68 GROWTH OF TEETH. 



upward movement 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 connection 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 cavity of reserve, already lengthened 

 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. folli- 

 cular ; 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 papillae 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 permanent, 

 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 de- 

 velopment from the primitive dental groove, by a papilla and follicle, 

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

 efficient grinder in the adult mouth, we must consider it a permanent 

 tooth." " It is a curious circumstance, and one which will readilv 

 suggest itself to the surgeon, that laying out of view the wisdom 

 teeth, which sometimes decay at an early period from other causes, 

 the anterior molars are the permanent teeth which most frequently 

 give way first, and in the most symmetrical manner and at the same 

 time, and frequently before the milk set." 



Growth of Teeth. Immediately that the dental follicles have been 

 closed by their opercula, the pulps become moulded into the form of 

 the future teeth ; and the bases of the molars divided into two or 

 three portions representing the future fangs. The dental sac is com- 

 posed of two layers, an internal or vascular layer, which was originally 



