P,02 



CLASS II. 



BIRDS (AVES). 



Birds 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 hi 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 vertebrae. The trunk, on the contrary, Avhich 

 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 Chiinie, I. 119. 



