278 HEAD AND NECK 



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 that communicating aperture is 

 of importance 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, an- 

 teriorly and laterally, by the gums and teeth, whilst, posteriorly, 

 it communicates, by means of the isthmus of the fauces, 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. It is into the mouth proper 

 that the ducts of the submaxillary glands and the ducts of 

 the sublingual glands open (p. 194). 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. 10). 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. The mucous membrane 

 is reflected from the inner aspect of the upper lip to the 

 alveolar margin of the maxillae, 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 

 the mucous membrane and the muscular fibres. The ducts 

 of those glands pierce the mucous membrane and open into 

 the vestibule. In each lip there is an arterial arch formed 

 by the corresponding labial arteries (p. 16). 



