The Cavity Of The Mouth is described as consisting of two parts, an anterior, or vestibule, and a posterior, the mouth proper, or buccal cavity, which are separated from one another by the gums and teeth. When the mouth is tightly shut, the only available opening between the two, if the teeth be intact, is behind the last molar tooth. Through this interval between the last molar and the ascending ramus of the jaw patients affected with tetanus are frequently fed by a tube, and a similar mode is frequently adopted in cases of fracture of the lower jaw. The lips are very mobile, the mobility rendering them liable to contraction deformity, and they are composed from without inwards of skin, superficial fascia, orbicularis oris, submucous tissue containing many mucous glands, and the coronary branches from the facial artery and mucous membrane. Thus the vessels are close to the inside of the lip, and may be wounded against the teeth, and, as they possess a free anastomosis, considerable bleeding may result, the blood frequently being swallowed and subsequently vomited, causing internal injuries to be suspected. The mucous membrane of the lips rests on a loose subcutaneous tissue, which readily becomes infiltrated in inflammatory affections, causing considerable swelling. The glands in the submucous tissue are numerous, and their ducts, when blocked, may give rise to mucous cysts, which occasionally attain a large size, while a general hypertrophy of the glands may cause a uniform enlargement of the lip. Naevi frequently occur on the lips, and the lower lip is the most frequent seat of epithelioma. As the lymphatics of both lips drain to the submaxillary glands, it is there that evidence of secondary extension should first be sought. A congenital enlargement of the lip due to lymphatic hypertrophy is rarely met with, and is called macrochilia. The upper lip derives its sensation from the second and the lower from the third division of the fifth nerve, labial herpes frequently occurring over the distribution of these nerves in cases of fever, gastric disturbance, etc. Stenson's duct opens on the mucous membrane of the cheek opposite the second upper molar tooth, and as it bends almost at a right angle just prior to opening, it is necessary, when passing a probe along it, to evert the cheek in order to remove the bend. The pterygo-maxillary ligament can be seen running from above downwards toward the last molar tooth, when the mouth is widely open. The lingual or gustatory nerve lies just in front of this, below the last molar tooth and close to the bone, where it has been injured by slipping of the forceps in extraction of the last molar.
Normally the lips and cheeks press against the teeth and gums, thus rendering the cavity of the vestibule only potential. In facial paralysis, on the other hand, the lips and cheeks fall away from the teeth, permitting the accumulation of food in the vestibule, while, owing to the inability to close the lips firmly, fluid is generally permitted to run out at the side of the mouth.
Hare-lip is a uni- or bi-lateral defect in the upper lip, varying from a slight notch in the margin of the lip to a gap extending into the nostril, due to imperfect fusion between the fronto-nasal and superior maxillary processes. A continuation of the defect by the side of the nose upwards to the lower eyelid, which may be involved, is called a coloboma facialis.
A median hare-lip is rare. In the upper lip it is due to persistence of a little cleft at the extremity of the fronto-nasal process (globular process), and in the lower lip to failure of fusion of the two mandibular processes.
Macrostoma, a lateral continuation of the aperture of the mouth, is due to imperfect fusion between the maxillary and mandibular processes, while microstoma is due to excessive fusion.
The gums are normally firm and vascular. They are frequently affected in scurvy and in mercurial and chronic lead-poisoning, a blue line forming at the junction of teeth and gum in the latter affection, especially if the mouth be not kept clean. The mucous membrane covering the gums is thin and adherent, save at the reflections, and is generally torn through in fracture of the lower jaw.
The buccal cavity communicates anteriorly with the vestibule, as already described, and posteriorly, through the isthmus of the fauces, with the pharynx. It contains the tongue, and presents the openings of several salivary ducts. When the mouth is shut, and breathing is conducted through the nose, the tongue practically fills the whole cavity, rendering it, like the vestibule, potential. Wharton's duct from the submaxillary gland opens on a soft papilla on the under surface of the tongue, at its base and close to the middle line. A ridge, the plica sublingualis, runs outwards and backwards from this point on either side. It is caused by the underlying sublingual gland, the numerous ducts from which (ducts of Rivini) open near the crest of the ridge. Rarely a large duct (Bartholin's) comes from the sublingual gland and opens close to or along with Wharton's duct. Wharton's duct is indistensible, and, when blocked by a calculus, causes considerable pain. Ranula, a mucous cyst found on the floor of the mouth, may be due to dilatation of Wharton's duct or of one of the sublingual ducts, or to occlusion of one of the mucous follicles which are numerous under the tongue.
Dermoids, due to persistence of the upper end of the thyro-glossal duct, sometimes occur on the floor of the mouth. An acute form of submaxillary cellulitis, called Ludwig's angina, sometimes occurs, involving both sides of the floor of the mouth, causing great swelling, which pushes the tongue upwards and backwards, and giving rise to danger from sloughing, haemorrhage, pyaemia, and even asphyxia when the larynx is involved. It is best treated by median external incision between the chin and hyoid, and Hilton's method of opening may often be employed with advantage.