802 



THE TEETH. 



the jaw, the permanent sac acquires at first a pear-shap.e, and is then con- 

 nected with the gum by a solid membranous pedicle. The recess in the jaw 

 has a similar form, drawn out into a long canal for the pedicle, which opens 



Fig. 560. 



II. 



III. 



IV. 



V. 



Fig. 560. SKETCHES SHOWING THE RELATIONS OP THE TEMPORARY AND PERMANENT 

 DENTAL SACS AND TEETH (after Blake, with some additions). 



The lower parts of the three first figures, which are somewhat enlarged, represent 

 sections of the lower jaw through the alveolus of a temporary incisor tooth : a, indicates 

 the sac of the permanent tooth ; c, its pedicle ; 6, the sac of the milk tooth or the milk 

 tooth itself ; a', b', indicate the alveolar recesses in which the permanent and temporary 

 teeth are lodged, and c, the canal by which that of the former leads to the surface of 

 the bone behind the alveolus of the temporary tooth. The fourth and fifth figures, which 

 are nearly of the natural size, show the same relations in a more advanced stage, in IV, 

 previous to the change of teeth, in V, when the milk-tooth has fallen out and tbe per- 

 manent tooth begins to rise in the jaw ; c, the orifice of the bony canal leading to the 

 place of the permanent tooth. 



on the edge of the jaw, by an aperture behind the corresponding milk-tooth. 

 The permanent tooth is thus separated from the socket of the milk-tooth by 

 a bony partition, against which, as well as against the root of the milk-tooth 

 just above it, it presses in its rise through the gum, so that these parts are in 

 a greater or less extent absorbed. When this has proceeded far enough, the 

 milk-tooth becomes loosened, falls out or is removed, and the permanent 

 tooth takes its place. The absorption of the dental substance commences 

 at or near the ends of the fangs, and proceeds upwards until nothing but 

 the crown remains. The cement is first attacked, and then the dentine : 

 but the process is similar in the two tissues. The change is not produced 

 merely by pressure, but through the agency of a special cellular structure 

 developed at the time, and applied to the surface of the tooth. Hollows or 

 indentations occur upon the latter, giving it a festooned appearance : and it 

 frequently happens that the dental tissues are deposited, absorbed, and 

 redeposited alternately in the same tooth (Tomes). The milk-teeth and 

 the permanent teeth are said by Serres to be supplied by two different 

 arteries, the obliteration of the one belonging to the temporary teeth being 

 regarded by him as the cause of their destruction ; but of this there is no 

 sufficient proof. 



The six posterior (or " supet added ") permanent teeth, that is, the three 

 permanent molars on each side, do not come in the place of other teeth. 

 They arise from successive extensions of the dental groove carried backwards 

 in the jaw, posterior to the milk-teeth, and named by Goodsir " posterior 

 cavities of reserve. " 



