STRUCTURE AND DEVELOPMENT OF THE TEETH. 



2 75 



v. Brunn the enamel extends along down the entire root of the tooth during the 

 process of development, but is subsequently lost in this situation. 



The dentine is formed on the uppermost surface of the protruding connective- 

 tissue dentinal papilla, the odontoblasts (Fig. 105; Fig. 106, k) arranged here in 

 a continuous layer becoming calcified, but in such a manner that uncalcified fibers, 

 the dentinal fibrils, remain. "By means of the process of the pulp each odonto- 

 blast is connected with the deeper lying, grad- 

 ually growing cells of the young pulp, so that 

 when an odontoblast is ossified down to the 

 rudiment of its fibril another takes its place, 

 without the continuity of the fibril being inter- 

 rupted. Accordingly, each dentinal fibril with 

 its anastomoses, must be considered as a rudi- 

 ment of several communicating odontoblasts." 

 In the hardening of dentine the same process 

 occurs as in that of ossification by osteoblasts. 



The cement is derived from the soft connec- 

 tive tissue of the alveolar process by ossification. 

 This connective tissue arises from the entire 

 base of the dental sac. 



The Shedding of the Teeth. Even during the 

 development of the milk-teeth, a special enamel 

 organ (Fig. 105, c) for the permanent teeth 

 forms by the side of that for the temporary 



teeth ; but its growth is held in check until the FlG - 105. a, Dental ridge; 6, enamel or- 



time for the shidding of the teeth. The papilla ^^^S^^S 



of the permanent tOOth IS absent at the begin- a mel cell layer; c, dentinal papilla, 



ning. As the permanent tooth grows, its dental with blood-vessels and layer of elon- 

 sae first breaks through the alveolar wall of the ffitSdtSfcS^ 

 temporary tooth from below. The tissue of this 

 dental sac, acting as an eroding granulation- 

 tissue, causes absorption of the root of the temporary tooth and later also of 

 its body, up to the crown, without its blood-vessels undergoing atrophy. The 

 ameboid cells of the granulation-tissue engage in a process of undermining in 

 the absorption of the temporary teeth by means of processes they send out, taking 

 up, like phagocytes, calcareous fragments of the disintegrating tooth. 



From the ninth month to the second year the twenty temporary teeth appear 



in the following order: lower internal incisors, 

 a upper internal incisors, upper external inci- 



sors, lower external incisors, first molar, 

 canine and second molar teeth. 



The shedding of the teeth begins in the 

 seventh year, in the same order (the decidu- 

 ous molars being replaced by the bicuspids) . 

 Then three new molars appear behind the 

 bicuspid teeth, the most posterior at about 

 the age of twenty years, therefore called 

 "wisdom-teeth." They may, however, ap- 

 pear as late as the eightieth year. Thus, 

 the adult has thirty-two teeth. 



According to Zuckerkandl, epithelial re- 

 mains are found in the gums behind the last 

 molar teeth, which must be regarded as the 

 rudiment of a fourth undeveloped molar tooth. 

 An analogous condition has been noted in 

 animals. 



The uninterrupted growth of the incisor 

 teeth may be readily observed in rodents, 

 replacing the free ends worn off by chewing. 

 If the opposing incisor teeth of a rodent are extracted, the remaining teeth, no 

 longer worn off by mutual attrition, grow from the jaw in the form of a long arch. 

 That in human beings also a continual replacement of the teeth must occur canno 

 be doubted. Landois has observed the advance toward the masticating si 

 and the final disappearance in from 8 to 9 years of rachitic, atrophic, circul 

 zones that must have formed on the permanent teeth of a boy even before 



FIG. 106. a, Dental ridge; b, enamel organ; 

 c, dentinal papilla; /, enamel; g, dentine; 

 h, gap between enamel organ and den- 

 tinal papilla; k, odontoblast layer. 



