The oral cavity [cavum oris] represents the first segment of the alimentary canal. Its walls are exceedingly specialized in structure, corresponding to its manifold functions (mastication, insalivation, taste, speech, etc.).
Boundaries of the mouth
The oral cavity communicates anteriorly with the exterior through the transverse oral fissure [rima oris], and posteriorly with the pharynx through the isthmus of the fauces [isthmus faucium]. The anterolateral walls are formed by the flexible lips and cheeks. The roof is chiefly immovable and is formed by the upper jaw with the hard and soft palate. The movable floor is formed by the lower jaw and the tongue.
Subdivisions of the mouth
The oral cavity is subdivided by the alveolar and dental arches into an inner cavity, the oral cavity proper [cavum oris proprium], and an outer vestibule [veigtibulum oris] adjacent to the lips and cheeks. When the upper and the lower teeth are in apposition, the vestibule communicates with the oral cavity proper (aside from the small interdental spaces) only through a space behind the last molar teeth on each side. Opening into the oral cavity are certain accessory glands, the salivary glands.
Structure of the mouth
Of the typical layers of the alimentary canal, only the mucous membrane can be recognized as a continuous layer in the mouth cavity. Even this is greatly modified and in structure somewhat resembles the skin, from which it is derived and with which it is continuous at the rima oris. The submucosa is a strong fibrous layer connecting the mucosa with adjacent structures, and lodging numerous racemose mucous glands. The muscles in the walls of the mouth cavity are not homologous with the typical muscularis of the alimentary canal. The outer fibrous tunic is also wanting.
The development of the oral cavity
As stated in the section on Morphogenesis, the oral cavity has its origin in a depression, the oral fossa, situated between the ventrally bent, developing head and the region occupied by the developing heart. This fossa is bounded anteriorly by the fronto-nasal process, and laterally by the maxillary and mandibular processes, portions of the first branchial arches. The fossa is lined by ectoderm. Its floor is in apposition with the cephalic end of the archenteron, lined by entoderm, the ectoderm of the oral fossa and the entoderm of the archenteron being in immediate contact and forming the pharyngeal mem- brane. The oral fossa deepens with further development, and becomes the oral sinus. The pharyngeal membrane becomes perforated in embryos about 2 mm. in length and disappears, leaving a free communication between the oral sinus and archenteron. On each side of the developing head and in a latero-ventral position there is early developed an area of thickened ectoderm, known as the nasal area. These areas soon develop into depressions, the nasal fossae, and assume a position, one on either side of the fronto-nasal process; on each side of the fronto- nasal process there is developed a prominent protuberance, the globular process, each process forming the median wall of a nasal fossa. The lateral wall of each nasal fossa also thickens to form the lateral nasal process. With the further development, the ventral portion of each lateral nasal process fuses with the corresponding globular process, the maxillary processes also uniting with the globular processes, in this way separating the nasal fossae from the oral sinus. With the further growth toward the median line of the maxillary processes the fronto-nasal process becomes narrower, ultimately forming the nasal septum and a small median portion of the upper jaw, the remainder of the upper jaw being formed by the maxillary processes, and the lower jaw having its origin in the mandibular processes.
The mouth is rarely absent, due to failure of the stomatodeal invagination, or imperforate, due to atresia of the pharyngeal membrane. Other variations will be mentioned in connection with the various mouth organs.
The phylogenetic origin of the mouth cavity from the integument is indicated not only by the ectodermal origin of its lining epithelium, but by its general structure and its appendages. Among the latter may be noted the teeth (representing modified dermal papillae), sebaceous glands, and (in some rodents) even hairs in the mucosa lining pouches in the cheeks.
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