217 



CLASS II. 



AVES. 



Birds are Oviparous Vertebrata, with double systems of cir- 

 culation and respiration, organized for flight. 



Their lungs, undivided and attached to the ribs, are enve- 

 loped 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 of the interior of the bones, 

 so that not only is the surface of the pulmonary vessels bathed 

 in the ambient fluid, but that also of an infinitude of vessels 

 in other parts of the body. Thus, in certain respects. Birds 

 respire by the branches of the aorta, as well as by those of 

 the pulmonary artery, and the energy of their irritability is 

 in proportion to their quantum of respiration. (1 ) The whole 

 body is so constructed as to profit by this energy. 



Their anterior extremities, destined to sustain them in 

 flight, could neither serve them for standing, nor for pre- 

 hension; they are bipeds then, and pick up objects from the 

 earth with their mouth ; their body, consequently, is inclined 

 before their legs, the thighs directed forwards, and the toes 

 elongated, to form a suflicient base for it. The pelvis is very 

 much extended in length, in order to furnish points of attach- 

 ment to those muscles which support the trunk upon the 

 thighs. There is even a suite of muscles reaching from the 

 pelvis to the toes, passing over the knee and heel, so that the 

 simple weight of the bird flexes the toes ; it is thus that they 



(1) Two common Swallows consume as much pure air as a Guinea-Pig. Lavoi- 

 sier, Mem. de Chimie, I, 119. 

 Vol. I. 2 C 



