Sternum and Ribs.
The breast-bone or sternum (fig. 8, p. 13) of birds shows the same relation to the power of flight that is shown by so many, if not by all, parts of the skeleton. It is relatively a very large bone, and is in all perfectly flying birds furnished in the middle line, below, with a strongly marked keel, the presence of which has given its name to the great group of birds called carinates. The ostrich tribe, from whose sterna the keel is absent, are termed ‘ratite,’ or ‘raftlike.’ The reason for the keel is the attachment of the great pectoral muscle, which is the most important muscle of flight. The sternum often offers useful characters to the systematist. The surface of the bone is sometimes in various degrees fenestrate, or more or less deeply incised, the one condition being an exaggeration of the other, and both the conditions being due to defective ossification. The sternum is attached to the vertebral column by the ribs, which are well developed in all birds, but vary very much in number. A highly characteristic feature of the ribs of birds is a small bony projection of the hinder margin of a certain number of them, called the uncinate processes. These are present in all birds, with the single and remarkable exception of the South American Screamers (Chauna, Palamedea), a group of birds occupying a rather isolated position, and showing resemblances to a great many different groups.
Pelvis.
Fig. 13.—Pelvis and Hind Limb of Diver.
c, d, ilium; 63, ischium; 64, pubis; 65, femur; 66, tibia; 67, fibula; 68, tarso-metatarsus; i.-iv. digits with phalanges numbered.
The hind limbs are attached to the vertebral column by means of a considerable bony structure known as the pelvic girdle (fig. 13). This mass of bone is in reality composed of three pairs of elements, though they are in the adult strongly compacted together. The main bone, which is firmly attached to the vertebral column, is the ilium; with this is almost completely fused the ischium; the very slender pubis is to a large extent free from these bones. The pelvis is in its form one of the most characteristic of the bones of the bird’s skeleton. In other animals the three bones are present, but they are directed away from each other; in the bird, as already described, the pubis is directed backwards, parallel to the ischium; in correspondence, perhaps, with its position it has become a feeble bone, and has but few muscles attached to it. The interest of the matter, however, is mainly in the fact that among the extinct Dinosaurs, a race of mesozoic reptiles, there were some in which the pelvis had a very bird-like structure, with the same feeble and recurrent pubis. This has been urged as a mark of affinity between the Dinosaurs and birds. The several bones of the pelvis are free from each other at the extremity, or almost so, in all the Ratites, and in the Tinamous, which are supposed to bear some relationship to the Ratites. The fact is interesting as being an example of the retention of a character by one group of birds which is only transitional and embryonic in another, for in all young birds the bones of the pelvis are separate; it is not until some time before hatching that they become fused together as we see them in the adult.
Hind Limb.
At first sight there appears to be a considerable difference between the fore limb and the hind limb. In both there is a long proximal bone, called humerus in the one case and femur in the other, followed by a pair of bones—the tibia and fibula—corresponding to the radius and ulna of the fore limb. But in the hind limb (fig. 13), the foot proper, consisting of metatarsals and phalanges, appears to come immediately after the tibia and fibula. In a sufficiently young bird, what is the apparent lower end of the tibia, and what is equally apparently the upper end of the metatarsus, are detachable; these two halves which are thus detachable are the tarsus, which is the equivalent of the carpus of the wing. The lower bone of the leg is on this account usually spoken of as the tarso-metatarsus. The lower part of this bone is made up of three fused elements, the separation of which from each other is clearly apparent at the lower end of the bone, where the phalanges are attached. In the Penguins the three bones are separated by grooves of a very marked character throughout. In some birds there is a fourth toe, the hallux; in these cases there is a small separate metatarsal loosely fixed to the lower end of the large conjoint metatarsals.