Dass sein Gefühl hinauf und vorwärts dringt,
Wenn über uns, im blauen Raum verloren,
Ihr schmetternd Lied die Lerche singt,
Wenn über schroffen Fichtenhöhen
Der Adler ausgebreitet schwebt,
Und über Flächen, über Seen
Der Kranich nach der Heimat strebt.”[139]
Inspired by feelings such as those so powerfully expressed in Goethe’s lines, numerous naturalists have treated of the bird as though the powers of flight were confined to it, and were not shared by Bees and Bats in the present, and by Pterodactyles in the past. With this word of comment, which it is even still necessary to insist upon, attention should be given to the following avian characters:—The anterior limbs do not touch the ground, and the bones which compose them are adapted for carrying the feathers of the wing; the breast-bone is not only elongated, but has its central portion developed (except in the Ratitæ) into a strong keel, the better to permit insertion of the muscles by which the fore-limb is moved; the small bones (vertebræ) in the region of the back are fixed firmly together, and are not, as in man or in the Ostrich, movable on one another; while those which succeed them are often welded into one mass with the greatly-developed upper bones (ilia) of the pelvic girdle; and the hinder vertebræ develop an upstanding plate (ploughshare bone) which gives support to the rectrices, or so-called steering feathers of the tail. It will have been seen that the ordinary seizing organ of man (the hand) has in birds been modified to serve another purpose; but this is made up for, not only by the character of the beak, but by the long and flexible neck, and in some by the grasping toes.
Before describing in detail the characters of the different parts of the skeleton, it is to be noted that many of the bones are not, as in the Mammalia, filled with marrow, but with air; a large cavity may, for example, be seen in the upper bone (humerus) of the wing of the common fowl. It is obvious that the specific gravity, or weight of the bird, is thus largely reduced, while the connection between these air-spaces and those which are derived from the lungs enables the contained air to undergo the necessary exchanges with the surrounding medium.
It was long ago observed by the famous German anatomist, Johannes Müller, that “it has often been a subject for complaint that the anatomical characters of birds are so constant that they are of but slight assistance in the labours of the zoologist.” The truth of this will very forcibly strike any one who comes to the study of the skulls of birds, after having examined a series of skulls in mammals, so that the seemingly trivial variations to which anatomists have directed attention are in truth those which are, in birds, often of the most importance.