154 



AVES. 



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

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

 from their parent. These are called false Ovipara. 



THE SECOND CLASS OF VERTEBRATED ANIMALS. 



THE BIRDS (AVES),— 



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

 for flight. 



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

 pierced with large holes, and which 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 fluid 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 respects, by 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.f Their total conformation is arranged to participate in this energy. 

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



them for 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 sufficient base for standing. The pelvis is longitudi- 

 nally much extended, to furnish attachment to the muscles 

 which support the trunk upon 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 flexes the toes : it is thus that they 

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

 especially the ossa pubis, are lengthened out behind, and 

 widened in their span, to allow the necessary space for 

 the developement of the eggs. 



The neck and the beak are elongated to reach the 



ground ; but the former has also the requisite flexibility for 



doubling backward when at rest. It has therefore numerous 



vertebra?, [varying from twelve to twenty-three, which latter 



number is attained only in the genus Cy gnus'] . The trunk, 



on the contrary, which serves as a fulcrum to the wings, 



has but little mobility ; the sternum especially, to which 



Fig. er.-sueicton of Jcr Falcon. are attached the muscles which effect the propulsive stroke 



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



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



* In the Hornbills, even the phalanges of the toes are hollow, and 

 communicate with the lungs. The opposite extreme occurs in the 

 Apteryx, which has no accessory air-cavities.— Ed. 



t Two Sparrows consume as much air as a Guinea-pig-.. — Lavoi- 

 sieb, Memuires de Chimie, i. 110. 



