IV. VERTEBRATA: AVES. 



609 



united to the ribs. On entrance to the lung the bronchus (fig. 639, br) loses 

 its cartilage supports and enlarges into a 

 vestibule (0) and extends thence as a 

 mesobronchus (6m) backwards, termi- 

 nating in the abdominal air sac (.5). A 

 side branch supplies the hinder sub- 

 costal sac (4). Secondary bronchi arise 

 from the vestibule and mesobronchus; 

 of these there are three to five euto- 

 bronchi (I-IV) supplying the remaining 

 air-sacs and six or more ectobronchi. 

 Arising from the mesobronchi and 

 secondary bronchi are tertiary bronchi, 

 or air pipes, running parallel to each 

 other and anastomosing frequently. 

 Each air pipe has a thick spongy wall 

 (tig. 640) composed of numerous thin- 

 walled sacs, the lung vesicles, closely 

 enveloped by capillaries, and connected 

 with the central air-conducting tube, 

 the lumen of the pipe. 



Inspiration is .effected by raising the framework of the chest, this 

 causing a straightening of the hinged ribs and an increase of the sterno- 

 vertebral diameter ; expiration by the reverse motion. By this the lungs, 

 attached to the ribs, are alternately enlarged and contracted in spite of 

 their slight elasticity. This is also true of the lung sacs, which, on account 

 of their poor blood supply, are not respiratory but serve as accessory air 



FIG. 639. Right lung of hen, some- 

 what diagrammatic. A window- 

 shows a mesobronchus with its- 

 branches, a, artery ; brn, meso- 

 bronchus, arising from the vesti- 

 bule : br, bronchus swelling to 

 vestibule ; eb, ectobronchus : I, 

 lung pipes ; 7-7F, mesobronchi - t 

 1-5, ducts of lung sacs. 



FIG. 640. Section of lung pipe. (After Schulze.) 



pumps. It is probable that in flight this air-pump action occurs espe- 

 cially with the subpectoral and axillary air sacs, drawing air through the 

 lungs and rendering other respiratory motions superfluous, thus enabling 

 the thorax to remain quiet, an important matter. If the trachea be 

 closed and the air canal in the humerus opened, the bird can breathe 

 through the latter. 



