Figure 12

The sternotracheal muscle stabilizes the syrinx during song. (A) Lateral view of a μCT-based volume rendering showing the position of the syrinx in the skeletal framework of the upper thorax. A spine (orange dashed line) projects ventro-caudally from the second thoracic vertebra, thereby providing an anchor point for the lungs (green) and a pivot point (white circle) for the syrinx. On its ventral side, the syrinx fits into a dorsally oriented protrusion of the sternum, the external spine (EXS, *). The force (F_st) exerted by contraction of the ST (blue) rotates the syrinx ventrally (arrow) into the external spine. (B) Lateral view of a dissected syrinx with the ST muscles (left ST, black arrowheads) and their attachments intact. The external spine is continuous with a collagenous band (CB) that connects to the CASM (dotted white line). Also visible are the arteria syringealis, which supplies blood to the syrinx (white arrow), and the left syringeal nerve (black arrows). (C) Caudal view of a μCT-based volume rendering looking up from the sternum showing the position of the syrinx (yellow circle) and the attachment sites of the ST muscles (blue lines) in the intact skeletal framework. The ST attaches to tracheal ring T1 and on two lateral protrusions of the sternum. Contraction of the ST muscles pulls the syrinx onto the EXS. (D) Virtual slice through a 3D MRI dataset showing the syrinx (yellow dotted line) and EXS. Abbreviations as listed in Table 1.