400 ZOOLOGY sect. 



end (Figs. 1041 and 1042), dividing into two branches, each 

 of which enters a bladder-like air-sac, formed as a dilatation of the 

 mucous membrane of the bronchus. One of these, the abdominal 

 air-sac (Fig. 1042, A, abd. a. s), lies among the coils of the intestine, 

 the other, or posterior thoracic air -sac {post: th.a. s), is closely 

 applied to the side-wall of the body. The bronchus also gives off, 

 near its entrance into the lung, three short branches, one of which 

 becomes connected with an anterior thoracic air-sac {ant. th. a. s), 

 situated just in front of the posterior thoracic ; another with an 

 interclavicular air-sac {int. clav- a. s), which is median and unpaired, 

 and connected with both lungs ; the third enters a cervical air-sac 

 {cerv. a. s.) placed at the root of the neck. Each side of the inter- 

 clavicular gives off an axillary air-sac, lying in the arm-pit. All 

 these sacs are paired except the interclavicular, which is formed by 

 the fusion of right and left moieties. The sacs are in communi- 

 cation with the pneumatic cavities of the bones. 



The ventral or free walls of the. thoracic air-sacs of each side 

 are covered by a sheet of fibrous tissue, the oblique septum {obi. 

 sepit.), which is continued forwards to the pericardium, and is 

 united with its fellow of the opposite side in the middle dorsal 

 line : it divides the ccElome into two compartments ; one containing 

 the lungs with the interclavicular and thoracic air-sacs, the other 

 {ahd. cav.) the heart, liver, stomach, intestine, etc., with the ab^ 

 dominal air-sacs. 



Besides the branches to the air-sacs, the main bronchus gives 

 off secondary bronchi, and these branch again, sending off tubes 

 which give rise to a system of fine branching and anastomosing 

 tubules, the "lung-capillaries," which make up the main substance 

 of the lunff. 



When the Pigeon is standing, the alternate elevation and de- 

 pression of the sternum, produced partly by the abdominal, partly 

 by the intercostal muscles, causes an alternate enlargement and 

 diminution of the capacity of the coelome, and thus pumps air in 

 and out of the lungs. During flight, when the weight is supported 

 by the wings, and the sternum is thus rendered relatively im- 

 movable, the same effect seems to be produced by the elevation 

 and depression of the back. In either case the inspired air 

 rushes through the lungs into the air-sacs and thence by diffusion 

 into the pneumatic cavities of the bones. Thus, while in other 

 animals a certain amount of unchanged or residual air is always 

 left in the lungs after each expiration, in Birds the residual air is 

 confined to the air-sacs and to the smaller branches of the bronchi, 

 every respiratory movement drawing a current of fresh or tidal air 

 through the lungs. As a result of this the aeration of the blood is 

 very complete and its temperature correspondingly high. It is 

 worthy of notice that Birds agree with Insects, the only other 

 typically aerial class, in having the inspired air distributed all 



