386 



RESPIRATION OP BIRDS. 



the venous blood coming in [contact with the oxygen, when 



passing through the capillary vessels of the general system, as 



well as when it passes through the pulmonary capillaries. The 



lungs are not divided into lobes as in the Mammalia, and are far 



from filling the thorax; they adhere to the ribs; and they 



present at their inferior surface many openings, belonging to the 



bronchial tubes, which traverse them in different directions, and 



carry the air into the air-cells distributed among the different 



organs of the animal. These cavities are formed by membranous 



partitions, or by laminae of cellular tissue, and communicate with 



one another ; some are found in 



the trunk, of very considerable 



size; and others are prolonged 



towards the head, and between 



the muscles of the limbs ; the air 



thus finds its way into every part 



of the body, and generally also 



into the substance of the bones. 



An examination of the air-cells 



in different Birds, shows that the 



quantity of air thus distributed 



into the different parts of the 



body, varies, other things being 



equal, in proportion to the energy 



and continuity of the movements 



of the animal; thus in Eagles, 



Sparro wha Wks, and Other Birds Of d > the trachea ; e, the lungs ; ///, air-cells, 

 ~ r i .a- i_j. J.1-* a '3 in which are also seen the tubes by which 



powerful flight, this fluid pene- these air-c^lls communicate with the 



trates into all the bones; while lungs * 



in those which are destitute of the power of flight, and which 

 walk but slowly, as Penguins, &c., it is excluded from the 

 greater part, or even from the whole of the skeleton. The air 

 is generally in greatest abundance in the bones of the limbs 

 most employed in locomotion ; thus in the Ostrich, the femur 

 (thigh-bone) presents a remarkable development of the air-cells. 

 349. Birds have a more constantly-active respiration than 

 any other animals ; they consume more oxygen in proportion, 



