DEVELOPMENT OF THE PEEMANENT TEETH. 



321 



glands {dental ffl/rnd.s), and were supposed by him to secrete the tartar of the 

 teeth. Meckel thought they were small abscesses, because no aperture could be 

 detected in them. In a foetus of sis months, they were found by Sharpey to be 

 small round pearl-like bodies, in fonn of small spherical capsules of various sizes, 

 filled with epithelium. They are probably the prominences, or sprouts of the 

 outer epithelial layer of the enamel organ, already referred to. 



Development of the permanent Teeth. — The preceding description 

 of the structure of the dental sacs and pulps and of the mode of 

 formation of the several parts of a tooth, applies to the permanent as 

 well as to the milk-teeth. The origin and progressive development of 

 the sacs and pulps of the permanent teeth have still to be considered. 

 Ten permanent teeth in each jaw succeed the milk-teeth, and six are 

 superadded fui'ther back in the jaw. It will be convenient to treat 

 first of the ten anterior teeth or teeth of succession. 



The sacs and pulps of these teeth have their foundations laid before 

 birth, behind those of the milk set. Eeverting to the follicular stage 

 of the temporary teeth, which is completed about the fourteenth week, 

 it will be remembered that behind each milk-follicle there is formed a 

 small recess (fig. 214, c,/p), which is filled with epithelium derived from 

 the common enamel-germ, and this forms the germ of the corresponding- 

 permanent tooth. As already stated, these recesses escape the general 

 adhesion of the sides of the dental groove, so that when the latter 

 closes they are converted into so many cavities, enclosing epithelium, 

 which were called by Goodsir, " cavities of reserve." They are ten in 

 number in each jaw, and are formed successively from before back- 

 wards. These cavities soon elongate and recede into the substance of 



Fig. 222. 



Fig. 222. — Sketches showing the Relations of the Temporary and Permanent 

 Dental Sacs and Teeth (after Blake, with some additions). 



The lower parts of the first three 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 ; b, the sac of the milk tooth or the milk 

 tooth itself ; a', b', indicate the bony 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 the per- 

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

 place of the permanent tooth. 



VOL. II. Y 



