226 



ERUPTION OF THE TEETH. 



unaffected, and remain as the fibres of Tomes. Exactly the same process occurs as in the 

 formation of bone, the odontoblasts forming around themselves a calcified matrix. The cement 

 is formed from the soft connective-tissue of the dental alveolus. 



Dentition. Duriqg the development of the first temporary or milk-teeth a special enamel 

 organ (fig. 169, c) is formed near these, but it does not undergo development until the milk- 

 teeth are shed ; even the papilla is wanting at first. When the permanent tooth begins to 



Fig. 168. Fig. 169. Fig. 170. 



Fig. 168. a, Dental ridge ; b, enamel organ ; c, beginning of the dentine germ ; d, first indication 

 of the tooth-sac. Fig. 169. a, Dental ridge ; b, enamel organ with (1) outer epithelium, 

 (2) middle stellate layer, (3) enamel prism-cell layer ; c, dentine germ with blood-vessels 

 and the long osteoblasts on the surface ; d, tooth-sac ; e, secondary enamel germ. Fig. 

 170. a, Dental ridge ; b, enamel organ ; c, dentine germ ; /, enamel ; g, dentine; h, inter- 

 val between enamel organ and the position of the tooth ; k, layer of odontoblasts. 



develop, it opens into the alveolar wall of the milk-teeth from below. The tissue of this dental 

 sac causes erosion, or eating away of the fang and even of the body of the milk-teeth, without 

 its blood-vessels undergoing atrophy. The chief agents in the absorption are the amoeboid cells 

 of the granulation tissue. [Multinuclear giant-cells also erode the fangs of the teeth.] 



Eruption of the Milk-Teeth. The following is the order in which the twenty milk-teeth cut the 

 gum, i.e., from the seventh month to the second year: Lower central incisors, upper central 

 incisors, upper lateral incisors, lower lateral incisors, first molar, canine, the second molars. 



[The figures indicate in months the period of eruption of each tooth.] 



[The permanent teeth succeed the milk-teeth, the process beginning about the seventh year. 

 Ten teeth in each jaw take the place of the milk-teeth, while six teeth appear further back in 

 each jaw. Thus the total number of permanent teeth is thirty-two. As the sacs, from which 

 the permanent teeth are developed, are formed before birth, they merely undergo the same 

 process of development as the temporary teeth, only at a much later period. The last of the 

 permanent molars the wisdom-tooth may not cut the jaw until the seventeenth to the twenty- 

 fifth year. At the sixth year the jaw contains the largest number of teeth, as all the temporary 

 teeth are present, and, in addition, the crowns of all the permanent teeth, except the wisdom- 

 teeth, making forty-eight in all.] 



[Eruption of Permanent Teeth. The age at which each tooth cuts the gum is given in years 

 in the following table : 



Molars. 



Bicuspid. 



Canines. 



Incisors. 



Canines. 



Bicuspid. 



Molars. 



17 12 

 to to 6 

 25 13 



10 9 



11 to 12 



8 7 7 8 



11 to 12 



9 10 



12 17 

 6 to to 



13 25 



[Action of Drugs on the Teeth. All the conditions for putrefaction are present in the mouth; 

 and when putrefaction occurs, the products (often acid) attack the dentine and hasten its decay. 

 Hence, the necessity for thorough daily cleansing of the teeth and mouth. The teeth may be 

 cleaned by means of a soft tooth-brush and water, with or without the use of any of the 



