OVIPARA. 113 



a sac attached to its intestinal canal ; this is what is called the vitellus, or yolk 

 of the egg, to which the young animal is a sort of appendage, at first imper- 

 ceptible, but which is nourished and augmented by absorbing the fluid of that 

 yolk. Such of the Ovipara as breathe with lungs have the egg furnished with 

 a highly vascular membrane, which appears to serve for the purposes of 

 respiration ; it is connected with the bladder, and is analogous to the allan- 

 toid of the Mammalia. It is neither found in Fishes nor in the Batrachians 

 which latter, when young, respire, like Fishes, by bronchiae. 



CLASS II. 



AVES. 



BIRDS are Oviparous Vertebrata, with a double system of circulation 

 and respiration, organised 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 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 re- 

 spects, 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 propor- 

 tion to their quantity of respiration*. The whole body is so con- 

 structed as to profit by this energy. 



Their anterior extremities, destined to sustain them in flight, could 

 neither serve them for standing, nor for prehension ; 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 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 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 some species are enabled to sleep in security, while 

 perched on one foot. 



The neck and the beak are elongated so as to reach the ground, but the 

 former has the requisite flexibility for bending backwards when at 

 rest, consequently, it has many vertebrae. The trunk, on the con- 



* Two common swallows consumed as much pure air as a Guinea Pig. 

 I 



