CHARACTERS OF AVES. 



467 



back of the thorax, extending along each side of the spine, 

 from the second dorsal vertebra to the kidney. They differ 

 from the lungs of the Mammals in not being freely suspended 

 in a pleural membrane. The pleura, on the other -hand, is 

 reflected only over the anterior 

 surface of the lungs. The bronchi, 

 or primary divisions of the wind- 

 pipe (fig. 184), diminish in size as 

 they pass through the lung, by 

 giving off branches, which, in 

 turn, give off the true air-vesicles 

 of the lung. When the bronchial 

 tubes reach the surface of the 

 lung, they open, by a series of 

 distinct apertures, into a series of 

 " air-sacs." These are a series of 

 membranous sacs formed by the 

 continuation of the lining mem- 

 brane of the bronchi, and sup- 

 ported by reflections of the serous 

 membrane of the thoracico-abdo- 

 minal cavity. In those aquatic 

 birds which, like the Penguin, do 

 not enjoy the power of flight, the 

 air-cells are restricted to the ab- 

 domen; but in most birds they 

 are continued along the sides of 

 the neck and limbs. In some 

 cases — as the Pelican and Gannet 

 — air-receptacles are situated be- 

 neath almost the whole of the in- 

 tegument. 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 re- 

 spiration, a,nd 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 



-Lung of Goose (after Owen). 



ichu 



Fig. 184.- 

 a Main bronchus dividing into secon- 

 dary blanches as it ent^s 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 fy which the bronchi communi- 

 cate with the air-receptacles. 



