a? mv é we: . 
Pits 6 . 4 
és MES sp Re 9 ' 217 
ip et 
CLASS II. 
is AVES. 
Cites are Oviparous Vertebrata, with double systems of cir- 
elation and respiration, organized for flight. 
Their lungs, undivided and attached to the ribs, are enve- 
; Tope 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 respects, 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 proportion to their quantum of respiration.(1) The whole 
, body is so constructed as to profit by this energy. 
Their anterior extremities, destined to sustain them in 
flight, could neither serve them for standing, nor for pre- 
hension; 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 asufficient base for it. The pelvis is very 
much extended in length, in order to furnish points of attach- 
-ment 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 they 
(1) Two common Swallows consume as much pure air as a Guinea-Pig. Layoi- 
sier, Mem. de Chimie, I, 119. 
Vor. I.—2 C 
