THE SECOND CLASS OF VERTEBRATED ANIMALS. 
BiRDS 
Are vertebi'ated oviparous animals, with double respiration 
and circulation, organized for flight. Their lungs, not divided, 
fixed against the ribs, (i) are enveloped by a membrane pierced 
with large holes, which allow the air to pass into several cavities 
in the breast, the lower stomach, arm-pits, and even the interior 
of the bones ; so that this fluid not only bathes the surface of the 
pulmonary vessels, but also that of an infinity of vessels in the 
rest of the body. (2) Thus, Birds breathe in certain respects 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 the 
quantity of their respiration. Their whole body is so disposed as 
to partake of this energy. (3) 
(1) The lungs of MammaUa are free and divldetl, and the dia- 
phragm, which separates the cavity of their Ijreast from that of their 
Jielly, does not exist in Birds. ' 
(2) In 7!/«77j/7ia/i!:« the inspired atmospheric air proceeds no further 
than the lungs, where it is immcdiatelj decoBiiposed, the oxygen re- 
maining to purify the blood which has been retm^ned by the veins, 
and the nitrogen escaping, or (according to recent conclusions on 
this long contested point) entering into the body in very small 
portions : we know that nitrogen is found by chemists in all animal 
substances except fat (which when burned produces water and car- 
bonic acid in the same proportions as vegetables) \ in plants, it is only 
found in the Fungi and in the perisperm of the G?mninecs. The at- 
mospheric air not only performs its office in the lungs of Birds, but a 
portion of it proceeds to the aerian cavities (which are membranous 
bags, pervading even the substance of the beak and the skins of the 
feathers), insinuates itself between the muscles, and exercises the same 
salutary action on the Jjlood contained in the vessels of these various 
parts of the body, as on that which has reached the lungs. The blood 
o£ Birds, from this greater proportion of oxygen, is of a much brighter 
red than that of Mammalia, 
(3) The fact that two common Sparrows consume as much air as a 
Guinea Pig, demonstrates the greater capacity of the smaller frame of 
Birds, [)y means of liie aeriaii cavities. Their system is conse(|ueatlY 
