The mouth, or oral cavity, is the beginning of the alimentary canal. Its roof is formed by the palate, the tongue rises up out of its floor, and the cheeks and lips form its boundaries laterally and anteriorly. The mouth communicates anteriorly with the external environment by the rima oris, or oral orifice, and posteriorly with the pharynx through the isthmus of the fauces. The oral cavity is divided into the vestibule and oral cavity proper by the teeth and alveolar processes of the mandible and maxilla. When the mouth is closed, these two parts are connected only by the small spaces between the teeth and a variable gap between the last molar tooth and the ramus of the mandible, through which a catheter can be passed for feeding when the jaws are closed tightly by muscle spasm.
When the lips are everted, a midline fold of mucous membrane, known as the frenulum, can be seen extending from each lip to the adjacent gum. These frenula may cause problems when fitting artificial dentures. Also in the vestibule, opposite the crown of the second maxillary molar tooth, is a small eminence through which the duct of the parotid gland opens. These structures of the vestibule are readily visible and can usually be felt by the tongue. Many small glands are located in the mucous membrane of the lips (labial glands) and of the cheeks (buccal glands), which empty their secretions directly into the vestibule.
The lips (upper and lower) are extremely mobile folds, which form the margins of the rima oris and meet laterally at the right and left angles of the mouth, where they become continuous with the cheeks. The framework of the lip is formed by the orbicularis oris muscle, external to which is skin with its subcutaneous tissue and internal to which is the mucous membrane. The red area of the lip has an intermediate appearance between the cheek skin and the mucous membrane.
The general structure of the cheek is similar to that of the lip. The framework is formed by the buccinator muscle, strengthened by a firm fascial layer, with skin and subcutaneous tissue external to it and a mucous membrane on the internal side. On the external surface of the buccinator muscle, at the anterior border of the masseter muscle, lies the buccal fat pad, which is especially prominent in the infant.
When the tip of the tongue is turned superiorly and posteriorly, several structures come into view. In the midline is the frenulum of the tongue. Immediately lateral to each side of the frenulum is a sublingual caruncle, at the apex of which is the opening of the submandibular duct. Running posterolaterally from the sublingual caruncle is a raised fold of mucosa caused by the underlying sublingual gland, with openings of several small ducts of this gland scattered along it. At each side of the undersurface of the tongue is the fimbriated fold and, medial to that, the deep lingual vessels are visible through the mucous membrane.
By direct examination of the open mouth, in addition to the structures described above, one can see the palate, the palatoglossal fold, and the palatopharyngeal fold, with the palatine tonsil between them, the teeth, and the tongue.
In an at-rest state, the upper and lower teeth are apt to be slightly separated from each other, the tongue is at least partially in contact with the palate, and the vestibule is nearly obliterated by the lips and cheeks lying against the teeth and gums.