Fig. 1. The songbird tracheobronchial syrinx is a complex bipartite structure situated where the connecting tubes to the lungs (the bronchi) join the windpipe (the trachea). (A) Schematic ventro-lateral external view of a songbird syrinx depicting the syringeal muscles. To illustrate the position of the angioscope used for internal views, parts of the trachea have been removed; the yellow colour indicates endoscope light. The external angioscope indicates the viewing angle in Fig. 4. (B) Schematic horizontal section through a songbird syrinx illustrating the two labial sound sources and the approximate position and field of view of the angioscope (yellow light) in Fig. 3. (For the syringeal views in Fig. 2, the angioscope was advanced towards the left side of the syrinx.) BC, bronchial cartilage; 3B, third bronchial cartilage; BL, bronchial lumen; dS, m. syringealis dorsalis; dTB, m. tracheobronchialis dorsalis; ICM, membrane of the interclavicular air sac; LL, lateral labium; M, syringeal muscle; ML, median labium; MTM, median tympaniform membrane; SM, semilunar membrane; ST, m. sternotrachealis; T, trachea; TL, m. tracheolateralis; TY, tympanum (consisting of four closely apposed or fused tracheosyringeal cartilages); vS, m. syringealis ventralis; vTB, m. tracheobronchialis ventralis.
