J26 



Comparative Morphology of Chordates 



ened by skeletal rings placed fairly close together and, except in the 

 syrinx, at nearly equal distances apart. These rings, cartilaginous in 

 the young bird, are more or less completely bony in the adult. They 

 usually encircle the tube, in contrast to the similar rings which, in 



Fig. 411. Section of syrinx of thrush, 

 Tardus (after Hacker). (b l -b 3 ) Bronchial 

 half-rings; (bd) bronchidesmus; (br) bron- 

 chus; (c) cavity opening ventrally; (el, il) 

 external and internal labia; (m) muscle; 

 (/•) resonator; (sm) semilunar membrane; 

 (t) trachea; (le, ti) external and internal 

 tympaniform membranes; (Ir) tracheal 

 rings. (Courtesy, Kingsley: "Compara- 

 tive Anatomy of Vertebrates," Philadel- 

 phia, The Blakiston Company.) 



mammals, are incomplete dorsally. In the wall of the syrinx the rings are 

 enlarged and so modified in form and arrangement as to leave certain 

 relatively large areas of the wall unoccupied by skeletal tissue. In 

 such areas the wall is a soft and flexible membrane, free to vibrate in 

 and out, therefore resembling the tympanic membrane of an ear. In a 

 syrinx of the bronchotracheal type, there may be two pairs of such 

 membranes, those of one pair (external tympaniform membranes) 

 situated in the right and left sides of the trachea, and those of the 

 other pair (internal tympaniform) in the adjacent inner walls of 

 the bronchi (Fig. 411). Where the walls of the two bronchi meet in 

 the median plane, their membranous lining is continued to form a fold 



