Skeleton.
A bird’s skeleton is wonderfully light and spongy in texture. It is full of air (see below, p. 27), but deficient in marrow. Its entire structure is pre-eminently suited to a flying creature, not only for the above reasons, but because the heaviest part (the sternum) lies in the middle, in the centre of gravity, and thus assists in preserving the balance, like Blondin’s pole.
The Skull.
The skull of a bird is composed of a large number of separate bones, which are very closely united in the adult bird, so much so that it is next to impossible to recognise that they are distinct bones. The bones are also thin and light, for to a flying animal any weight forward would be most disadvantageous. The weight of the bird should be, and is, concentrated in the middle of the body. We can divide the skull into two regions: behind is the smooth, rounded brain-case or cranium; in front is the face, which is largely ensheathed by the beak. It is chiefly formed by the maxillary and nasal bones above, and by the palatine and pterygoids below. The length of this part of the skull is subject to great variation in different birds. In the Storks, for instance, the face is extremely long, while in the Parrots it is comparatively short.
Professor Huxley, about thirty years ago, proposed to classify birds by the form of the bones of the palate. In the skull of the Hawk, it will be seen that two bones lying in the front region of the palate are fused with each other in the middle line, and to the type of skull which is thus characterised the name ‘desmognathous’ was given. It is found not only in the Hawks, but in a quantity of other birds; for instance, in the Stork tribe, and in the Hornbills and Toucans. The second form of skull distinguishes the gallinaceous birds; in them the two maxillo-palatines remain unconnected, and the palate is therefore in a way cleft; this is termed the ‘schizognathous’ skull. In the finch tribe there is a slight modification of this, called, from the Greek word for a finch, ‘ægithognathous.’ In these birds a median bone, called the vomer, from the fact that the bone to which it corresponds in the human skull is shaped somewhat like a plough-share, is truncated in front, instead of tapering, as it does in the schizognathous skull of the common fowl. There is a fourth variety, which marks out the Ostrich tribe and the American Tinamous, in which the two pairs of bones called the pterygoids and palatines do not, as they do in the types of skull that have been hitherto considered, reach the middle line of the skull, but are kept off from it by the vomers, which extend backwards. The term ‘dromæognathous,’ or emu-like, is applied to this form of skull. If the back of any bird’s skull be examined, it will be noticed that just below the great hole or foramen, through which the medulla passes to join the spinal cord in the canal of the vertebral column, is a rounded, rather kidney-shaped boss. This is the occipital condyle, by means of which the skull articulates with the first vertebra. If you look at the same region in a mammal, you will find that there are two of these, one on each side, though also below the foramen magnum. This is one of the many points of structure that distinguish a bird from a mammal and ally it to the reptiles; but it must be remembered that in some reptiles there is a commencing division of the single condyle into two.
The Vertebral Column.
Like all other backboned animals, birds have a chain of small bones running along the back, and enclosing a canal in which runs the spinal marrow. In most vertebrates some of the individual vertebræ in the region of the hind limb, the sacral region, are somewhat intimately fused together, forming a more solid structure for the support of the pelvis. In birds the strong coupling of the vertebræ is more marked, and extends to the dorsal region. The mechanical value of this to a flying animal is clear; it is analogous to the tight coupling of an express train, and prevents the back from bending from side to side under the strain produced by the powerful movements of the muscles in flight. The tail vertebræ show some curious modifications in different birds. In the typical carinate bird, the last few vertebræ are fused into a piece which is called the ‘plough-share bone,’ or ‘pygostyle.’ The name of this bone sufficiently indicates its shape; the expanded end of the bone serves as a firm base, upon which rest the strong tail feathers. Now, in the ostrich tribe there are no rectrices comparable in size to those of the flying carinates. Here there is no pygostyle, but the individual vertebræ are small and disconnected. They are, however, few in number, whereas in the Archæopteryx they are numerous, though, oddly enough, not so numerous altogether as are the tail vertebræ of some flying birds. Each individual vertebra in the Archæopteryx supports a pair of rectrices, which are thus arranged in a series, and not in one row. A very distinctive peculiarity of the vertebræ of birds is the saddle-shaped centrum. The centrum of the vertebra is the solid piece which underlies the canal of the spinal cord, the walls of the latter being formed by the neural arches, which unite above to form a neural spine. In other vertebrates the centra are flat (mammals), or procœlous (the concavity being forward), or opisthocœlous (the concavity posterior), or amphicœlous (concave on both sides). This latter form of vertebra is frequently met with in archaic forms belonging to various groups. It occurs, for example, in many fishes. Such reptiles as Hyperodapedon and the Geckos have the same kind of vertebræ. Among birds there is no existing genus or species which is to be thus characterised; but the extinct Ichthyornis had clearly biconcave vertebræ.
Shoulder Girdle.
Fig. 8.—Sternum of Shrike.