332 



THE TEETH. 



the gum behind the cjerms of the milk teeth, above and behind in the 

 upper jaw, below and behind in the lower. In the meantime, a papilla 

 appears in the bottom of each, (that for the central incisor appearing 

 first, at about the sixth month,) and they become closed in above in 

 a similar manner to the germs of the temporary teeth as already 

 described. When these changes have taken place, the sac of the per- 

 manent tooth adheres to the back of that for the temporary tooth. 

 Both of them then continue to grow rapidly, and after a time it is 

 found that the bony socket not only forms a cell for the reception of 

 the milk-sac, but also a small posterior recess or niche for the permanent- 

 tooth-sac, with which the recess keeps pace in its growth. In the lower 

 jaw, to which our description may now, for convenience, be confined, 

 it is found that at length the permanent sac so far recedes in the bone 

 as to be lodged in a special osseous chamber at some distance below and 

 behind the milk-tooth, the two being completely separated from each 

 other by a bony partition. In descending in the jaw, the sac for the 

 permanent tooth acquires at first a pear-shape, and is then connected 

 with the gum by a solid pedicle (fig. 222, I., II., c). The recess in the 

 jaw (a') has a similar form, drawn out into a long canal for the pedicle 

 which opens on the edge of the jaw, by an aperture behind the corre- 

 sponding 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 be- 

 comes 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 pro- 

 duced merely by pressure, but, as in the case of the absorption of bone, 

 through ' the agency of multi-nucleated absorbing cells or ostoclasts, 

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



Fi". 223 —Part of the Lower Jaw op a Child of Three or Four Years Old, 



SHOWING THE RELATIONS OF THE TEMPORARY AND PERMANENT TeETH. 



The specimen contains all the inilk-teeth of the right side, together with the incisors of 

 the left ; the inner plate of the jaw has been removed, so as to expose the sacs of all the 

 permanent teeth of the right side, except the eighth or wisdom tooth, which is not yet 

 formed. The large sac near the ramus of the jaw is that of the first permanent molar, 

 and above and behind it is the commencing rudiment of the second molar. 



