DEVELOPMENT OF THE TEETH. 



183 



Fig. 87. 



217. As it is of much practical importance to understand the origin of the 

 several kinds of Human Teeth, and the times of their appearance, some de- 

 tails upon these subjects will be given ; those which relate to the mode of de- 

 velopment being principally derived from the researches of Mr. J. Goodsir.* 



a. At the sixth week of Festal life, a deep narrow groove may be perceived, in the upper 

 jaw of the Human embryo, between the lip and the rudimentary palate; this is speedily 

 divided into two by a ridge, which afterwards becomes the external alveolar process; and 

 it is in the inner groove, that the germs of the teeth subsequently appear. Hence this may 

 be termed the primitive dental groove. At about the seventh week, an ovoidal papilla, 

 consisting of a granular substance, makes its appearance on the floor of the groove, near 

 its posterior termination ; this papilla is the germ of the Anterior superior Milk Molar 

 tooth. About the eighth week, a similar papilla, which is the germ of the Canine tooth, 

 arises in front of this; and during the ninth week the germs of the Incisors make their ap- 

 pearance under the same form. During the tenth week, processes from the sides of the 

 dental groove, particularly the external one, approach each other, and finally meet before 

 and behind the papilla of the anterior Molar ; so as to inclose it in a follicle, through the 

 mouth of which it may be seen. By a similar process, the 

 other teeth are gradually inclosed in corresponding follicles. 

 The germ of the Posterior milk Molar also appears during 

 the tenth week,.as a small papilla. By the thirteenth week, 

 the follicle of the Posterior Molar is completed ; and the 

 several papilla? undergo a gradual change of form. Instead 

 of remaining, as hitherto, simple, rounded, blunt masses of 

 granular matter, each of them assumes a particular shape ; 

 the Incisors acquire in some degree the form of the future 

 teeth; the Canines become simple cones; and the Molars 

 become cones flattened transversely, somewhat similar to 

 carnivorous molars. During this period, the papillae grow 

 faster than the follicles ; so that the former protrude from 

 the mouth of the latter. At this time, the mouths of the 

 follicles undergo a change, consisting in the development of 

 their edges, so as to form Opercula; which correspond in 

 some measure with the shape of the crowns of the future 

 teeth. There are two of these opercula in the Incisive follicles, three for the Canines, and 



a 



Upper jaw of human embryo 

 at Gth week ; showing b, the primi- 

 tive Dental Groove, behind a, the 

 Lip. 



1 Diagrams illustrative of the formation of a Temporary, and its corresponding Permanent Tooth, from 



a Mucous Membrane. 



four or five for the Molars. At the fourteenth week, the inner lip of the dental groove has 

 increased so much, as to meet and apply itself in a valvular manner to the outer lip or 



* Edin. Med. and Surg. Journal, vol. li. 



