pessulus

from The Century Dictionary.

• noun In ornithology, the cross-bone of the syrinx; the gristly or bony bar across the lower end of the windpipe, at the point where the trachea forks into right and left bronchi.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) A delicate bar of cartilage connecting the dorsal and ventral extremities of the first pair of bronchial cartilages in the syrinx of birds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A delicate bar of cartilage connecting the dorsal and ventral extremities of the first pair of bronchial cartilages in the syrinx of birds.