112 THE ALIMENTARY SYSTEM. 



The lower or germinal cells are rounded and spheroidal, but 

 as the layers approach the surface the cells become gradually 

 flatter and more squamous in character. This layer is very 

 similar to the Malpighian layer of the skin. On its free 

 surface the squamous cells approach a hard and horny condi- 

 tion and form an imperfect representative of the stratum 

 corneum of the epidermis. 



The epithelial layer rests on the tunica propria, a firm 

 fibrous connective-tissue structure entirely analogous to the 

 corresponding layer of the cutis vera. The outer surface of 

 the tunica propria, as in the skin, is thrown up into numerous 

 small rounded elevations, or papUlce, projecting upward into 

 the epithelium ; the intervals between the papillae are filled 

 with epithelium-cells. 



The submucosa consists of a variable amount of areolar and 

 connective tissue extending from the tunica propria to the 

 underlying structures. It contains numerous small racemose 

 mucous glands, whose ducts open on the surface of the 

 mucous membrane. Diffuse h/mphoid tissue is present in the 

 submucosa in places, especially in the vicinity of the fauces. 



The Teeth. 



General structure : The teeth present three divisions : the 

 crown, projecting above the gum;- the fane/, buried in 

 the gum; and the neck, at the junction of the two (Figs. 46 

 and 47). 



Each tooth is made up chiefly of dentine, which is covered 

 on the crown by enamel and on the fang by crusta petrosa or 

 cement. The interior of the tooth is a cavity inclosed by the 

 dentine, called the pulp-cavity, filled with a soft tissue 

 or pulp. 



The enamel is of epiblastic and epithelial origin ; the den- 

 tine and crusta petrosa are forms of connective tissue and are 

 of mesoblastic derivation. 



The fangs of the teeth rest in sockets of the jaw-bones 

 lined by the periodontal membrane, which thus separates 

 the tooth from the bone. This membrane is a vascular 

 fibrous structure continuous with and similar in structure to 

 the periosteum of the maxillary bones ; it possesses transverse 



