THE ORAL CAVITY. 



247 



milk-teeth, which are due, as in like conditions of the bone, to the 

 action of certain cells, which are here known as " odontoclasts." 

 The crowns of the milk-teeth are then thrown off, one by one, by 

 the growing permanent teeth. 



For further information as to the teeth and their development, 

 see the articles by v. Ebner (Scheff 's " Handbuch der Zahnheil- 

 kunde" and in Kolliker's "Handbuch der Gewebelehre," Bd. iii), 

 whose studies we have to a great extent followed on this subject. 



2. THE TONGUE. 



The Lingual Mucous Membrane and its Papillae. The 



mucous membrane of the tongue differs in general very little from 



Fig. 191. Fungiform papilla from human tongue. 



that lining the rest of the oral cavity. It must, however, be borne 

 in mind that in the greater part of the tongue the submucosa is 

 poorly developed, and as a consequence the mucous membrane on 

 the upper surface and base of the tongue is scarcely movable. 

 Other peculiarities of the lingual mucous membrane are the absence 

 of glands in the mucosa on the upper surface of the tongue, 

 although glands are found in the musculature of the tongue, their 

 ducts passing through the mucosa, the presence of epithelial 

 papillae, and of lymph-follicles at the base of the tongue. 



The upper surface of the tongue is roughened by the presence 

 of epithelial projections, the lingual papilla. The latter are almost 

 entirely epithelial structures, and should not be confused with those 

 papillae which are composed exclusively of connective tissue. There 



