The anatomy of the modiolus and surrounding muscles is complicated and contributes to the morphology of the mouth corner. The authors used iodine staining and micro–computed tomography to observe the anatomy of the modiolus and surrounding muscles. Based on the micro–computed tomographic images and the three-dimensional model construction, the authors found that the muscle fibers in the modiolus were oriented continuously in the running direction of the muscle itself and that there were no chaotic clumps in the region of muscle fiber convergence. The pars marginalis of the orbicularis oris muscle connects with the deep buccinator muscle, which may account for how the pars marginalis fibers act to press the upper lip against the maxillary teeth or invert it closer to the oral cavity. These findings may be helpful for plastic surgeons who perform commissuroplasty.