154 
AVES. 
Many of the cold-blooded Ovipara do not bring forth their young until they are 
developed and extricated from their shell, or other membranes which separated them 
from their parent. These are called Ovipara. 
THE SECOND CLASS OF VERTEBRATED ANIMALS. 
THE BIRDS {JVES),~ ^ 
Are oviparous vertebrates with double circulation and respiration, [mostly] organized 
for flight. 
Their lungs, undivided and attached to the ribs, are enveloped by a membrane 
pierced with large holes, and which allows the air to pass into many cavities of the 
chest, the abdominal region, arm-pits, and even of the interior of the bones* ; so that 
the ambient fluid not only bathes the surface of the pulmonary vessels, but also tbati 
of an infinitude of vessels traversing the rest of the body. Thus Birds respire, in§ 
certain respects, by the ramifications of their aorta, as well as by those of their* 
pulmonary artery, and the energy of their irritability is in proportion to their amount | 
of respiration.]' Their total conformation is arranged to participate in this energy. 
Their anterior extremities, destined to sustain them in flight, could neither serve 
them for standing, nor for clutching : they are bipeds, then, 
and pick up objects from the earth with their mouth ; their 
body, consequently, is balanced upon the legs ; the thighsl 
are directed forward, and the toes are lengthened to form 
a sufficient base for standing. The pelvis is longitudi- 
nally much extended, to furnish attachment to the muscles 
which support the trunk upon the thighs : there is even 
a suite of muscles proceeding from the pelvis to the toes ; 
and passing over the knee and heel, so that the simple 
weight of the bird flexes the toes : it is thus that they 
are enabled to sleep perched on one foot. The iscJiia, 
especially the ossa pubis, are lengthened out behind, and* 
widened in their span, to allow the necessary space forj 
the developement of the eggs. 
The neck and the beak are elongated to reach the 
ground ; but the former has also the requisite flexibility for. 
doubling backward when at rest. It has therefore numerous 
vertebrae, [varying from twelve to twenty-three, which latter 
number is attained only in the genus Cygnus'\ . The trunk, |V 
on the contrary, which serves as a fulcrum to the wings,' 
has but little mobility ; the sternum especially, to which 
are attached the muscles which effect the propulsive stroke | 
in flying, is of great extent, its surface [except in the Ostrich and allied genera, which do J 
not fly,] being further augmented by a projecting ridge along its middle. It is [mostly] 
* In the Honibills, even the phalanges of the toes are hollow, and t f Two Sparrows consume as much air as a Guinea-pig..— 
eominunicate with the lungs. The opposite extreme occurs in the I sikk, Memuires de Chimie, i. 110. 
Apteryx, which has no accessory air-cavities. — E d. I 
Fig. 6". — Skeleton of Jer Falcon. 
