THE TONGUE 131 



enamel-germ become transformed into branching corpuscles (GJ com- 

 municating by their processes, and thus forming a continuous net- 

 work. The enamel-germ, after it is thus modified, is known as the 

 enamel -organ. 



The dentine of the tooth is formed by calcification of the surface of 

 the papilla. At this surface there is a well-marked layer of odonto- 

 blasts (fig. 157), and these produce a layer of dentinal matrix which 

 forms a sort of cap to the papilla, and which soon becomes calcified by 

 the deposition of globules of calcareous matter. Processes of the 

 odontoblasts remain in the dentine as it is forming, and thus the 

 dentinal tubules are produced. Subsequently other layers of dentine 

 are formed within the first by a repetition of the same process, and in 

 this way the papilla gradually becomes calcified. A part, however, 

 remains unaltered in the centre of the tooth, and with its covering of 

 odontoblasts forms the pulp. 



The ten milk-teeth are formed in each jaw in this manner. 

 These, however, become lost within a few years after birth, and are 

 replaced by permanent teeth in much the same way that a new succes- 

 sion of hairs occurs. A small outgrowth takes place at an early period 

 from the enamel-germ of each of the milk-teeth (fig. 155, D,/_p), and 

 this eventually becomes the germ of the corresponding permanent 

 tooth. It gradually enlarges, acquires a papilla, forms an enamel- 

 organ, in short, passes through the same phases of development as its 

 parent germ, and when the milk-tooth drops out of the jaw in conse- 

 quence of the absorption of its roots (by osteoclasts) the permanent 

 tooth grows up into its place. 



But there are six permanent teeth in each jaw which do not suc- 

 ceed milk-teeth ; these are the permanent molars. They are developed 

 from an extension backwards of the original epithelial thickening 

 (common enamel-germ) and the downgrowth from this into the corium 

 of three successive special enamel-germs at comparatively long intervals 

 of time. Within these the tissues of the permanent molars become 

 formed in a manner exactly similar to that in which the milk-teeth are 

 developed. 



THE TONGUE. 



The tongue is mainly composed of striated muscular fibres, running, 

 some longitudinally, and others transversely. It is covered by a mucous 

 membrane, the epithelium of which, like that of the rest of the mouth, is 

 thick and stratified, and conceals microscopic papillae (fig. 158) like 

 those of the skin. Besides these, the upper surface of the organ is 

 covered with larger papillae, which give it a rough appearance. These, 

 which are termed the lingual papilla, are of three kinds: (1) About 

 twelve or thirteen comparatively large circular projections, each of which 

 is surrounded by a narrow groove (fossa), external to which the mucous 



