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The vestibulum oris.

The vestibulum oris (also termed the buccal cavity) is a narrow, somewhat semicircular space situated between the cheeks and lips and the teeth. When the upper and lower teeth are in apposition it communicates with the oral cavity proper behind the last molar tooth, and it communicates with the outer world through the oral orifice (rima oris).
This is bounded by the lips, which are connected at the angles of the mouth by the labial commissures and form the greater portion of the anterior wall of the vestibule. The upper lip is longer than the lower and its external surface presents a rather broad, shallow, median furrow, known as the philtrum, which runs downward toward the vermilion border and terminates in the tubercle of the upper lip. The upper lip is separated from the cheek by the nasolabial groove, which passes outward and downward in a slight curve from the ala of the nose. The outer surface of the lower lip is traversed by the metUolabial groove, a transverse furrow which separates it from the chin.
The lips are composed of the skin, the labial muscles, and the labial mucous membrane, the last containing the labial glands, which are mucous glands varying in size from that of a lentil to that of a small pea.
The posterior surfaces of the lips are connected with the mucous membrane (gingiva) covering the alveolar processes of the maxillae and mandible by thin folds of mucous membrane known as the frenula of the lips. The frenulum of the upper lipp is always longer and more distinct than that of the lower.
Lateraly from the Ups the cheeks (buccce) form the external boundaries of the vestibulum oris. Like the Ups, they consist of integument (with large hairs in the male), of muscles, and of mucous membrane which in this situation is thin and contains the buccal glands partly embedded in the buccinator muscle or even lying upon its outer surface. In the angle between the buccinator and masseter muscles is situated a marked accumulation of fatty tissue, the buccal jot mass.
The portion of oral mucous membrane w r hich envelops the alveolar processes and passes between the teeth to be attached to the interalveolar septa is of considerable thickness and is known as the gum or gingiva. It is firmly attached to the periosteum by its submucous layer and is distinguished from the remainder of the oral mucous membrane by its firm structure.
The posterior wall of the vestibulum oris is formed by the alveolar processes enveloped by the oral mucous membrane and by the anterior or anterolateral teeth.
The buccal mucous membrane also presents the orifice of the parotid duct.

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