274 STUDIES IN ADVANCED PHYSIOLOGY. 



cavity is the product of a' single odontoblast. In an adult 

 tooth these odontoblasts in reduced number may still be 

 found next to the pulp cavity. 



The growth of a tooth begins with the crown and ex- 

 tends to the fang. As this growth proceeds downwards the 

 crown is gradually pushed upwards and so soon appears 

 above the gums. While the tooth is thus developing the 

 jaw-bone around it is also developing, and so when the 

 tooth appears it finds itself firmly placed in its bony socket. 

 In the case of the milk dentition the cement which forms 

 seems to be removed by the action of osteoclasts, and so 

 these teeth soon drop out and are replaced by the perma- 

 nent set. These permanent teeth develop in a way perfectly 

 analogous to that of the milk dentition. From the epi- 

 thelium groove from which the lateral enamel outgrowths, 

 or milk teeth arise, secondary outgrowths arise which are 

 later on to form the enamel coverings of the permanent 

 teeth. These epithelial cells concerned in the formation 

 of the enamel are called adamantoblasts . The epithelium 

 groove of course soon disappears after the lateral out- 

 growths from it have resulted, and no vestige of it remains 

 in the adult jaw. 



2. THE TONGUE. 



The tongue is a muscular organ with its base attached to 

 the floor of the mouth and to the hyoid bone, and covered 

 over with a sensitive mucous membrane. On this mucous 

 membrane there are developed three kinds of papillae. 

 Scattered all over the tongue are small pointed projections 

 known as the filliform papilla. They are not very well 

 developed in man, but are in many of the lower animals. 

 The intense roughness of a cow's tongue is due to these 

 filliform papillae, while with many of the carnivora these 

 papillae enable them to scrape the bones and remove from 

 them by their rasping action all shreds of flesh. They 

 function mainly in man to give the tongue a certain rough- 

 ness so that it may more readily manipulate the food in the 

 act of mastication and in swallowing. 



