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.

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THE RHINOCEROS BIRD.

This curious bird is of the order Picae, 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 Linnaeus, commonly called the Rhinoceros Bird.

[Illustration:  The Rhinoceros Bird.]

Its general size is that of a Turkey, but with a much more slenderly proportioned body.  Its colour is black, with the tail white, crossed by a black bar:  the beak is of enormous size, of a lengthened, slightly curved, and pointed shape, and on the upper mandible, towards the base, is an extremely large process, equal in thickness to the bill itself, and turning upwards and backwards in the form of a thick, sharp-pointed horn, somewhat resembling the horn of the rhinoceros.  The use of this strange proboscis is by some supposed to be that of enabling the bird more easily to tear out the entrails of its prey; but others affirm that it is not of a predaceous nature, feeding only on vegetable substances.  This bird is principally found in the East Indian Islands.  A remarkably fine specimen was preserved in the Leverian Museum.

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THE SKETCH-BOOK.