AVES. 261 



receptacles which are scattered through various parts of the 

 body. The lungs are two in number, of a bright-red color, 

 and spongy texture, and they are confined to the back part 

 of the chest. They differ from the lungs of Mammals in not 

 being freely suspended in a membranous bag (pleura), but in 

 being fixed to the back wall of the chest. The thoracic and 

 abdominal cavities are not separated from one another by a 

 complete partition (midriff or diaphragm) as the Mammals, but 

 the common thoracico-abdominal cavity is subdivided by means 

 of membranous partitions into a series of cavities or sacs, which 

 are termed the " afr-receptacles." These air-sacs are filled 

 with air from the lungs, and vary considerably in number and 

 size in different birds. They not only serve greatly to reduce 

 the specific gravity of the body, but also assist largely in the 

 aeration of the blood. Connected with the air-receptacles, 

 and supplementing their action in both of these respects, 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 in the bones do not exist, and the bones are 

 simply filled with marrow, as in the Mammals. In the Pen- 

 guin, which does not fly, none of the bones contain air-cavities 

 or are " pneumatic ; " and in the Ostrich only a few of the 

 bones contain air. In the Pelican and Gannet all the bones 

 of the skeleton, except the phalanges of the toes, are per- 

 meated by air ; and in the Hornbill even these are pneu- 

 matic. 



The heart in all birds consists of four chambers, and the 

 two sides of the heart are completely separated from one 

 another. In all essential details, as regards the structure of 

 the heart and great vessels, and the course of the circulating 

 fluid, Birds agree with Mammals. The impure venous blood 

 which has traversed the body is returned by the great veins 

 to the right auricle. From the right auricle it passes into the 

 right ventricle, from which it is driven by the pulmonary artery 

 to the lungs. Having been submitted to the action of the air 

 contained in the lungs, and having been thereby changed into 

 arterial blood, the blood is sent back to the left auricle by 

 means of the pulmonary veins. Thence it passes into the 

 left ventricle, by which it is again propelled throughout the 

 whole body, to retirm again as venous blood to the ri^ht side 

 of the heart. The heart, therefore, of birds, differs from that 

 of reptiles in consisting of two sides, each composed of an 

 auricle and ventricle, the right side being wholly concerned 

 with sending the venous blood to the lungs, and the left side 



