LUNGS 295 



communicating with the anterior intermediate air-sac. A branch of the 

 second entobronchium opens externally to the hilum of the lung into the 

 sub-bronchial sac. 



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. These come off in a 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, or parabronchia : the walls of these are raised into numerous 

 transverse net-like folds, into which the pulmonary capillaries extend. 



The air-sacs arise from the embryonic pulmonary vesicles as 

 delicate-walled hollow processes, lined by pavement epithelium: these 

 grow rapidly, and soon exceed the lung proper in size, extending 

 amongst the viscera. Their form and extent depend largely upon 

 their surroundings : they consist simply 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, pass- 

 ing in between the muscles, beneath the skin, and even into most 

 of the bones. The latter are thus rendered pneumatic, and con- 

 sequently the specific gravity of the body is lessened, and the power 

 of flight increased. The pneumaticity of the bones is not, however, 

 an essential peculiarity connected with flight, for in many Birds 

 which are extremely good fliers (e.g., Larus, Sterna) the bones are 

 not pneumatic. 1 In these cases, however, a compensation is 

 effected by a more marked development of the muscles, and the 

 abdominal (posterior) air-sac, which in no Birds appears to be 

 entirely wanting, is here well developed. In the cursorial Ratitse, 

 on the other hand, the bones are markedly pneumatic. 



The air-sacs must be looked upon as integral parts of the 

 respiratory apparatus : a greater amount of air can, by their means, 

 pass in and out during inspiration and expiration, especially through 

 the larger bronchi, and consequently there is less necessity for the 

 expansion of the lung-parenchyma. The function of the prolonga- 

 tions of the air-sacs lying towards the outer surface of the body 

 consists in the giving off of watery vapour and in regulating the 

 heat of the body. Those which extend in between the muscles, 

 and supplant the connective and fatty tissue in these regions, have 

 a further importance in causing less power to be lost in friction. 



But by far the greatest importance of the air-sacs situated towards 

 the periphery consists in the enlargement of the anterior thoracic 

 region, principally that surrounded by the pectoral arch. A larger 

 development of the skeleton can thus take place, giving an increase 



1 The pneumaticity of the bones is not a special peculiarity of Birds : amongs 

 Mammals, frontal, maxillary, and sphenoidal sinuses are present in Anthropoids 

 Elephants, and Marsupials for instance ; the skull of Crocodiles is also strongly 

 pneumatic. All these sinuses communicate with one another, and also with the 

 tympanic cavity. They are in many cases developed in order to give a greater 

 surface for the attachment of muscles, and also to effect a saving of material and 

 a lightening of the skull. 



