CLASS SECOND. 
BIRDS. 
All the animals of this class are Oviparous; that is, they pro¬ 
duce their young by eggs. They are distinguished from the 
other divisions of the animal kingdom, by being covered with 
feathers. To aid in their classification, the feathers have been 
distinguished by different names, according to the particular part 
of the animal which they cover. 
Birds have a double respiration and circulation,—these are 
for adapting the animals for flight. Another admirable contriv¬ 
ance for this purpose is the general diffusion of air through 
their bodies. Their lungs are not divided, but adhere to the 
ribs, and are composed of a membrane penetrated by orifices, 
which permit the free passage of the air into the cavities of the 
chest, lower abdomen, the arm-pits, and even to the bones. 
Birds respire not only by branches of the aorta, but also by 
means of the pulmonary artery. 
Birds are true bipeds, and can only lift things from the 
ground by means of their bill. The neck is much elongated, as 
well as the beak, which is covered with a horny substance. 
They are always destitute of teeth. The upper mandible is 
a prolongation of the intermaxillary bones: these are pro¬ 
longed into two arches; the internal one of which consists of 
the palate bones, and the external one of the maxillary and 
