YII.] 



THE EXTERNAL SKELETON. 



249 



The epithelial lining of the stomach may take on a dense 

 horny structure, as in the gizzards of Birds, but this will be 

 again noticed under the head of Alimentary Organs. 



14. We come now to enderonic 

 appendages. As has been said, that 

 part of the dermis which underlies 

 the epidermis is devoid of hard 

 structures in man, who possesses 

 them in the sub-epithelial enderon 

 only. Such structures are the 

 TEETH. Each tooth consists of a 

 " crown," which is visible, and of a 

 "fang" or " fangs/' which are buried 

 in the gum. 



15. The mode of formation of the 

 teeth has been described in the 

 Twelfth Lesson of "Physiology:" 

 how the teeth first arise as little 

 processes (or papillae) of the dermis, 

 which appear at the bottom of a 

 groove running along each side 

 of the jaw ; how the walls of the 

 groove grow together, between and 

 over each of the papillae, and thus 

 enclose the papillae in what are 

 called the dental sacs. 



Each such sac is a pouch of the 

 dermis, enclosing a "pinched off" 

 bit of the epidermis. 



The papilla, assuming the form of 

 the future tooth, becomes hardened 

 by a deposit within it of calcareous 

 salts, the central part of the papilla 

 remaining soft (as the pulp) but 

 diminishing with the growth of the 

 tooth, till, when the tooth is full 

 grown, there is but a minute aper- 

 ture at the bottom of each root 

 to give entrance to very delicate 

 vessels and nerves. 



16. The STRUCTURE of each tooth, how it consists of three 

 substances, " dentine, enamel, and cement," and the nature of 

 each of these, have also been described in the " Elementary 

 Physiology." 



FIG 217. FOUR PLATES OF 

 BALEEN SEEN OBLIQUELY 

 FROM WITHIN. 



