The Three Divisions of the Class Aves—ANATOMY OF A BIRD—The Skeleton—Distinctive Features—Peculiar Bone Character—The Skull—Difference between the Skull of Birds and that of Mammals—The Jawbones—Vertebral Column—Sternum—Fore-limbs—Hind-limbs—Toes—The Muscular System—How a Bird remains Fixed when Asleep—The Oil-gland—The Nervous System—The Brain—The Eye—The Ear—The Digestive System—The Dental papillæ—The Beak—Tongue—Gullet—Crop—Stomach—Uses of the Gizzard—Intestine—The Liver, Pancreas, and Spleen—The Blood and Circulatory System—Temperature of Blood of a Bird—Blood Corpuscles—The Heart—The Respiratory System—Lungs—Air-sacs—The Organs of Voice—The Egg—Classification of the Class Aves.
BIRDS may be separated into three great divisions: the Carinatæ, or birds with a keeled sternum, the Ratitæ, or birds having a raft-like sternum, and the Saururæ, or lizard-like birds. The last of these orders links the birds with the reptiles, and does not concern us here, as it contains only one genus, and that a fossil one, the Archæopteryx lithographica, respecting which a few words will be found at the end of this article (Vol. IV., pp. 236–8). The other two divisions are of great importance, and are easily recognisable, although the characters which separate them are chiefly anatomical. The principal point of difference lies in the sternum, or breast-bone, and the name Carinatæ is given to all those which have a keel (carina) or sternal ridge largely developed, as in the common fowl; and this is present in the great majority of birds. The Ratitæ have not got this keel, and in this division are found the Struthious birds—Ostriches, Cassowaries, &c. They are all species which cannot fly; and although the number at present existing is small, the fact of their being found at widely distant parts of the earth—in South America, in Africa, and again in Australia—would seem to indicate that they were once more plentifully distributed, and that they are remains of what was formerly a large and important group. To these Ratite birds belonged also the extinct gigantic Moas of New Zealand, and the Æpyornis of Madagascar.
Before proceeding further, it will be necessary to give a brief outline of the principal anatomical features of a bird’s body. On examining either the general features of the skeleton, or the minute characters of many of the bones of which it is made up, in such a bird as a Hawk, for example, we are arrested by those remarkable arrangements by which this part of the body is adapted to the mode of life of its possessor. Here, however, as in so many instances, we have to distinguish between what is characteristic of the bird as a flying animal, and what is more or less common to it and other vertebrate animals, and does not especially relate to peculiar habits. We may well be struck by the marvellous power which birds have, and which man has not, and it is curious to notice how man’s aspirations have ever been associated with it. Without pausing to observe that painters always endow spirits with wings, or that the imaginative genius of the French has emboldened them to form a “Société d’Encouragement pour la Locomotion Aérienne,” we may find in the words of Faust definite expression of what man feels with regard to the law by which he is held down to earth:—
“Ich eile fort, ihr ew’ges Licht zu trinken,
Vor mir den Tag und hinter mir die Nacht,
Den Himmel über mir, und unter mir die Wellen.
Ein schöner Traum, indessen sie entweicht.
Ach! zu des Geistes Flügeln wird so leicht
Kein körperlicher Flügel sich gesellen.
Doch ist es jedem eingeboren,