MOUTH 3 6 5 



Above and below, it is bounded by the reflection of the 

 mucous . membrane from the lips and cheeks on to the 

 alveolar'' margins of the maxilla and mandible. Anteriorly, 

 it opens upon the face by means of the oral fissure ; whilst 

 posteriorly, it communicates, on each side, with the cavity 

 of the mouth proper through the interval between the last 

 molar tooth and the anterior border of the ramus of the 

 mandible. The existence of this communication is of import- 

 ance in cases of spasmodic closure of the jaws when all the 

 teeth are in place, because through it fluids may be introduced 

 into the posterior part of the mouth proper. 



In paralysis of the facial muscles the lips and cheeks fall away from the 

 dental arches and food is apt to lodge in the vestibule. 



Cavum Oris Proprium. The mouth proper is bounded 

 anteriorly and laterally by the gums and teeth, whilst posteriorly 

 it communicates by means of the isthmus faucium with the 

 pharynx. The floor is formed by the tongue and the mucous 

 membrane which connects it with the inner aspect of the 

 mandible ; the roof is vaulted, and is formed by the hard 

 and the soft palates. Into this part of the buccal cavity the 

 ducts of the submaxillary glands and the ducts of the sub- 

 lingual glands open (p. 288). When the mouth is closed 

 the dorsum of the tongue is usually applied more or less 

 closely to the palate and the cavity is almost completely 

 obliterated. 



The various parts which bound the oral cavity may now 

 be examined in turn. 



Labia Oris. The structure of the lips has in a great 

 measure been examined already in the dissection of the face 

 (p- I 33)- Each lip is composed of four layers: (i) Cuta- 

 neous; (2) muscular; (3) glandular; and (4) mucous. The 

 skin and mucous membrane become continuous with each 

 other at the free margin of the lip. From the inner aspect 

 of the upper lip the mucous membrane is reflected to the 

 alveolar margin of the maxilla and from the inner aspect of the 

 lower lip to the mandible. In each case it is raised in the 

 median plane in the form of a free fold termed the frenulum. 

 The muscular layer constitutes the chief bulk of the lips. It 

 is formed by the orbicularis oris and the various muscles 

 which converge upon the oral fissure. Numerous labial 

 glands lie in the submucous tissue which intervenes between 



