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:
[Illustration: Skeleton of a Turkey.]
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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