GROWTH OF TEETH. 97 



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. follictilar; 

 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 papillse 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 perma- 

 nent, 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 development 

 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 readily 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 por- 

 tions representing the future fangs. The dental sac is composed of two 

 layers, an internal or vascular layer, which was originally a part of the 

 mucous surface of the mouth, and a cellulo-fibrous layer, analogous to the 

 corium of the mucous membrane. Upon the formation of this sac by the 

 closure of the follicle, the mucous membrane resembles a serous mem- 

 brane in being a shut sac, and may be considered as consisting of a tunica 

 propria, which invests the pulp ; and a tunica reflexa, which is adherent 

 by its outer surface with the structures in the jaw, and by the inner surface 

 is free, being separated from the pulp by an intervening cavity. As soon 

 as the moulding of the pulp has commenced, this cavity increases and 

 becomes filled with a gelatinous granular 

 substance, the enamel organ, which is ad- 

 herent to the whole internal surface of the 

 tunica reflexa, but not to the tunica propria 

 and pulp. At the same period, viz., dur- 

 ing the fourth or fifth month, a thin lamina 

 of ivory is formed by the pulp, and occu- 

 pies its most prominent point ; if the tooth 



* a. Capsule of a temporary incisor with the rudiment of the corresponding perma- 

 nent tooth attached, b. Capsule of a molar in the same state. A part of die gum is 

 seen above it and in contact. 



9 G 



