126 



THE BIOLOGY OF BIRDS 



membranes function as vocal cords. Their tension is 

 regulated by muscles attached to the wall of the syrinx. 



A detailed account of the syrinx of the common fowl 

 has been given by A. O. V. Tymms (1913), who emphasises 

 the following features. There is an extensive syringeal 



-MTE 



Fig 32. — Vertical section of the syrinx of a male blackbird (after 

 Haecker). tr., trachea or windpipe ; tr.r., a tracheal ring cut across ; 

 TY., wall of tympanum or general cavity of the syrinx ; M., muscles on 

 the wall ; i.br., 1st bronchial ring; ii.br.r., 2nd bronchial ring; iii.br.r., 

 3rd bronchial ring ; br., bronchus ; pe., bony pessulus ; M.S., mcmbrana 

 seniilunaris ; a.s., cavity occupied by an extension of the anterior 

 thoracic air-sac ; le., external lip or labium; Li., internal lip or labium ; 

 M.T.I., membrana tympaniformis interna; m.t.e., mcmbrana tympani- 

 formis externa- 



chamber or " tympanum " with membranous walls ; the 

 last five tracheal rings are very vestigial, the first two 

 bronchial semi-rings are more strongly developed, the 

 pessulus is bolt-like, with ventral and dorsal triangular 

 plates ; very noticeable is the absence of muscles in direct 

 or intimate association with the syringeal membranes ; there 



