CHARACTERS OF AVES. 495 



Running birds (Cvrsores), 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 \vhich 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 o*f 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- 

 pels the aerated blood to all parts of the body through the 

 great systemic aorta. 



The chief difference between Birds and Reptiles as regards 

 the course of the circulation is, that in the Birds the two sides 

 of the heart are completely separated from one another, the 

 blood sent to the lungs being exclusively venous, whereas that 

 which is sent to the body is exclusively arterial. In Reptiles, 

 on the other hand, the pulmonary and systemic circulations 

 are connected together either in, or in the immediate neigh- 

 bourhood of, the heart ; so that mixed venous and arterial 

 blood is propelled both through the lungs and through every 

 part of the body. 



In accordance with their extended respiration and high mus- 

 cular activity, the complete separation of the greater and lesser 

 circulations, and the perfect structure of the heart, Birds main- 

 tain a higher average temperature than is the case with any 



