154 



AVES. 



Many of the cold-blooded Ovipara do not bring forth their young until they are 

 developed and extricated from their shell, or otlier membranes which separated them 

 from their parent. Tlicse are called /ff/ic Ovipara. 



THE SECOND CLASS OF VEUTEBRATED AXBIALS. 



THE BIRDS (JVES),— 



Are oviparous vertebrates with double circulation and respiration, [mostly] organized 

 for flight. 



Their lungs, undivided and attaclied to the ribs, are enveloped by a membrane 

 pierced with large holes, and \\'!ncli allows the air to pass into many cavities of the 

 chest, the abdominal region, arm-pits, and even of the interior of the bones* ; so that 

 the ambient tluid not only bathes the surface of the pulmonary vessels, but also that 

 of an infinitude of vessels traversing the rest of the body. Thus Birds respire, in 

 certain re.-pects, liy the ramifications of their aorta, as well as by those of their 

 pulmonary artery, and the energy of their irritability is in proportion to their amount 

 of respiration.! Their tnt;d conformation is arranged t(j participate in this energy. 

 Their anterior extremities, destined to sustain them in flight, could neither serve 

 ^ _ them fur standing, nor for clutching : they are bipeds, then, 



and pick up objects from the earth with their mouth ; their 

 body, consequently, is balanced upon the legs ; the thighs 

 are directed forward, and the toes are lengthened to form 

 a suflicient base for standing. The pelvis is longitudi- 

 nallv much extended, to furnish attachment to the muscles 

 \'\liieh suppoit the trunk u]Hin the thighs: there is even 

 a suite of muscles proceeding from the pelvis to the toes ; 

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

 weight of the bird Ilexes the toes : it is thus that they 

 are enabled to sleep perched on one foot. The iacliKi, and 

 especially the os.^ii piihig, are leng'thencd out behind, and 

 widened in their span, to allow the necessary space for 

 the developement of the eggs. 



Tlie neclc and the lieak are elongated to reach the 

 ground ; but the former has also the requisite flexibility fur 

 donliling Ijackward when at rest. It has tlierefore numerous 

 vertebra', [varving from twelve to twentv-thrce, which latti r 

 numljcr is attained only in the genus Cijgmis']. The trunk, 

 on the eiintrin-y, which serves as a fulcrum to the wings, 

 has but little mobility; the sternum especially, to which 

 arc attached the muscles wdiich cflect the propulsive stroke 

 in flying, is of great extent, its surface [except in the Ostrich and allied genera, vdiich do 

 not fly,] being further augmented by a projecting ridge along its unddle. It is [niostlyl 



-SkL-lctonof Jc 



i.c as niiicli .lir 



i, nil. 



