54 TOPOGRAPHIC AND APPLIED ANATOMY. 



through the choanae toward the naso-pharynx and into the mouth, into the maxillary sinus, into 

 the ethmoidal cells, into the orbit, and also through the spheno-palatine foramen into the spheno- 

 maxillary fossa, whence they may extend outward into the temporal fossa or through the spheno- 

 maxillary fissure into the orbit. Catarrh or new-growths in the lateral wall of the nose may lead 

 to disturbances in the vicinity of the nasal duct. 



THE ORAL CAVITY. 



As the function of the oral cavity is to aid in the ingestion of food, it is not surrounded by bony 

 walls to the same extent as the nasal fossas, but its boundaries are formed in many places by 

 movable and distensible soft parts [see Fig. 20 A. ED.]. The bony support is furnished by the 

 hard palate and by the inferior maxillary bone, with their alveolar processes containing the teeth. 

 The dividing-line between the mouth and the pharynx is known as the isthmus 0} the fauces. It is 

 bounded below by the sulcus terminalis (commencing at the foramen caecum and dividing the 

 dorsum from the root of the tongue) and laterally and above by the palato-pharyngeal arch and 

 the uvula (see Fig. 21). The roof of the mouth is formed by the hard and soft palates; it is 

 covered by a thick mucous membrane, rich in glands and fat, which is immovable on account of 

 its firm attachment to the periosteum. The mylohyoid muscle is to be looked upon as the floor 

 of the mouth, although posteriorly the oral contents seem to extend into the neck without there 

 being any sharp dividing-line. [Abscesses or tumors above the mylohyoid project into the mouth, 

 those below present in the neck. ED.] 



We differentiate the vestibule (vestibulum oris) from the oral cavity (cavum oris). 



The vestibule is a horseshoe-shaped cleft between the mucous membrane of the cheeks and 

 lips and the teeth (see Fig. 16). If the finger is introduced into the vestibule and pushed back- 

 ward, when the teeth are closed, it strikes upon the hard anterior margin of the ramus of the 

 mandible, and the tip of the finger may be passed into the mouth (cavum oris) through the space 

 between the ramus of the mandible and the last molar tooth. If the denture is perfect, this space 

 is the only communication between the vestibule and the oral cavity (with the exception of the 

 narrow slits between the teeth), and, in cases of trismus, it may be utilized for the introduction of 

 liquid food, if a more favorable route has not been furnished by the loss of one or more teeth. If 

 the tip of the index-finger is placed against the anterior border of the ramus and the teeth are 

 firmly pressed together repeatedly, the anterior margin of the contracting masseter muscle may be 

 distinctly felt. The orifice of the parotid duct (Stenson's) is situated opposite to the second upper 

 molar [about 4 mm. below the reflection of the mucous membrane from gums to cheek. ED.]; 

 although small, this duct may be probed in the living subject (see page 41). 



The marked elasticity of the lateral and anterior walls of the vestibule allows us to palpate 

 the rows of teeth throughout their entire extent; it also makes it possible to recognize the altered 

 position of the mandible in anterior dislocations of this bone. Under certain circumstances the 

 maxillary sinus may be opened through the vestibule (see page 52). 



The contents of the oral cavity, as well as the region between the mouth and the pharynx, 

 should be studied as far as possible in the mouth of the reader with the aid of a hand-mirror and 

 strong sunlight. If the tongue is raised, the fraenum linguae may be seen in the median line, 



