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