A model of the sagittal plane motion of the tongue, jaw, hyoid bone and larynx is presented, based on the version of equilibrium point hypothesis. The focus is on the organization of control signals underlying vocal tract motions. A number of muscle synergies or 'basic motions' of the system are identified. It is shown that systematic sources of variation in a X-ray data base of midsagittal vocal tract motions can be accounted for with six independent commands, each corresponding to a direction of articulator motion. It is further shown that hyoid position and orientation can be predicted from the application of other vocal tract commands and need not be explicitly controlled. The dynamics of individual commands are also assessed. It is shown that the dynamic effects are not neglectable in speech-like movements because of the different dynamic behavior of soft and bony structures.