Ooi' 



CHORD ATA 



The pneumaticity of the 1)ones is an important feature. In place of 

 marrow and bonv tissue, the ca\ity of the Ixmes in strong Hying birds is 

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

 possible lightness and strength to the skeleton. In Buccros and Palamcdca 

 all bones are pneumatic; in others [rcIcia)iHS,Snla, etc.) only the phalanges 

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

 mals, air spaces occur only in some of the craiiial Ixmes. 



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

 and tympanum, l.iut most of them, l.iy means of the air sacs, communicate 

 with the lungs. The long trachea forks at its 

 lower end into two bronchi. At its upper end is 

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

 vocal; the notes of Ijirds are produced by the 

 svriiix, which lies at the division of trachea into 

 bronchi. It is usually formed of both trachea 

 and lironchi, but more rarely of either trachea or 

 bronchi alone. Its vocal cords are regulated by 

 special muscles, which in the singing birds have 

 a complicated arrangement. The relatively small 

 lungs send out from their surface air sacs, espe- 

 cially well seen in embryos (lig. 5S9, 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 ccclom, but extending in l.ietween the 

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



Fig. 5S0. — r)evclop- 

 ment of tracViea, lungs, 

 and air sacs of a chick 

 (after Selenka). tr, 

 trachea; 1-5, lung sacs. 



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. 5Q0, hr) loses its cartilage 

 supports and enlarges into a vcslihiilc (■!') and extends thence as a incsohnnuinis 

 (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 ciilohroiniii (/-/T) 

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

 mesobronchi and secondary bronchi are tertiary bronchi, parahnviiiii, or luui^ 

 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, arc 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, dra\^^ng 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. 



