716 ORGANS OF DIGESTION. 



of the future tooth. The follicle for the first molar is complete about the tenth 

 week ; the canine follows next, succeeded by the follicles for the incisors, which 

 are completed about the eleventh or twelfth week ; and, lastly, the follicle of 

 the posterior molar is completed about the fourteenth week. These changes 

 constitute the second or follicular stage. 



About the thirteenth week, the papillae begin to grow rapidly, project from 

 the follicles, and assume a form corresponding with that of the future teeth ; 

 the follicles soon become deeper, and from their margins small membranous 

 processes, or opercula, are developed, which, meeting, unite and form a lid to 

 the now closed cavity (Fig. 392). These processes correspond in shape to the 

 form of the crown of the tooth, and in number to the tubercles on its surface. 

 The follicles of the incisor teeth have two opercula, the canine three, and the 

 molars four or five each. The follicles are thus converted into dental sacs, and 

 the contained papillae become pulps. The lips of the dental groove gradually 

 advance over the follicles from behind forwards, and, uniting, gradually oblite- 

 rate it. This completes the third or saccular stage, which takes place about the 

 end of the fifteenth week. 



The deep portion of the primitive dental groove is now closed in ; but the 

 more superficial portion, near the surface of the gum, still remains open ; it is 

 called, by Mr. Goodsir, the secondary dental groove ; from it are developed the 

 ten anterior permanent teeth. About the fourteenth week, certain lunated 

 depressions are formed, one behind each of the sacs of the rudimentary milk- 

 teeth. They are ten in number in each jaw, and are formed successively from 

 before backwards ; they are the rudimentary follicles of the four permanent 

 incisors, the two canine, and the four bicuspids. As the secondary dental 

 groove closes in, the follicles become closed cavities of reserve (Fig. 392). The 

 cavities soon elongate, and recede from the surface into the substance of the 

 gum, behind the sacs of the deciduous teeth, and a papilla projects from the 

 bottom of each, which is the germ of the permanent tooth ; at the same time, 

 one or more opercula are developed from the sides of the cavity ; and these 

 uniting, divide it into 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. 393). 



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 re- 

 serve 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-fibrous 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 



