386 



ZOOLOGY 



SECT. 



of which forms a cushion-like thickening on each side. At the 

 junction of the bronchi a bar of cartilage, the pessulus, extends 

 dorso-ventrally and supports an inconspicuous fold of mucous 

 membrane, the membrana semilunaris. The membranous inner 

 walls of the bronchi form the internal tympaniform membranes. 

 A pair of intrinsic syringeal muscles arise from the sides of the 

 trachea and are inserted into the syrinx, and a pair of sterno- 

 tracheal muscles arise from the sternum and are inserted into the 

 trachea. The voice is produced by the vibration of the semilunar 

 membrane : its pitch is altered by changes produced by the action 

 of the muscles. 



The lungs (Figs. 1051, Ing., and 1052) are very small in com- 

 parison with the size of 

 the Bird, and are but 

 slightly distensible, being 

 solid, spongy organs, not 

 mere bags with saccu- 

 lr>'" lated walls as in Am- 

 phibia and many Rep- 

 tiles. Their dorsal sur- 

 faces fit closely into the 

 spaces between the ribs, 

 and have no peritoneal 

 covering : their ventral 



sb.l 



O..TII 



ji.in 





F,o. 1052,-Columb, UvU. The Im with H,e 



faces are covered by a 

 strong sheet of fibrous 

 tissue, the pulmonary 

 aponeurosis or pleura 



posterior end of the trachea, ventral aspect, a. in. (Fig. 1053, B, pul. ap.}, 



aperture of anterior thoracic air-sac ; br. principal i i j: 



bronchus ; br', br", br'" , secondary bronchi ; p. aper- a Special development OI 



ture of abdominal air-sac; p.a. pulmonary artery f V,p npritnnpiiTn Tnrn fhiq 



entering lung ; p. in. aperture of posterior thoracic 



air-sac: p. v. pulmonary vein leaving lung; sb. b. membrane are inserted 



aperture of interclavicular air-sac; sp.b. aperture of ,, ., ,., 



cervical air-sac ; ay. syrinx; tr. trachea. (From Small tan-like COStO- 



pulmonary muscles, 

 which arise from the junction of the vertebral and sternal ribs. 



The bronchus, on entering the lung, is continued to its posterior 

 end (Figs. 1052 and 1053), dividing into two branches, each of 

 which enters a bladder-like air-sac, formed as a dilatation of the 

 mucous membrane of the bronchus. One of these, the abdominal 

 air-sac (Fig. 1053, A, abd. a. s.), lies among the coils of the intestine ; 

 the other, or posterior thoracic air-sac (post. th. a. s.), is closely 

 applied to the side-wall of the body. The bronchus also gives off, 

 near its entrance into the lung, three short branches, one of which 

 becomes connected with an anterior thoracic air-sac (ant. th. a. s.), 

 situated just in front of the posterior thoracic ; another with an 

 interclavicular air-sac (int. clav. a. s.}, which is median and unpaired, 

 and connected with both lungs ; the third enters a cervical air-sac 



