The oral cavity proper
The oral cavity proper is bounded above by the palate, which separates it from the nasal fossae. Its floor is formed chiefly by the tongue, which, when the mouth is closed, practically fills the cavity, only a relatively small space remaining between the dorsum of the tongue and the palate. The anterior and lateral boundaries are formed by the dental arches, while posteriorly the cavity is only partly bounded by the velum palatinum and the palatine arches, since it communicates in this direction with the pharynx through the isthmus of the fauces.
The palate, the roof of the oral cavity, is further divided into the hard and the soft palate. The former accurately corresponds to the relief of the hard palate of the skeleton, and its mucous membrane is thick and firm, like that of the gums, and is intimately connected with the periosteum by strong submucous fasciculi. It contains many mucous palatine glands which are of irregular shape and vary in size from 2 to 5 mm.
In the median line the mucous membrane of the hard palate forms a slightly elevated palatine raphe, the anterior extremity of which terminates in a small, rounded, wartlike projection, the incisive papilla* which corresponds to the position of the incisive foramen. Anteriorly it also presents three or four transverse palatine folds or ruga, the development of which is subject to considerable variation.
The soft palate or velum palatinum is a muscular plate, richly supplied with glands and covered upon both surfaces by mucous membrane, which separates the oral cavity from the nasal portion of the pharynx. It hangs obliquely downward and backward, its base being attached to the posterior border of the bony palate and its anterior surface directly continuous with the mucous membrane of the hard palate. At either side it is continuous with the palatine arches which form the lateral boundaries of the fauces and it terminates below and behind in a round conical appendage, the uvula, whose tip, when its muscles are at rest, is curved forward. The anterior surface of the soft palate is concave and directed toward the oral cavity, the posterior is convex and looks toward the pharynx. The mucous membrane of the soft palate is fairly smooth, although it is thrown into slight folds by the relaxation of the muscles; it is thinner than that of the hard palate and contains a much greater number of mucous palatine glands, which are also larger and more closely crowded together. The lateral portions of the soft palate constitute the two palatine arches, two folds of mucous membrane containing muscles, which form the lateral boundaries of the isthmus of the fauces.
The more anterior fold, the glosso palatine arch, passes in a curve from the lower margin of the soft palate to the mucous membrane of the lateral border of the tongue, where it broadens somewhat and terminates as the plica triangularis. The posterior or pharyngo palatine arch is thicker and straighter than the anterior one and extends between the soft palate and the outer wall of the oral portion of the pharynx.
Between the palatine arches upon either side is found the tonsillar sinus, in which is situated the palatine tonsil, a somewhat flat, oblong elevation upon the surface of which deep fissures or depressions, the fossulae tonsillares, are visible. Its borders are not very sharply circumscribed, but it fills more or less completely the space between the two palatine arches. Just above the tonsil there is frequently a deep triangular pit, the supraton-sillar fossa, which is believed to be the remains of the second pharyngeal pouch of the embryo.
It contains the orifices of mucous glands which are everywhere so plentiful in the tonsillar region.
Both the soft palate arid the palatine arches contain muscles which are termed the palatal and pharyngeal muscles. They are the following:
The azygos uvula (m. uvula) is a small, unpaired, flat, elongated muscle which arises from the posterior nasal spine and terminates in the apex of the uvula, being situated nearer its posterior than its anterior surface. It not infrequently shows indications of being a paired muscle.
The levator veli palatini (petrosal pin gostaphylinus) is a rather flat, elongated, paired muscle which arises from a rough area near the carotid foramen upon the inferior surface of the petrous portion of the temporal bone, and from the lower margin of the posterior extremity of the cartilaginous portion of the tuba auditiva. It passes downward and inward in the outer wall of the nasal portion of the pharynx to the soft palate, where it spreads out in a flat lamina, the fibers of which interlace with those of the opposite muscle and with the pharyngopalatinus and azygos uvulae, forming with these muscles an almost continuous muscular plate, situated nearer the posterior than the anterior surface of the soft palate, being separated from the latter surface by a thick mass of glands.
The tensor veli palatini (sphenosalpingostaphylinus) is a thin, flat, elongated muscle which arises by a short tendon from the spine of the sphenoid, from the scaphoid fossa of the internal pterygoid process, and from the outer wall of the cartilaginous tuba auditiva. It is intimately related with the internal surface of the pterygoideus internus, from which it is separated only by the buccopharyngeal fascia. The levator veli palatini is more internal and further posterior than the tensor veli palatini, from which it is separated by fatty tissue.
Above the hamular process of the internal pterygoid plate the posterior surface of the muscle passes into a narrow tendon which winds around the hamular process in the sulcus hamuli and broadens out into an aponeurosis which passes almost horizontally across the soft palate to join its fellow of the opposite side. A small bursa, the bursa m. tensoris veli palatini, separates the tendon from the bone. The aponeurosis formed by the tendons of the two tensores veli palatini is attached to the posterior margin of the bony palate and is situated in front of the radiating fibers of the levatores.
The glossopalaiinus is a flat muscular bundle which forms the anterior palatine arch. It arises from the transverse fibers of the tongue and inserts into its fellow of the opposite side in the base of the uvula, being also connected with the radiations of the levator veli palatini.
The pharyngopalatinus is better developed than the preceding muscle and forms the posterior palatine arch. It has manifold connections with the constrictors of the pharynx and may consequently be regarded as one of the pharyngeal muscles.
A portion of it comes directly from the constrictor medius and the remainder takes origin from the posterior margin of the thyreoid cartilage in connection with the inferior constrictor. In the soft palate the muscle has relations similar to those of the glossopalatinus and is particularly intimately connected with the radiation of the levator veli palatini.
The tensor veli palatini is innervated from the third division of the trigeminus through the otic ganglion; the remaining muscles are supplied from the pharyngeal plexus by fibers from the spinal accessory and pneumogastric nerves. The levator veli palatini and pharyngopalatini constrict the isthmus of the fauces.
The oral mucous membrane varies in character and thickness in different localities. In the floor of the mouth, in the sublingual region, it is thin and is separated from the underlying tissue by a loose submucous layer. In the gums and hard palate it is especially thick, and the sub-mucous layer in these situations is firmer and immovably connects the membrane with the peri- osteum. It is also thick upon the dorsum of the tongue, beneath which the submucosa becomes the lingual fascia.
A considerable portion of the oral cavity is developed from the so-called oral invagination, an ectodermal invagination which is at first separated from the endodermal intestinal tube by the pharyngeal membrane. At a certain period of development the oral invagination represents both the future oral and nasal cavities, but after the disappearance of the pharyngeal membrane and the union of the invagination with the anterior portion of the primitive gut, the oral and nasal cavities are separated by the paired palatal plates which develop from the superior maxillary processes and fuse in the median line. The lips are formed anteriorly during the formation of the face from the maxillary and mandibular processes of the first visceral arch.