SKELETON OF EAGLE. (Reduced. After Milne-Edwards.)
(p) Pelvis; (c) Coracoid; (dr) Dorsal ribs; (sr) Sternal ribs; (up) Uncinate processes; (co) Coccyx; (r) Radius; (u) Ulna; (d) first phalanx of chief digit of the wing; (d′) second phalanx of chief digit of the wing; (d″) Phalanges of lower digit; (d‴) Pollex; (ca) Carpus; (f) Femur; (fu) Furcula; (h) Humerus; (pp) Postorbital process; (tm) Tarso-metatarsus; (m) Metatarsus; (ma) Lower jaw; (me) Metacarpus; (s) Scapula; (pa) Phalanges of foot; (fi) Fibula; (pt) Patella; (st) Sternum; (ti) Tibia.
The skull, then, is, as compared with the rest of the body, small; but that portion which contains the brain is relatively larger to the face than it is in any living mammal. The orbits, or cavities in which lie the eyes, are very deep, in consequence of the small extent to which the walls of the brain-case extend forwards. The cavities of each side are separated by a partition (inter-orbital septum), which is more or less bony; the nasal bones are short, so that the nasal orifices (anterior nares) are placed near to where the beak joins the face. Of the four bones which bound the great opening at the back of the skull for the passage of the spinal cord, three take part in the formation of the single ball-like projection, or condyle, by which the skull is hinged on the vertebral column. In this point, the skull of birds offers a striking point of dissimilarity to that of mammals, in which there are two condyles, one on each side of the great opening (of). Another point in which birds do not resemble mammals is in the mode by which the lower jaw is hinged on the skull. This is in the case of birds effected by a bone, which, being more or less square in shape, has gained the name of the quadrate (q). In mammals the skull proper and the lower jaw are directly connected. This quadrate bone is connected by a long narrow bar (quadrato-jugal) with the bones which go to form the “beak,” and also, by a narrow bone directed inwards, with the bones which lie in the middle line of the base of the skull, and form the hard palate. The connections between these bones are often of such a kind as to allow of the upper jaw, or upper half of the beak, being movable on the rest of the skull, the upper bones of which are so completely united together as to form a very firm point of support. In the Parrots this arrangement is carried to an extreme, for the slender bones (nasals and processes of the pre-maxillaries) which connect the upper jaw with the bones of the brain-case form with them a distinct joint, and so allow of that large amount of vertical movement which will have been observed in these birds. The pre-maxillary bones (pm), which are so small in mammals, are very largely developed in birds, giving off, as they do, three processes: one to the frontal bone (or fore-bone of the brain-case), one along the hard palate, and another externally to form the margin of the beak. The parts that vary most in this bone-group are the bones which make up the hard palate. Of these, the chief are the so-called palatines (p) and the maxillaries; the former are united by an articular surface with the bone which forms the anterior part of the base of the brain-case, while there is in the middle a narrow bone, which, from its shape in man, has received the name of the vomer (ploughshare, v). The maxillary bones develop horizontal plates, which have the palate below and the nasal chamber above them.[140]
The lower jaw (ma) is composed of six pieces of bone on each side—the dentary, angular, surangular, coronoid, splenial, and articular. The upper part of the joint is concave.
The tongue is in relation, as regards its support and movements, to the hyoid bones, which will be especially noticed in describing the Woodpecker.
SKULL OF YOUNG OSTRICH FROM ABOVE (A) AND FROM BELOW (B).
(After Owen.)
(of) Occipital foramen; (so) Supraoccipital; (eo) Exoccipital; (q) Quadrate; (pa) Parietal; (pp) Pterygoid process; (f) Frontal; (e) Ethmoid; (n) Nasal; (pm) Premaxillary; (m) Malar; (p) Palatine; (v) Vomer; (im) Intermaxillary; (l) Lachrymal bones.
Turning to the vertebral column, we find a number of small bones, complicated in form, and more or less movable on one another. For convenience of description they may be divided into those which belong to the neck (cervical vertebræ), to the trunk (dorsal vertebræ), to the sacrum (so-called because it was offered in sacrifices!), or to the tail (caudal vertebræ). As has been observed already, the first of these, or the region of the neck, is very long, and is always long enough for the beak to be able to reach to the base of the tail. In birds, unlike mammals, the number of these cervical vertebræ may be as low as nine, or as high as twenty-four. The first of them, which is known as the atlas, has on its front face a rounded cavity into which fits the single projecting condyle, which was spoken of as being found at the back of the skull; and this condyle, being well rounded, is easily able to turn in the cavity which it fits, and the head is thereby capable of a large amount of movement. In the succeeding vertebræ it is possible to make out a body, an upper arch, through which passes the spinal cord, which meets above in the middle line, and is produced into a more or less long spinous process set horizontally to the “body,” and others directed forwards and backwards, so as to connect each vertebra with its neighbours; and lastly, a lower arch, the two halves of which are not connected below, but are converted into the more or less long ribs. As these vertebræ are so small it is clear that if their spines were long the free movement of the neck would be greatly impeded, and they are therefore in many cases little more than projecting processes. This free movement is further greatly aided by the characters of the two faces of the body (or centrum) of each vertebra; the face of each is saddle-shaped, that is to say, the anterior face is concave from side to side, and convex from above downwards, while the reverse of this is seen on the posterior face; in addition to this the vertebræ are separated by a disc of cartilage from one another. The region of the neck is, broadly speaking, distinguished from that which succeeds it by the fact that the ribs connected with its vertebræ do not reach to the sternum, or breast-bone. In all birds which are capable of flight this dorsal region has its parts firmly united together, and the same holds for the parts which follow, till we reach the region of the tail, where the more anterior vertebræ are movable on one another, so as, perhaps, to serve in aid of the steering organ formed by the rectrices, or feathers (co). In all living birds the caudal vertebræ are a good deal shorter than the body, but in the fossil Archæopteryx they are longer.