158 THE DIGESTIVE SYSTEM 



begins. This lining is a mucous membrane, or mucosa. A mucosa 

 consists of at least two different tissues: an epithelial membrane 

 and the connective tissue, or tunica propria, upon which it rests. 

 A third membrane of smooth muscle may also be present and form 

 a muscularis mucosae. 



A typical mucous membrane, therefore, consists of an epithelium, 

 a tunica propria, and a muscularis mucosa. Mucous membranes 

 rest upon a submucosa, which is a loose fibroelastic connective-tissue 

 layer. The latter is extensible, thus allowing for the passage or pres- 

 ence of quantities of food in the tract and passively contracts as the 

 food disappears. The apparent thickness of the submucosa depends 

 usually on the distention of the canal. Where no muscularis mucosa 

 is present the connective tissue just underneath the epithelium is 

 more dense and corresponds to a tunica propria; the deeper zone 

 which corresponds with the submucosa is more loosely organized. 



THE LIPS. 



A longitudinal vertical section through the lip, perpendicular to 

 the surface, shows that the outer surface consists of skin tissue 

 with an epidermis of stratified epithelium and a dermis of a dense 

 fibroelastic connective tissue. Hairs, sebaceous glands, and sweat 

 glands or other typical skin features are present, depending on the 

 animal. 



These structures change at the inner surface of the lip proper, 

 where the epithelium becomes gradually thicker and loses the 

 integumental features. Small sulicpithelial connective tissue papillae 

 appear. \Yhere the inner surface is continually moistened by the 

 fluids present in the mouth, the epithelium is still thicker and the 

 connective tissue papillae are more pronounced. The epithelium 

 of the oral surface rests upon a denser connective tissue (tunica 

 propria) which emerges into a more loosely organized, deeper con- 

 nective tissue (submucosa), supporting the larger blood vessels, 

 lymph vessels, and nerves. Small compound glands of the mucous- 

 serous type are usuall\- present, those grouped in the submucosa 

 near the margin of the skin area being called labial glands, and 

 others further l)ack are known as buccal glands. 



The region between the dermis of the skin surface and the sub- 

 mucosa of the oral mucosa is composed mainly of voluntary muscles. 



