xiii PHYLUM CHORDATA 371 



organs, not mere bags with sacculated walls as in Amphibia and 

 many Reptiles. Their dorsal surfaces fit closely into the spaces 

 between the ribs, and have no peritoneal covering, their ventral 

 faces are covered by a strong sheet of fibrous tissue, the pulmonary 

 aponeurosis or pleura (Fig. 983, B, pul. ap.), a special development 

 of the peritoneum, Into this membrane are inserted small fan- 

 like costo-pulmonary muscles, which arise from the junction of the 

 vertebral and sternal ribs. 



The bronchus, on entering the lung, is continued to its posterior 

 edge (Figs. 982 and 983), where it divides 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. 983, 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-walls of the bod}^. The bronchus also gives off, 

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

 becomes connected with an anterior thoracic air-sa6 (ant. th. a. s\ 

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

 inter clavicular 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. 

 sept.) 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 coelome into two compartments ; one containing 

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

 (abd. 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 end blindly near the surface of the lung and give off blind 

 dilatations commonly know as alveoli. The ultimate branches 

 are given off at right angles from those of a higher order. 



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 



B B 2 



