116 THE ALIMENTARY SYSTEM. 



sort of hood over it. The connective-tissue cells at the 

 periphery of the papilla become modified and arranged in a 

 layer of odontoblaste, which then produce the dentine. 



The enamel-organ consists of epithelium cells at first like 

 those of the lower or germinal layers of the mucous mem- 

 brane. The outermost layer of cells of the enamel-organ 

 become of a columnar or cuboidal type, while the interior 

 cells degenerate into a pulpy mass of branched cells with 

 anastomosing processes; the cells ( a adamantoblasts ") on the 

 lower surface of the enamel-organ become especially long and 

 columnar, and these cells secrete, or become transformed into, 

 columns of enamel resting on the surface of the dentine. 



The crusta petrosa is bony matter subsequently deposited 

 on the fangs by the osteogenetic action of the periodontal 

 membrane. 



The permanent teeth are formed in a similar manner to the 

 temporary teeth, their enamel-organs being developed from 

 buds given off from the epithelial processes which form the 

 enamel-organs of the temporary teeth. 



The Tongue. 



General structure : The tongue consists of a mass of volun- 

 tary muscle, covered with mucous membrane, and contains 

 small glands, lymphoid tissue, blood and lymphatic vessels, 

 the terminals of the special sense of taste, and other nerves. 



The muscular substance of the tongue is made up of fas- 

 ciculi of voluntary muscle-fibres interlacing and running in 

 all directions, supported and bound together by connective 

 tissue. A vertical fibrous septum divides the tongue into two 

 lateral halves. 



The mucous membrane, which is continuous with that of 

 the mouth and pharynx, is made up of the usual three layers, 

 of surface-epithelium, tunica propria, and submucous tissue. 



The epithelial covering consists of stratified squamous epithe- 

 lium, the surface-cells becoming rather horny ; it is very 

 similar to that lining the mouth and to the Malpighian layer 

 of the skin. The tunica propria is a firm fibrous membrane 

 supporting the epithelium and giving strength to the mucous 

 membrane. The submucosa consists of areolar tissue con- 



