THE PIGEON. 245 



the inner face of each bronchus is flat and membranous, and 

 constitutes the membrana tympaniformis interna. 



IT" 



FIG. 57. Columba livia. The lung?, filled from the trachea with 

 a solid injection and viewed from the ventral aspect (nat size). 



In the left lung the bronchus (br, br 1 } is traced to its termination by 

 dissecting a\vay the overlying substance of the lung. 



br, principal bronchus : br 1 , br", br"', secondary bronchi : p. a, 

 pulmonary artery : p.v, pulmonary vein : sy, syrinx : tr, trachea : sp.b, 

 aperture for prebronchial air-sac: sb.b, for sub-bronchial: a.in, for 

 anterior-intermediate : p.in, for posterior-intermediate : /, for posterior 

 (abdominal). 



284. The modification of the tracheal rings in the syrinx : the two 

 posterior rings are more widely separated from each other than the rest, 

 and their bony ventral portions send median processes towards one 

 another, which are only separated by a small cartilaginous interval : on 

 the dorsal side these rings are incomplete, but are united with one 

 another on each side of the middle line by a longitudinal cartilaginous 

 bar : the bony first pair of bronchial half-rings are in contact with one 

 another dorsally and ventrally, and, with the distal tracheal rings just 

 described, inclose a chamber, the tympanum. 



285. If the air-sacs, and consequently the lungs, are injected, the 

 secondary bronchi (br', br", br'"} will be seen branching out over the 

 anterior and internal regions of the ventral surface of the lungs : they 



