DIGESTION. 223 



Permanent Teeth. 

 The age at which each tooth is cut is indicated in this table in years. 



Biscupid or Biscupid or 



Molars. Prsemolars. Canines. Incisors, Canines. Praemolars. Molars. 



The times of eruption given in the above tables are only approximate, 

 the limits of variation being tolerably wide. Some children may cut 

 their first teeth before the age of six months, and others not till nearly 

 the twelfth month. In nearly all cases the two central incisors of the 

 lower jaw are cut first ; these being succeeded after a short interval by 

 the four incisors of the upper jaw, next follow the lateral incisors of the 

 lower jaw, and so on as indicated in the table till the completion of the 

 milk dentition at about the age of two years. 



The milk-teeth usually come through in batches, each period of 

 eruption being succeeded by one of quiescence lasting sometimes several 

 months. The milk-teeth are in use from the age of two up to five and 

 a half years; at about this age the first permanent molars (four in num- 

 ber) make their appearance behind the milk-molars, and for a short 

 time the child has four permanent and twenty temporary teeth in posi- 

 tion at once. 



It is worthy of note that from the age of five years to the shedding 

 of the first milk-tooth the child has no fewer than forty-eight teeth, 

 twenty milk teeth and twenty-eight calcified germs of permanent teeth 

 (all in fact except the four wisdom teeth). 



Structure of a Tooth. 



A tooth is generally described as possessing a crown, neck, and fang 

 or fangs. 



The crown is the portion which projects beyond the level of the gum. 

 The neck is that constricted portion just below the crown which is em- 

 braced by the free edges of the gum, and the fang includes all below 

 this. 



On making a longitudinal section through its centre (Figs. 166, 

 167), a tooth is found to be principally composed of a hard material, 

 dentine or ivory, which is hollowed out into a central cavity which 

 resembles in general shape the outline of the tooth, and is called the 

 pulp cavity, from its containing the very vascular and sensitive tooth 

 pulp which is composed of connective tissue, blood-vessels, and nerves. 



The blood-vessels and nerves enter the pulp through a small opening 

 at the extremity of the fang. 



