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