VERTEBRATA: AVES. 



587 



" air-sacs." These are a series of membranous sacs formed by 

 the continuation of the lining membrane of the bronchi, and 

 supported by reflections of the serous membrane of the thora- 

 cico-abdominal cavity. There are nine proper air-sacs two 

 abdominal (the only ones present 

 in some birds, such as the Pen- 

 guin), two in the hinder part of 

 the thorax, two in the front part 

 of the thorax, two on the sides 

 of the neck, and one between the 

 branches of the furculum. The 

 air-cells not only greatly reduce 

 the specific gravity of birds, 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-re- 

 ceptacles, arid as an extension of 

 them, is a series $f cavities occu- 

 pying the interior of a greater or 

 less number of the bones, and 

 also containing air. In young 

 birds these air-cavities do not ex- 

 ist, and the bones are filled with 



i -. .- i rm 



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. 



Fig. 327. Lung of Goose (after Owen). 

 a Main bronchus dividing into sec- 

 ondary branches as it enters the lung, 

 these giving off smaller branches, the 

 openings of which are seen on the 

 back of the bronchial tubes : b b Bris- 

 tles passed from the bronchi through 

 the apertures on the surface of the 

 lung by which the bronchi communi- 

 cate with the air-receptacles. 



