942 THE ORGANS OF DIGESTION. 



is enclosed in a loculus of bone which has developed around it and at some distance 

 from it ; the loculus is open at the top toward the gums, where it is closed by 

 fibrous tissue ; the developing permanent tooth is contained in the same loculus, 

 but is later separated from the temporary tooth by a growth of bone. The alveolar 

 process is not completed until after the eruption of the teeth. During eruption 

 that portion of the process overlying the crown undergoes absorption, and as soon 

 as the immature tooth has erupted the alveolar process is developed about the root, 

 whose formation is also completed after eruption. 



Development of the Permanent Teeth. The permanent teeth as regards their 

 development may be divided into two sets : (1) those which replace the temporary 

 teeth, and which, like them, are ten in number : these are the successional per- 

 manent teeth ; and (2) those which have no temporary predecessors, but are 

 superadded at the back of the dental series. These are three in number on either 

 side in each jaw, and are termed the superadded permanent teeth. They are the 

 three molars of the permanent set, the molars of the temporary set being replaced 

 by the prernolars or bicuspids of the permanent set. 



The development of the successional permanent teeth the ten anterior ones 

 in either jaw will be first considered. As already stated, the germ of each 

 milk tooth is a special thickening of the " free " edge of the common dental germ 

 or dental lamina. In like manner is formed the special dental germ of each 

 of the successional permanent teeth. But these thickenings are not at the 

 "free" edge of the dental lamina, but occur behind and lateral to each of the 

 milk-tooth germs (Fig. 564). There are ten of these, and they appear in order, 

 about the sixteenth week, on each side, the central incisor germs being the 

 first. 



These special dental germs now go through the same transformations (and 

 become enamel-organs) as were described in connection with those of the milk 

 teeth ; that is, they recede into the substance of the gum behind the germs of the 

 temporary teeth. As they recede they become flask-shaped, form an expansion 

 of their distal extremity, and finally meet a papilla, which has been formed in 

 the mesoblast, just in the same manner as was the case in the temporary teeth. 

 The apex of the papilla indentates the dental germ, which encloses it, and form- 

 ing a cap for it, undergoes analogous changes to those described in the develop- 

 ment of the milk teeth, and becomes converted into the enamel, whilst the papilla 

 forms the dentine, of the permanent tooth. In its development it becomes en- 

 closed in a dentinal sac which adheres to the back of the sac of the temporary 

 tooth. The sac of each permanent tooth is also connected with the fibrous tissue 

 of the gum by a slender band or gubernaculum, which passes to the margin of 

 the jaw behind the corresponding milk tooth (see above). 



The superadded permanent teeth three on each side in each jaw arise from 

 successive extensions backward i. e. along the line of the jaw of the common 

 dental germ from the back part of the special dental germ of the immediately 

 preceding tooth. During the fourth month or seventeenth week, in that portion 

 of the common dental germ which lies behind i. e. lateral to the special dental 

 germ of the last temporary molar tooth, and which has hitherto remained unal- 

 tered, there is developed the special dental germ of the first permanent molar 

 into which a papilla projects. In a similar manner, about the fourth month 

 after birth the second molar is formed, and about the third year the third 

 molar. 



Eruption. When the calcification of the different tissues of the milk tooth 

 is sufficiently advanced to enable it to bear the pressure to which it will be after- 

 ward subjected, its eruption takes place, the tooth making its way through the 

 gum. The gum is absorbed by the pressure of the crown of the tooth against it, 

 which is itself pressed up by the increasing size of the fang. At the same time 

 the septa between the dentinal sacs, at first fibrous in structure, ossify and thus 

 form the loculi or alveoli ; these firmly embrace the necks of the teeth and afford 

 them a solid basis. 



