A deficiency in the mylohyoid muscle, usually with a herniation of part of the sublingual gland and fat tissue through it, was present in 82 (27.3%) of 300 half-heads dissected. In 17 (11.3%) of the cadavers the condition was bilateral. The hiatus appeared as a transverse fissure between the fibers or as a broader round or oval opening. In some cases the herniation was found constricted at its base; in others the base was broad. The importance of these herniations for diagnostic and surgical purposes is stressed, and the possible factors in their development are analyzed.