262 COMPARATIVE ANATOMY. 



air-cavities which lie between the pericardium and sternum, and in the axilla, 

 outside the body-cavity (axillary sac) (Fig. 209, p, p). 



The lower surface of each lung is closely invested by a thin fibrous 

 membrane, the pulmonary aponeurosis, 1 into which are inserted a variable 

 number of muscular bands (costo-pulmonary muscles). These arise from 

 the vertebral ribs, and are supplied by the intercostal nerves (Fig. 210). 



The main bronchus (mesobronchium) runs close to the ventral 

 surface of the lung surrounded by the lung-parenchyma, and extends to its 

 posterior end, where, as a rule, it opens directly into the abdominal air-sac 

 (Fig. 210, au). From it a large lateral bronchus branches off, which opens 

 into the posterior intermediate sac by one or two (e.g. in Passeres) apertures (Fig. 

 210, &, &'). Besides this there are from four to six other lateral bronchi (Fig. 

 210, I to 2V), all of which become broadened out in a fan-like manner on the 

 ventral surface of the lung. These may be called entobronchia (bronchi 

 divergentes, Sappey) : they all arise from the anterior portion of the meso- 

 bronchium. The first of these radiates out anteriorly to the hilum of the lung, 

 and gives off internal, external, and anterior branches, one of which opens 

 into the prebronchial sac (Fig. 210, c). The other entobronchia give rise to 

 two series of branches, one of which extends inwards and backwards between 

 the factors of the pulmonary vein, and the other outwards between the 

 arterial branches. Almost without exception a large aperture or ostium is 

 present on the wall on the third entobronchium, communicating with the 

 anterior intermediate air-sac (Fig. 210, e). A branch of the second ento- 

 bronchium opens externally to the hilum of the lung into the sub-bronchial 

 sac (Fig. 210, lie, d). 



The lateral bronchi considered as yet have to do with the ventral surface 

 of the lung only ; but besides these there are a variable number of ecto- 

 bronchia, arising from the dorsal side of the main bronchus posteriorly to 

 the entobronchia (see Fig. 210). These come off in a double longitudinal row, 

 those of the outer row being larger than those of the inner. They pass 

 dorsally to the costal face of the lung. Both ecto- and entobronchia give off 

 numerous bronchi of a third order, orparabronchia: the walls of these are 

 raised into numerous transverse net-like folds, into which the pulmonary 

 capillaries extend. 



The following points must be noticed as regards the genesis 

 and function of the air-sacs. 



Early in the embryonic period, delicate-walled hollow pro- 

 cesses, lined by pavement epithelium, arise from the pulmonary 

 vesicles : these grow rapidly, and soon exceed the lung proper in 

 size, so that they extend amongst the viscera. Their form and 

 extent depend entirely upon their surroundings, and they simply 

 consist of interstitial cavities lined by the membrane of the air- 

 sacs. Moreover, they are not confined to the body-cavity, but in 

 numerous places extend beyond it, passing in between the muscles, 

 beneath the skin, and even into most of the bones. The latter 

 are thus rendered pneumatic, 2 and consequently the specific 



ivity of the body is lessened, and the power of flight increased, 

 pneumaticity of the bones is not, however, an essential pecu- 

 liarity connected with flight, for in many Birds which are extremely 



1 The pulmonary aponeurosis, as well as the oblique septum, is often spoken of 

 as a "diaphragm" (comp. p. 122). 



2 This cannot of course take place until the marrow of the bones has performed 

 the greater part of its bone-forming function. 



