252 ORIGIN AND DEVELOPMENT OF THE TEETH. 



time, as the jaw elongates still further, this molar also de- 

 scends to its appropriate place ; and then the remainder of 

 the cavity behind dilates for the development of the dens 

 sapientice. 



Ahout the fourth or fifth month of intra-uterine life, 

 when the sacculated stage is complete, and the pulps have 

 taken the form of the future teeth, we find the growth of 

 the dentine, or ivory, commencing by being deposited first 

 on the most prominent parts of the surface of the pulp, as 

 on the cutting and grinding surfaces. This we have 

 already explained under the head of dentine. 



At birth, and previous to it, each maxillary bone pre- 

 sents, on its border, a whitish and dense tissue continuous 

 with the gum ; on raising it, membranous and bony septa, 

 though imperfect, are found between the alveoli; and in 

 the latter are seen, extending from this fibrous, white tis- 

 sue, prolongations enclosing the dental follicle in a distinct 

 sac, which is perforated at the bottom for the vessels and 

 nerves to enter. 



On examination, these dental sacs are found to resemble 

 serous sacs, and to hold some fluid. Their external or 

 parietal layer, being fibro-cellular, is connected with the 

 periosteum, lines the alveolus, and is reflected from the 

 bottom, round the vessels and nerves over the pulp, where 

 it is called the tunica propria. It is highly vascular, no- 

 ticed in the description of the pulp. This reflected portion 

 was originally, in the follicular stage, mucous membrane, 

 which the papilla and pulp, in their ascent and develop- 

 ment carry, or push before it. 



It has been stated that, on the surface of the pulp, the 

 ivory is first deposited; and that, according to Mr. Na- 

 smyth's discoveries, the superficial set of cells contained 

 in this pulp become elongated, and are first formed into 

 tooth bone ; and as these become "calcified," the layer next 

 beneath takes a similar arrangement, is in like manner 

 formed into bone, and so on, layer within layer, till the 

 greater part of the pulp is ossified. In this way a complete 

 shell is formed for the pulp, except where the vessels enter. 



