4 CLASS AVES. 



the surface of the pulmonary vessels, but also the sur- 

 faces of an infinity of vessels of the rest of the body. 

 Thus birds respire, in some respects, by branches of 

 their aorta, as well as by those of the pulmonary 

 artery, and the energy of their irritability is in pro- 

 portion to their quantity of respiration*. All their 

 body is arranged to participate in this energy. 



Their anterior extremities, destined to sustain them 

 in flight, could neither serve the purposes of standing 

 or holding: hence they are biped, and take things 

 from the ground with thd mouth : thus their body is 

 inclined before the legs, the thighs carry them for- 

 ward, and the toes are elongated to form a sufficient 

 base; the pelvis much extended in length, to fur- 

 nish attachment to the muscles which support the 

 trunk on the thighs. There is, moreover^ a set of 

 muscles proceeding from the pelvis to the toes, and 

 passing over the knee and the heel, so that the weight 

 alone of the bird closes the toes, and thus they are 

 enabled to sleep perched on one foot. The ischia, 

 and especially the ossa pubis, are elongated behind, 

 and widen, to leave sufficient place for the develop- 

 ment of the egg. 



The neck and the beak are elongated, to reach the 

 ground, and the former has pliability enough to be 

 bent backward when at rest. It has therefore many 

 vertebrae. On the other hand, the trunk which 

 supports the wings has very little motion ; the ster- 



* Two sparrows consume as much pure air as a guinea-pig. — 

 Lavoisier, Memoires de Chimit', i. 119. 



