

THE TEETH. 



DENTES. 



1113 



Permanent canine 



1st permanent premolar 

 2nd permanent premolar 



1st permanent molar 



Each tooth is a calcified papilla of the mucous membrane of the mouth, and 

 consists, like that membrane, of two chief portions namely, the substantia eburnea 

 or ivory (O.T. dentine) derived from the connective tissue, and the substantia adaman- 

 tina or adamant (O.T. enamel) from the epithelial layer of the mucous membrane. 

 The substantia eburnea constitutes the chief mass of the tooth, whilst the sub- 

 stantia adamantina forms a cap for the portion which projects above the gum. 

 There is also found in the teeth another special tissue the substantia ossea (O.T. 

 cement), a form of modified bone encasing the roots, which are formed chiefly of 

 substantia eburnea. 



Both ivory and 

 adamant, but parti- 

 cularly the latter, 

 are the hardest and 

 most resistant struc- 

 tures in the body, 

 and are thus specially 

 fitted for the func- 

 tions which they 

 have to perform. 



Dentes Decidui 

 and Dentes Per- 

 manentes (Decidu- 

 ous and permanent 

 teeth). The mouth 

 of the infant at 

 birth contains no 

 teeth, although a 

 number, partly de- 

 veloped, lie em- 

 bedded in the jaWS 2nd decidupus molar 



> 1st deciduous molar 



beneath _ the gum. Mental forameil 



months FlG 879. TEETH OF A CHILD OVER SEVEN YEARS OLD (modified from Testut). 



Mandibular 

 canal 



later, teeth begin to 

 appear, and by the 

 end of the second 

 year a set, known as 

 the deciduous teeth 

 (O.T. milk teeth), 

 twenty in number, 

 has been " c u t." 

 Then follows a pause 

 of about four years, 

 during 



By the removal of the bony outer wall of the alveoli, the roots of the teeth which 

 have been erupted, and the permanent teeth which are still embedded in the 

 mandible and maxilla, have been exposed. The deciduous teeth are coloured 

 blue, the permanent teeth yellow. It will be seen that the first permanent 

 molars have appeared, the central and lateral deciduous incisors have been 

 replaced by the corresponding permanent teeth in the maxilla, but the 

 deciduous canine and molars have not yet been shed. In the mandible 

 the central deciduous incisor has been replaced by the permanent central ; 

 the lateral has not yet been shed, but its permanent successor is making its 

 way up to the surface on its lingual side. In addition, the canine and two 

 molars of the deciduous set persist. The position of the crowns of the 

 permanent teeth between the roots of the deciduous molars, and the deep 

 situation occupied by the permanent canines, should be noted. Observe also 

 the absorption of the root of the lower lateral incisor. 



which no 

 visible change takes 



place in the mouth, although in reality an active preparation for further develop- 

 ment is going on beneath the gum. 



At the end of this period, namely, about the sixth year, the next stage in the 

 production of the adult condition begins. It consists in the eruption of four new 

 teeth the first permanent molars one on each side, above and below, behind those 

 of the deciduous set. This is followed by the gradual falling out of the twenty teeth 

 which have occupied the mouth since the second year (Fig. 879), and the sub- 

 stitution for them of twenty new teeth, which take up, one by one, the vacancies 

 created by the dropping out of each of the deciduous set.. Finally, the adult condition 

 is attained by the eruption of eight additional teeth the 2nd and 3rd molars 

 two on each side, above and below, behind those which have already appeared. 

 11 of these the permanent teeth have appeared by the end of the twelfth or 

 thirteenth year, except the four dentes serotini (O.T. wisdom teeth), which are usually 



