DEVELOPMENT OF THE TEETH. 



317 



like tissue interposed between the two. The inner coat is lined with 

 the epithelium of the enamel organ to be hereafter described. Their 



Fig. 218. 



Fig. 218.— The Dental Sacs 



EXPOSED IN THE JaW OF A 



Child at Birth. 



a, the left half seen from 

 the inner side ; b, the right 

 half seen from the outer 

 side ; part of the bone has 

 been removed so as to ex- 

 pose the dental sacs as they 

 lie l)elow the gum ; the lower 

 figure shows the sacs of the 

 milk-teeth and the first per- 

 manent molar, exposed by 

 removing the bone from the 

 outside ; the iipper figure 

 shows the same from the in- 

 side, together -with the sacs 

 of the permanent incisor and 

 canine teeth adhering to the 

 gum. 



blood-vessels are derived partly from the dental arteries which course 

 along the base of the sacs, and partly from those of the gums. 



The papilljB, now the dental pulps, acquire a perfect resemblance to 

 the crowns of the future teeth, and then the formation of the hard sub- 

 stance commences in them, as will be immediately described. This 

 process begins very early, and by the end of the fourth month of foetal 

 life thin shells or caps of dentine (fig. 219, 1) are found on all the 

 pulps of the milk-teeth, and a little later on that of the first per- 

 manent molar, while at the same time the coating of enamel begins to 

 be deposited on each. 





Fig. 219.— DlFFERElTT StAGES IN THE FORMATION OF A MoLAR ToOTH WITH T\TO 



Fangs (from Blake). 

 1, the distinct caps of dentine for five cusps in the earliest stage of formation ; in 2, 

 and the remaining figures, the crown is do\rawards ; in 2 and 3, the formation of the 

 crown having proceeded as far as the neck, a bridge of dentine stretches across the base 

 of the tooth-pulp ; and in 4, tlie division of the fangs is thus completed ; in 5, 6, and 

 7, the extension takes place in the fangs. 



The cap of dentine increases in extent by a growth around its edges, 

 and in thickness by additions in its interior, while the substance of the 

 pulp decreases in proportion. This growth of the tooth continues until 

 the crown is completed of its proper width, and then the pulp under- 

 goes a constriction at its base to form the cervix of the tooth, and 



