BLOOD-VESSELS OF DENTINE OR IVORY. 245 



twenty; and the four posterior molars is from eighteen 

 to thirty-six months. 



Professor Harris, who has devoted much attention to 

 this point, thinks Mr. Bell in error, in regard to the ap- 

 pearance of the posterior molars. His observations fix 

 the period from twenty-four (instead of eighteen) to thirty- 

 six ; though he admits exceptions to the general rule. 



The teeth have sometimes made their appearance at 

 birth, showing extreme forwardness in their eruption ; and 

 on the other hand, cases are stated, where the eruption did 

 not occur till ten years after birth. 



Eruption of the Permanent Teeth. The accompanying 

 figure (70) shows the relative position of the two sets of 

 teeth. The permanent, it will FIG 7Q 



be seen, are behind the tempo- 

 rary set; and the period of their 

 eruption is regarded much more 

 variable than the temporary. 

 The following order in the 

 time of their eruption, is given 

 by Professor Harris: "First 

 molars, from fifth to sixth year; 

 central incisors, from sixth to 

 eighth ; lateral incisors, from 

 seventh to ninth ; first bicuspids, ninth to tenth ; second 

 bicuspids, tenth to eleventh and a half; cuspidati, eleventh 

 to twelfth; second molars, twelfth to fourteenth; and the 

 third molars (dentes sapientise), from eighteenth to twen- 

 tieth." 



The power, agency, and manner in which the teeth are 

 made to rise out of their prison-houses, the alveolar cells, 

 and come through the gums, are not settled. Some sup- 

 pose the process entirely mechanical, and that the tempo- 

 rary set are pushed out of their places by the advance of 



FIG. 70 represents the two sets of teeth in a child four years old. The ex- 

 terior walls of each jaw have been removed to show the relative position of 

 each set. 



The illustrations of the temporary, together with those of the permanent 

 teeth, and several others, were kindly furnished the author by Professor 

 Harris. 



