THE STKUCTURE OF THE ELEMENTARY TISSUES. 



71 



Permanent Teeth. 



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



The times of eruption given in the above tables are only approxi- 

 mate: 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 inter- 

 val by the four incisors of the upper jaw; next follow the lateral in- 

 cisors of the lower jaw, and so on as indicated in the table till the com- 

 pletion of the milk dentition at about the age of two years. Certain 

 diseases affecting the bony skeleton, e.g., Rickets, retard the eruptive 

 period considerably. 



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 should be in use from the age of two up to 

 within a few months of the time for their successors to appear. Their 

 retention serves the purpose of preserving the necessary space sufficient 

 for the succeeding permanent teeth to occupy. 



It is important to notice that it is a molar which is the first tooth to 

 be cut in the permanent dentition, not an incisor as in the case of the 

 temporary set, and also that it appears behind the last deciduous molar 

 on each side. 



The third molars, often called Wisdoms, are sometimes unerupted 

 through life from want of sufficient jaw space and the presence of the 

 other teeth: and in highly civilized races there are evidences to show 

 that they are in process of suppression from the dental series; cases of 

 whole families in which their absence is a characteristic feature being 

 occasionally met with. 



When the teeth are fully erupted it will be observed that the upper 

 incisors and canines project obliquely over the lower front teeth and the 

 external cusps of the upper bicuspids and molars lie outside those of 

 the corresponding teeth in the lower jaw. This arrangement allows to 

 some extent of a scissor-like action in .dividing and biting food in the 

 case of incisors; and a grinding motion in that of the bicuspids and 

 molars when the side to side movements of the lower jaw bring the ex- 

 ternal cusps of the lower teeth into direct articulation with those of the 



