Birds may be said to constitute an isolated class of beings. They are distinguished by certain characters from all other animals: their classification does not pass into any other, and cannot, therefore, be consistently introduced into the supposed chain or gradation of natural bodies.
The skeleton or bony frame of birds is in general lighter than in quadrupeds. They have the largest bones of all animals, in proportion to their weight; and their bones are more hollow than those of animals that do not fly: air-vessels also enable them to blow out the hollow parts of their bodies, when they wish to make their descent slower, rise more swiftly, or float in the air. The spine is immovable, but the neck has a greater number of bones, (never less than nine, and varying from that to twenty-four,) and consequently of joints, and more varied motion, than in quadrupeds. The breast-bone is very large, with a prominent keel down the middle, and is formed for the attachment of very strong muscles: the bones of the wings are analagous to those of the fore-legs in quadrupeds, but the termination is in three joints or fingers only, of which the exterior is very short. This will be better understood by the annexed:
The muscles that move the wings downwards, in many instances, are a sixth part of the weight of the whole body; whereas those of a man are not in proportion one hundredth part so large. The centre of gravity of their bodies is always below the insertion of their wings to prevent them falling on their backs, but near that point on which the body is, during flight, as it were, suspended. The positions assumed by the head and feet are frequently calculated to accomplish these ends, and give to the wings every assistance in continuing the progressive motion. The tail also is of great use, in regulating the rise and fall of birds and even their lateral movements. What are commonly called the legs are analogous to the hind legs in quadrupeds, and they terminate, in general, in four toes, three of which are usually directed forwards, and one backwards; but in some birds there are only two toes, in others three.
Birds exceed quadrupeds in the quantity of their respiration, for they have not only a double circulation, and an aerial respiration, but they respire also through other cavities beside the lungs, the air penetrating through the whole body, and bathing the branches of the aorta, or great artery of the body, as well as those of the pulmonary artery.
Birds are usually classed according to the forms of their bills and feet, from those parts being connected with their mode of life, food, &c. and influencing their total habit very materially.
THE RHINOCEROS BIRD.
This curious bird is of the order Picæ, or Pies, and of the genus Buceros, consisting of birds of rather large size, and distinguished by the disproportionate forms of their beaks, which are often still further remarkable for some kind of large prominence on the upper mandible. The most conspicuous species is the Buceros Rhinoceros of Linnæus, commonly called the Rhinoceros Bird.
