comparison of this bone. When the wing is folded the long glenoid surface of the head of the humerus is bordered above by the tuberculum externum or superius, in the middle and below by the tuberculum medium or inferius for the insertion of the coraco-brachialis posterior muscle. From the outer tuberculum extends the large crista superior (insertion of pectoralis major and of deltoideus major muscles). The ventral portion of the neck is formed by the strong crista inferior, on the median side of which is the deep fossa subtrochanterica by which air sacs enter the humerus. On the outer side of the humerus between the head and the crista inferior is a groove lodging one of the coraco-humeral ligaments. The distal end of the humerus ends in a trochlea, with a larger knob for the ulna and a smaller oval knob for the radius. Above this knob is often present an ectepicondylar process whence arise the tendons of the ulnar and radial flexors. The radius is the straighter and more slender of the two forearm bones. Its proximal end forms a shallow cup for articulation with the outer condyle of the humerus; the distal end bears a knob which fits into the radial carpal. The ulna is curved and rather stout; it articulates with both carpal bones; the cubital quills often cause rugosities on its dorsal surface. Of wrist-bones only two remain in the adult bird; the original distal carpals coalesce with the proximal end of the metacarpals. These are reduced, in all birds, to three, but traces of the fourth have been observed in embryos. The first metacarpal is short and fuses throughout its length with the second. This and the third are much longer and fuse together at their upper and distal ends, leaving as a rule a space between the shafts. The pollex and the third finger are as a rule reduced to one phalanx each, while the index still has two. The first and second fingers frequently carry a little claw. The greatest reduction of the hand-skeleton is met with in Dromaeus and in Apteryx, which retain only the index finger. It is of importance for our understanding of the position of the Ratitae in the system, that the wing-skeleton of the ostrich and rhea is an exact repetition of that of typical flying birds; the bones are much more slender, and the muscles are considerably reduced in strength also to a lesser extent in numbers, but the total length of the wing of an ostrich or a rhea is actually and comparatively enormous. Starting with the kiwi and cassowary, people have got into the habit of confounding flightless with wingless conditions. It is absolutely certain that the wings of the Ratitae bear the strongest testimony that they are the descendants of typical flying birds.