538 CHORDATA 



The pneumaticity of the bones is an important feature. In place of 

 marrow and bony tissue, the cavity of the bones in strong flying birds is 

 more or less completely occupied by air spaces. This gives the greatest 

 possible lightness and strength to the skeleton. In Buceros and Palamedea 

 all bones are pneumatic; in others (Pelecamis,Sula, etc.) only the phalanges 

 of the toes contain marrow, while in the penguin and Apteryx, as in mam- 

 mals, air spaces occur only in some of the cranial bones. 



The air spaces of the bones are in part (skull) connected with the nose 

 and tympanum, but most of them, by means of the air sacs, communicate 



with the lungs. The long trachea forks at its 

 1 lower end into two bronchi. At its upper end is 

 a larynx, as in other vertebrates, but this is not 

 vocal; the notes of birds are produced by the 

 * svrinx, which lies at the division of trachea into 

 5 bronchi. It is usually formed of both trachea 

 FIG ?8o Develop- an< ^ bronchi, but more rarely of either trachea or 

 merit of trachea, lungs, bronchi alone. Its vocal cords are regulated by 

 f^^ Qoi C ^i^ a ^K special muscles, which in the singing birds have 



^dl Lei OtlC IlK.il ) . (' , 



trachea; 1-5, lung sacs. a complicated arrangement. The relatively small 



lungs send out from their surface air sacs, espe- 

 cially well seen in embryos (fig. 589, 1-5). These later become large, 

 thin-walled spaces, easily torn away in dissection, leaving large openings 

 on the surface of the lungs (fig. 590, 1-5). Usually five pairs of these air 

 sacs are present, largely in the ccelom, but extending in between the 

 muscles (breast and axillary region), and also into the bones. 



The spongy lungs lie on either side of the vertebral column and are attached 

 to the ribs. On entrance to the lung the bronchus (fig. 590, br) loses its cartilage 

 supports and enlarges into a vestibule (v) and extends thence as a mesobronchus 

 (bin) backwards, terminating in the abdominal air sac (5). A side branch 

 supplies the hinder subcostal sac (4). Secondary bronchi arise from the vesti- 

 bule and mesobronchus; of these there are three to five cntobronchi (I-IV) 

 supplying the remaining air-sacs and six or more ectobro-nchi. Arising from the 

 mesobronchi and secondary bronchi are tertiary bronchi, parabronchi, or lung 

 pipes, running parallel to each other and anastomosing frequently. Each air 

 pipe has a thick spongy wall (fig. 591). 



Inspiration is effected by raising the framework of the chest, thus causing 

 an increase of the sternovertebral 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 air sacs, which, on 

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

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

 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. 



