FORMATION OF THE TEETH. 479 



SECT. IV. OF THE FORMATION OF THE TEETH. 



The teeth, before they become visible, are formed in the in- 

 terior of the maxillary bones. Their rudiments consist in a 

 vascular pulpy substance, having somewhat the shape of the 

 future tooth, and surrounded by two membranes or sacs. 



The external sac is soft, fibrous, and spongy, and, according 

 to Mr. Hunter, is destitute of vessels. It lines the interior of 

 the socket, thereby forming its periosteum;* adheres closely by 

 its deepest end to the dental nerves and blood vessels, and by 

 its superficial one to the cartilaginous thickening which exists 

 on the margins of the gums of infants. Fox, Blake, and Meckel, 

 consider this sac vascular, which I think more probable, from 

 its being a continuation of the periosteum, or acting as such. 

 Mr. Hunter might, therefore, mean that it was comparatively 

 destitute of vessels, and not totally. It is more spongy, loose, 

 and soft, than the internal sac, and owing to its adhesion to the 

 gum may, by pulling at the latter, be readily drawn out entire 

 with all its contents. The internal sac is extremely vascular, 

 and when successfully injected appears red all over; it is very 

 thin and transparent, and was considered by Bichat as a serous 

 membrane. It adheres to the external sac where the latter 

 corresponds with the gum; but is elsewhere detached from it 

 with the exception of its base, where it is united by the medium 

 of the vessels that penetrate to the pulp, and in doing so it ob- 

 tains its extreme vascularity from these vessels. Between it 

 and the pulp there is a mucilaginous fluid like the synovia of 

 the joints;! which causes the internal sac to protrude like a 

 hernia, if a small puncture be made through the parietes of the 

 external one. The internal sac forms an envelope to the ves- 

 sels and nerves of the pulp, and being reflected along them, 

 terminates by adhering to the base of the pulp. When the 

 tooth protrudes through the gum, the" capsule thus formed by 

 the two sacs is perforated at its apex; and wastes away, like 

 the gum, till the body of the tooth is sufficiently advanced. 

 The two capsules which are then to be considered as the peri- 

 osteum of the socket and of the root of the tooth, adhere close- 



* Serres, loc. cit. t Hunter, loc. cit. 



