THE DIGESTIVE SYSTEM 225 



I. THE HEADGUT 

 The Mouth 



The Mucous Membrane of the Mouth. — This consists of 

 stratified squamous epithelium lying upon a connective-tissue stroma 

 or tunica propria. The latter is thrown up into papillce, which do 

 not, however, appear upon the free surface of the epithelium. The 

 submucosa is a firm connective-tissue layer A\dth few elastic fibres. 

 The thickness of the epithelium, the character of the stroma, and the 

 height of the papillae vary in different parts of the mouth. There is 

 no muscularis mucosas/| 



At the junction of the skin and mucous membrane (red margin of 

 the hps) the epithehal layer is much thickened, the stroma is thinned, 

 and the papillae are very high. At this point the stratum corneura 

 of the skin passes over into the softer nucleated epithelium of the 

 mouth, while the stratum lucidum and stratum granulosum of the 

 skin terminate (see skin, page 389). 



The mucous membrane of the gums has prominent, long, 

 slender papillae, the summits of which are covered by a very thin 

 layer of epithelium. This nearness of the vascular stroma to the 

 surface accounts for the ease with which the gums bleed. That 

 portion of the gums which extends over the teeth is devoid of papil- 

 lae. The submucosa of the gums is firmly attached to the underlying 

 periosteum. 



//The mucous membrane fining the cheeks has low, small papillae, 

 and the submucosa is closely adherent to the muscular fibres of the 

 buccinator. ,1 



Covering the hard palate, the mucous membrane is tliin and the 

 short papillae are obliquely placed, their apices being directed ante- 

 riorly. The submucosa is firmly attached to the periosteum. 



Over the soft palate the papillae of the mucous membrane are low 

 or even absent. They are somewhat higher on the uvula, the pos- 

 terior surface of which shows a transitional condition of its epithe- 

 lium, areas of stratified squamous alternating with areas of stratified 

 columnar ciliated epithelium. Throughout the mucous membrane of 

 the soft palate, uvula, and fauces, the stroma and submucosa contain 

 difl'use lymphatic tissue. In some places the lymphoid cells are so 

 closely placed as to form distinct nodules. 



Glands of the Oral Mucosa.^— Distributed throughout the 



^ For description of the larger salivary glands see page 281. 

 15 



