The =lips= (=labia oris=) are thick folds of skin bounding the entrance to the mouth cavity. The outer surface is covered with hair; the inner surface is covered with the mucous membrane. The upper lip is marked in the median line by a deep external groove which extends upward to the septum of the nose. Along the inner surface of this groove the lip is closely united to the jaw by a thick fold, the =frenulum= of the upper lip. For some distance on each side of the frenulum the inner surface of the lip bears numerous large papillæ. The =lower= lip is also united to the jaw by a frenulum in the median line; it is again united to the jaw just caudad of the canine tooth, in the space between the latter and the first premolar. Caudad the two lips pass into each other (forming the =commissura labiorum=), and unite with the cheek. The muscles of the lips have been described (page 105).
The =cheeks= (=buccæ=) in the cat are comparatively thin and small, extending from the lips caudad to the ramus of the mandible. The outer surface is covered with hair; the inner surface is smooth and somewhat folded. The buccal cavity is rather small. On the inner surface of the cheek open the duct of the parotid (Steno’s duct), the ducts of the molar gland, and that of the infraorbital gland.
The roof of the mouth cavity is formed by the hard and soft palates. The hard palate (=palatum durum=) forms the cranial part of the roof; it is supported by the palatal plates of the maxillary and palatine bones. The mucosa of the hard palate is elevated to form seven or eight curved transverse ridges, which are concave caudad. Between the ridges are rows of papillæ. In front of the most cranial ridge is a papilla in the middle line, and at each side of the papilla is the opening of a small duct (the =incisive duct= or =Stenson’s duct=), which leads dorsad through the incisive foramen to the =vomeronasal= organ (or =organ of Jacobson=), which lies on the floor of the nasal cavity. Caudad of the hard palate the roof of the mouth is formed by the soft palate or =velum palatinum=, described below.
The floor of the mouth cavity is formed chiefly by the tongue, which extends as far caudad as the isthmus faucium. Ventrad of the free edge of the tongue the mucosa forms a prominent median vertical fold which unites the tongue with the floor of the mouth beneath it; this fold is the frenulum linguæ. On each side of the median line at the cranial border of the floor of the mouth is a prominent papilla, at the apex of which open the ducts of the submaxillary and sublingual glands, the former on the lateral side of the apex, the latter on the medial side.