398 



ZOOLOGY 



SECT. 



as it enters the body-cavity, where it divides into the right (r. br.) 

 and left bronchi. The rings supporting the trachea are not 

 cartilaginous but bony, as also is the first ring of each bronchus, 

 those of the trachea completely surrounding the tube, those of the 

 bronchi incomplete mesial ly. 



At the junction of the trachea with the bronchi occurs the 

 characteristic vocal organ, the syrinx (syr.) } found in no other class. 

 The last three or four rings of the trachea (Fig. 1041, tr.), and the 

 first or bony half-ring of each bronchus (br.), are modified to form 

 a slightly dilated chamber, the tympanum, the mucous membrane 

 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 sup- 



8 y f* ^i ports an inconspicuous 



fold of mucous mem- 

 brane, the membrana 

 semilunaris. The mem- 

 branous 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 semi- 

 lunar membrane : its 

 pitch is altered by 

 changes in the form of 

 the tympanum produced by the action of the muscles. 



The lungs (Figs. 1040 and 1041, Ing.) are very small in comparison 

 with the size of the Bird, and are but slightly distensible, being solid, 

 spongy organs, not mere bags with sacculated walls as in Amphibia 

 and many Reptiles. Their dorsal surfaces fit closely into the spaces 

 between the ribs, and have no peritoneal covering : their ventral 

 faces are covered by a strong sheet of fibrous tissue, the pulmonary 

 aponeurosis or pleura (Fig. 1042, B, pul.ap.), a special development 

 of the peritoneum. Into this membrane are inserted small fan- 

 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 



Irr" 



FIG. 1041. Columba livia. The lungs with the 

 posterior end of the trachea, ventral aspect. a. in. 

 aperture of anterior thoracic air-sac ; b.r. principal 

 bronchus ; W. br". br'". secondary bronchi ; p. aperture 

 of abdominal air-sac ; p. a. pulmonary artery entering 

 lung ; p. in. aperture of posterior thoracic air-sac ; p. v. 

 pulmonary vein leaving lung ; sb. b. aperture of inter- 

 clavicular air-sac ; sp. b. aperture of cervical air-sac ; 

 sy. syrinx ; tr. trachea. (From Parker's Zootomy.) 



