902 BIRDS. 
CLASS IL. 
oe 
BIRDS (AVES). 
Birps are Oviparous Vertebrata, with double systems of circulation and 
respiration, organized for flight. 
Their lungs, undivided and attached to the ribs, are enveloped by a 
membrane pierced with large holes, which allow the air to pass into several 
cavities of the chest, lower part of the abdomen, arm-pits, and even into 
the interior of the bones, so that not only is the surface of the pulmonary 
vessels bathed in the external fluid, but that also of an infinitude of ves- 
sels in other parts of the body. Thus, in certain respects, Birds respire 
by the branches of the aorta, as well as by those of their pulmonary 
artery, and the energy of their irritability is in proportion to their quan- 
tum of respiration.* The whole body is so constructed as to profit by 
this energy. 
Their anterior extremities, destined to sustain them in flight, could 
serve them neither for standing, nor for prehension; they are therefore 
bipeds, and pick up objects from the earth with their mouth; their body, 
consequently, must necessarily be inclined before their feet, the thighs 
directed forwards, and the toes elongated, to form a sufficient base for it. 
The pelvis is very much extended in length, in order to furnish points of 
attachment to those muscles which support the trunk upon the thighs. 
There is even a series of muscles reaching from the pelvis to the toes, 
passing over the knee and heel, in such a way as that the simple weight 
of the bird flexes the toes; it is thus that they are enabled to sleep in 
security, while perched on one foot. The ischia, and particularly the ossa 
pubis are lengthened out behind, and the interval between them is widen- 
ed, in order to allow the necessary space for the development of the eggs. 
The neck and the bill are elongated to reach the ground, but the former 
has the requisite flexibility for bending backwards when at rest,—con- 
sequently, it has many vertebre. The trunk, on the contrary, which 
serves as a point d’appui to the wings, must need have but little mobility; 
the sternum, particularly, to which are attached the muscles for depressing 
the wings in flight, is of great extent, and has its surface still more- 
* Two common Swallows consume as much pure air as a Guinea-Pig. Lavoi- 
sier, Memsires de Chimie, I. 119. 
