1564 HUMAN ANATOMY. 



completed tooth-development the tissue of the dental sac becomes denser, the part 

 opposite the root persisting as the pericementum which intimately connects the 

 cementum with the alveolar wall, while the more superficial part blends with the 

 tissue forming the gum. 



The development of the permanent teeth is early provided for by the dif- 

 ferentiation of the anlages of the secondary dental germs during the growth of the 

 first. This provision includes the thickening and outgrowth of the dental bar to form 

 the enamel-organ of second dentition, and later the appearance of a new dental pa- 

 pilla beneath the epithelial cap. The enamel-organ for the first permanent molar 

 appears about the seventeenth week of foetal life, followed soon by the corresponding 

 dental papilla. The germs of the permanent incisors and canines, including the 

 papillae, -are formed about the twenty-fourth week ; those for the first bicuspids are 

 seen at about the twenty-ninth week, and those for the second bicuspids about one 

 month later. The interval between the formation of the enamel-organ and the asso- 

 ciated dental papilla increases in the case of the last two permanent molars. While 

 the enamel-germ of the second molar appears about four months after birth and the 

 corresponding papilla two months later, the enamel-organ for the third molar, or 

 wisdom-tooth, which is visible about the third year, precedes its papilla by almost 

 two years. 



The First and Second Dentition and Subsequent Changes. — At birth 

 the jaws contain the twenty crowns of the milk-teeth, the still separate cusps of the 

 first permanent molars, one of which has begun to calcify, and the uncalcified rudi- 

 ments of the permanent incisors and canines behind and above the corresponding 

 milk-teeth of the upper jaw, behind and below those of the lower. At birth the bony 

 plate above the alveoli of the upper jaw is separated by a little diploe from the floor 

 of the orbit. The milk-teeth come through the gum in five groups at what are called 

 dental periods, separated by intervals of rest. The grouping is more regular than the 

 time of eruption. The teeth of the lower jaw have a tendency to precede their fellows 

 of the upper. 



TABLE OF ERUPTION OF MILK-TEETH.^ 



Dental Periods. Groups of Teeth. 



I. Six to eight months. Two middle lower incisors. 



II. Eight to ten months. Four upper incisors. 



III. Twelve to fourteen months. Two lateral lower incisors and four first molars. 



IV. Eighteen to twenty months. Four canines. 



V. Twenty-eight to thirty-two months. Four second molars. 



The interval between the first and second periods is practically nothing. It is 

 very common to have the first two groups appear together. After this every interval 

 is longer than the preceding one. In the matter of time no part of development is 

 more irregular than that of the teeth. The first incisors occasionally appear early in 

 the fifth month and sometimes not till the tenth, or even later. The first dentition is 

 sometimes complete at or shortly after the close of the second year. The roots are 

 not fully formed when the crowns pierce the gums. The first set of teeth is in its 

 most perfect condition between four and six years. 



Calcification of the second set begins in the first molar before birth, in the incisors 

 and canines at about six months, the bicuspids and the second upper molar in the third 

 year, the second lower molar at about six, and the wisdom-tooth at about twelve. 



The first permanent molars come into line with the milk-teeth, piercing the gums 

 before any of the latter are lost. Before eruption the upper first molars lie nearer the 

 median line and farther forward than the lower. The roots of the incisors are absorbed 

 and the crowns fall out to make way for their successors. The molars do the same 

 for the bicuspids which grow between their roots. The permanent superior canines 

 are developed above the interval between the lateral permanent incisors and the first 

 bicuspid, which are almost in contact. An expansion of the jaw is necessary for them 

 to come into place. The inferior ones have more room. Both are somewhat external 

 to their predecessors. The second upper molar comes down from above and behind, 



* From Retch's Pediatrics. 



