CHARACTERS OF AVES. 437 



restricted to the abdomen; but in most birds they are con- 

 tinued along the sides of the neck and limbs. In some 

 cases as the Pelican and Gannet air-receptacles are situated 

 beneath almost the whole of the integument. The air-cells 

 not only greatly reduce the specific gravity of the bird, and 

 thus fit them for an aerial life, but also* assist in the mechanical 

 work of respiration, and must also greatly promote the aeration 

 of the blood. 



In connection with the air-receptacles, and as an extension 

 of them, is a series of cavities occupying the interior of a 

 greater or less number of the bones, and also containing air. 

 In young birds these air-cavities do not exist, and the bones 

 are filled with marrow as in the Mammals. The extent also 

 to which the bones are " pneumatic " varies greatly in different 

 birds. In the Penguin which does not fly all the bones 

 contain marrow, and there are no air-cavities. In the large 

 Running Birds (Cursores), such as the Ostrich, the bones of 

 the leg, pelvis, spine, ribs, skull, and sternum are pneumatic; 

 but the bones of the wings, with the exception of the scapular 

 arch,, are without air - cavities, and permanently retain their 

 marrow. All birds which fly, with the singular exception of 

 the Woodcock, have air admitted to the humerus. In the 

 Pelican and Gannet, all the bones of the skeleton, except the 

 phalanges of the toes, are penetrated by air; and in the Horn- 

 bill even these are pneumatic. The functions discharged by 

 the air-cavities of the bones appear to be much the same as 

 those of the air-receptacles namely, that of diminishing the 

 specific gravity of the body and subserving the aeration of the 

 blood. 



The heart in all Birds consists of four chambers, two auricles 

 and two ventricles. The right auricle and ventricle, constitut- 

 ing the right side of the heart, are wholly concerned with the 

 pulmonary circulation ; the left auricle and ventricle, forming 

 the left side of the heart, are altogether occupied with the 

 systemic circulation ; and no communication normally exists 

 in adult life between the two sides of the heart. In all essen- 

 tial details, both as regards the structure of the heart itself 

 and the course taken by the circulating fluid, Birds agree with 

 Mammals. The venous blood namely, that which has circu- 

 lated through the body is returned by the venae cavae to the 

 right auricle, whence it is poured into the right ventricle. The 

 right ventricle propels it through the pulmonary artery to the 

 lungs, where it is aerated, and becomes arterial. It is then 

 sent back by the pulmonary veins to the left auricle, whence it 

 is driven into the left ventricle. Finally, the left ventricle pro- 



