A close observation of tongue muscle geometry using Magnetic Resonance Imaging (MRI) gives us insight into the lingual function in vowel production. The muscle length measurement based on 3D data from four Japanese males showed that the genioglossus (GG) and hyoglossus (HG) were shortened as expected from our previous knowledge. In contrast, the styloglossus (SG) tended to differ: the extra-lingual bundles of this muscle were nearly constant in all vowels, while the intra-lingual bundles varied across vowels. The width measurement of the tongue tissue indicated that the central part of the tongue body was significantly narrower in /o/ and /a/ than in /i/, /e/, and /u/. The SG used to be considered to pull the tongue body in back vowels directly upward and backward, which now appears errornous. Our data showed evidence that the SG intra-lingual bundles and certain intrinsic muscles along with the genioglossus anterior fibers bunch up the back of the tongue to raise the tongue dorsum. Thus, the MRI analysis of tongue tissue deformation revealed a complex hydrostat mechanism involving the GG, SG intra-lingual fibers, and intrinsic muscles for vowel production.