The lower boundary of the oral cavity is the base of the mouth. The basis of the base of the mouth is formed by the mylohyoid muscle (musculus mylohyoideus), which is horizontally positioned between the lower jaw and the hyoid.

In the frontal area of the mouth base, a mucosal fold, known as the lingual frenulum (frenulum linguae), runs towards the alveolar process. There are two small mounds on the right and left side of the lingual frenulum, the outlet points of the submandibular salivary glands. A mucosal fold (plica sublingualis) runs along the sides of the outlet points. Here the sublingual salivary gland (glandula sublingualis) opens into several outlet passages.