172 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



papilla, which later become differentiated into the 

 dentin and dental pulp. The developing papillae 

 gradually become invested by the enamel organ and 

 one by one erupt on the surface of the oral cavity 

 either as deciduous or permanent teeth. 



It will be observed that the enamel organs are sac- 

 like structures consisting of an outer and inner layer 

 of epithelial cells. The inner layer envelops the 

 dental papillae and is destined to form the enamel 

 prisms. The outer layer becomes associated with an 

 investing sheath of connective tissue, the dental sac, 

 and serves as a temporary protection while the 

 enamel is being formed. When the tooth erupts the 

 outer lining of epithelial cells disappears. 



The tooth papillae are thus all preformed at the 

 time of birth. They remain latent and develop 

 regularly into the different teeth according to the 

 table on page 144. A serious illness of a child just 

 before their eruption may affect their healthy 

 growth by interfering with proper nutrition, and 

 imperfect and pitted teeth result, which often ac- 

 counts for an early decay. 



THE TONGUE* 



The tongue is a voluntary muscular organ that 

 occupies the floor of the mouth. In lower verte- 

 brates the tongue is a prehensile organ. In many 

 fishes it is covered with teeth, its function being to 

 capture and hold prey. In frogs and toads it is 

 covered with mucous and peculiarly modified to 

 capture insects. In reptiles it is often bifurcated, 

 very motile, and used to frighten an enemy. In 



