108 THE TEETH AND THEIR CARE 



cisors, two sharp canines, and four molars especially adapted for 

 crushing and grinding. The first teeth of the temporary set usu- 

 ally begin to appear when the child is six 

 months old and the last appear when he is 

 about two years old. The teeth of the 

 lower jaw usually develop before the cor- 

 nxst.-...i 14 w'.j.'z&m*** responding ones of the upper jaw. 



The permanent teeth push out the tem- 

 porary teeth. The milk molars are fol- 

 Biood vessels and nerves lowed by the permanent premolars, but 



enter the tooth cavity, which , , 



consists of connective tissue, there are no predecessors for the perma- 



through the root canal. This i 



cavity is well protected by the nent HlOlarS. 



enamel, dentine, and cement. ^ ^ permanent ^ft usually appear 



about the sixth year while the last ones may not appear until the 

 twenty-fifth year. The molars are the first to appear, then the 

 two central incisors about the seventh year, the two lateral in- 

 cisors at eight, the bicuspids at nine and ten, the canine at eleven 

 or twelve, the second molars at twelve or thirteen, and last are 

 the third molars or wisdom teeth. 



The first permanent teeth to appear are the upper first molars 

 which do not replace any of the milk teeth, but come down behind 

 the second temporary molars. When the temporary teeth of chil- 

 dren are neglected these six-year molars are often neglected, and 

 consequently have to be extracted early in life. 



Arrangement of teeth. When the mouth is closed the upper 

 front teeth protrude slightly over the lower front teeth. If the 

 teeth are irregular or out of position, this arrangement is changed. 

 Adenoids are sometimes responsible for narrowing the jaw and 

 throwing the teeth out of position. People with poor formations 

 of teeth can not chew their food properly and digestion is therefore 

 impaired. When teeth are crowded and out of place, the food may 

 remain lodged between the teeth and cause them to decay. The 

 normal contours of the face are changed by malformations of the 



