DEVELOPEMENT OF TEETH. 67 



the secondary dental groove upon the summit of its crown forms a 

 large cavity of reserve, lying in contact with the dental sac upon 

 the one side and with the gum upon the superficial side. At this 

 period the deciduous teeth, and the sacs of the ten anterior perma- 

 nent teeth, increase so much in size, without a corresponding 

 lengthening of the jaws, that the first permanent molars are gra- 

 dually pressed backwards and upwards into the maxillary tubero- 

 sity in the upper jaw, and into the base of the coronoid process of 

 the lower jaw ; a position which they occupy at the eighth and 

 ninth months of foetal life. In the infant of seven or eight months 

 the jaws have grown in length, and the first permanent molar 

 returns to its piy)per position in the dental range. The cavity of 

 reserve, which had been previously elongated by the upward move- 

 ment of the first permanent molar, now dilates into the cavity which 

 that tooth has just quitted; a papilla is developed from its fundus, 

 the cavity becomes constricted, and the dental sac of the second 

 molar tooth is formed, still leaving a portion of the great cavity of 

 reserve in connexion with the superficial side of the sac. As the 

 jaws continue to grow in length, the second permanent dental sac 

 descends from its elevated position and advances forwards into the 

 dental range, following the same curve with the first permanent 

 molar. The remainder of the oavity of reserve, already length- 

 ened backwards by the previous position of the- second molar, again 

 dilates for the last time, developes a papilla and sac in the same 

 manner with the preceding, and forms the third permanent molar 

 or wisdom tooth, which, at the age of nineteen or twenty, upon 

 the increased growth of the jaw, follows the course of the first and 

 second molars into the dental range. 



From a consideration of the foregoing phenomena, Mr. Goodsir 

 has divided the process of dentition into three natural stages: 

 1, follicular; 2, saccular; 3, eruptive. The first, or follicular 

 stage, he makes to include all the changes which take place from 

 the first appearance of the dental groove and papillae to the closure 

 of their follicles ; occupying a period which extends from the sixth 

 week to the fourth or fifth month of intra-uterine' existence. The 

 second, or saccular stage, comprises the period when the follicles 

 are shut sacs, and the included papilla?, pulps ; it commences at the 

 fourth and fifth months of intra-uterine existence, and terminates 

 for the median incisors, at the seventh or eighth month of infantile 

 life, and for the wisdom teeth at about the twenty-first year. The 

 third, or eruptive stage, includes the completion of the teeth, the 

 eruption and shedding of the temporary set, the eruption of the per- 

 manent, and the necessary changes in the alveolar processes. It 

 extends from the seventh month till the twenty-first year. 



" The anterior permanent molar" says Mr. Goodsir, "is the most 

 remarkable tooth in man, as it forms a transition between the milk 

 and the permanent set." If considered anatomically, i. e. in its 

 developement from the primitive dental groove, by a papilla and 

 follicle, " it is decidedly a milk tooth ;" if physiologically, " as the 



