DEVELOPMENT OF THE TEETH. 651 



each, which is the germ of the permanent tooth ; at the same time, one or more 

 operculse are developed from the sides of the cavity ; and these, uniting, divide it 

 into two portions ; the lower portion containing the papilla of the permanent 

 tooth, the upper narrower portion becoming gradually contracted in the same way 

 that the primitive dental groove was obliterated over the sacs of the deciduous 

 teeth (fig. 328). 



The six posterior permanent teeth in each jaw, three on each side, arise from 

 successive extensions backwards of the back part of the primitive dental 

 groove. During the fourth month, that portion of the dental groove which lies 

 behind the last temporary molar follicle remains open, and from it is developed 

 the papilla, the rudiment of the first permanent molar. The follicle in which it 

 is contained becomes closed by its operculum, and the upper part of the now- 

 formed sac elongates backwards to form a cavity of reserve, in which the papilla 

 of the second permanent molar appears at the seventh month after birth. After 

 a considerable interval, during which the sacs of the first and second permanent 

 molars have considerably increased in size, the remainder of the cavity of reserve 

 presents for the last time a series of changes similar to the preceding, and gives 

 rise to the sac and papilla of the wisdom-tooth, which appears at the sixth 

 year. 



Growth of the Teeth. As soon as the dental sacs are formed by the closing-in 

 of the follicles, they gradually enlarge, as well as their contained papillae. Each 

 sac consists of two layers ; an internal, highly vascular layer, lined by epithelium ; 

 and an external or areolo-fi brous membrane, analogous to the corium of the mucous 

 membrane. 



The dental pulps soon become moulded to the form of the future teeth, and are 

 adherent by their bases to the bottom of the dental sacs ; in the case of the molars, 

 the base of the pulp is divided into two or more portions, which form the future 

 fangs. During the fourth or fifth month of foetal life, a thin lamina or cap of dentine 

 is formed on the most prominent point of the pulp of all the milk-teeth. In the 

 incisor and canine teeth, this newly-formed lamina has the form of a hollow cone ; 

 in the molar teeth, as many separate lamina? are found as there are eminences upon 

 its crown. These lamina) grow at the expense of the pulp -substance, increasing 

 in breadth by a growth round their margins, and in thickness by a similar forma- 

 tion in its substance ; the separate cones (if a molar tooth) ultimately coalesce, and 

 the crown is completely formed. The pulp now becomes constricted, so as to form 

 the cervix ; and the remaining portion becomes narrow and elongated, to form the 

 fang. The growth of dentine takes place from the surface towards the interior, 

 until nothing but the small cavitas pulpse remains in the centre of the tooth, com- 

 municating by the aperture left at the point of each fang, with the dental vessels 

 and nerves. 



As soon as the formation of the dentine has commenced, there is developed 

 from the inner wall of the dental sac, a soft pulpy mass, the enamel organ, which 

 is intimately united to the surface of the dental pulp, or its cap of dentine. It 

 consists of a mesh of fibres, elastic and spongy, containing within its reticulations 

 fluid albumen; and at the point of junction of each fibre, a transparent nucleus is 

 visible. The surface towards the dentinal pulp is covered by a layer of elongated 

 nucleated cells, the enamel membrane. The deposition of the enamel takes place 

 on the outer surface of the cap of dentine. 



The cementum appears to be formed, at a later period of life, by the periodontal 

 membrane, extending from the margin of the enamel downwards. 



Eruption. When the calcification of the different tissues of the tooth is suffi- 

 ciently advanced to enable it to bear the pressure to which it will be afterwards 

 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 (fig. 329). Concurrent with 

 this, the septa between the dental sacs, at first fibrous in structure, soon ossify, and 



