290 



OF THE PRIMARY TISSUES OP THE HUMAN BODY. 



union of a linear series of cells which were once distinct, their cavities having 

 become continuous ; and it is to this cause, that we are probably to attribute the 

 transverse striae with which they are marked ( 276). * 



281. The Cemental pulp seems to be a substance very closely resembling that 

 which intervenes, in the growing bone, between its surface and the investing 



periosteum. It is composed of nucleated cells, 

 Fig. 82. scattered through a granular base, lying in the 



areolse of a fibrous tissue; and this tissue is 

 continuous externally with that of the dental sac. 

 According to Prof. Owen and Mr. Tomes, the 

 process of calcification, which begins in the part 

 nearest the dentine, consists in the absorption of 

 calcareous matter into the cavities of the cells, 

 in the more close aggregation of the cells with 

 each other, and in the changes which take place 

 coincidently in their nuclei. These, which are 

 at first large granular spots of a rounded form, 

 send out radiating prolongations, which extend 

 quite to the borders of the cell ; and as the calca- 

 reous salts which penetrate the cell, are not de- 

 posited in the space occupied by the nuclei, the 

 stellate cavities, or lacunae and diverging canali- 

 culi, are left, which are so analogous to those of 

 bone, as to serve to identify the two tissues. In 

 the Cementum, as in Bone and Dentine, the con- 

 solidating substance appears to consist of mineral 

 and organic matter in a state of chemical union. 

 The boundaries of the original cells disappear ; 

 so that nothing remains in the* fully-formed ce- 

 mentum, to mark its cellular origin, save the 



stellate lacunas which represent the positions of the formerly existing nuclei. 



The thin layer of cementum, which is affirmed by Mr. Nasmyth to cover the crown 



of the newly-formed tooth, would seem to be formed by the calcification of the 



tooth-capsule itself. 2 



282. Having thus considered the mode of development of the several compo- 

 nents of the Human Teeth, we are prepared to inquire into the history of 

 evolution of each tooth as a whole, and into the successional relations of the 

 different teeth to each other. This topic has been especially elucidated by Prof. 

 Groodsir, 3 whose account will be here followed. At the 6th week of Fo3tal life, 

 a deep narrow groove may be perceived, in the upper jaw of the Human embryo, 

 between the lip and the rudimentary palate ; this is speedily divided into two 

 by a ridge, which afterwards becomes the external alveolar process ; and it is in 

 the inner groove, that the germs of the teeth subsequently appear. Hence this 

 may be termed the primitive dental groove (Fig. 83). At about the 7th week, 

 an ovoidal papilla, consisting of a granular substance, makes its appearance on the 

 floor of the groove, near its posterior termination ; this papilla is the germ of 



1 A precisely similar set of appearances has been described by the Author in the prismatic 

 cellular structure forming the shells of certain Mollusks, and has been shown by him to be 

 probably due to the same cause the coalescence of flattened epithelial cells in vertical piles. 

 (See the "Reports of the British Association," 1844, 1847.) 



2 For an account of Prof. Owen's researches on the comparative structtire and develop- 

 ment of the Teeth in the lower Vertebrata, see his "Odontography," and his Art. Teeth in 

 the "Cyclop, of Anat. and Physiol. ;" also "Princ. of Phys., Gen. and Comp.," \\ 212- 

 219, 322 /, g, 324 o, p, and 326 o, p, q, Am. Ed. 



9 "Edinburgh Medical and Surgical Journal," vol. li. 



Formation of the Cementum : m, pri- 

 mary cells ; p, their granular nuclei ; n, 

 more minutely granular blastema; o, 

 the primary cell enlarged, and receiving 

 the hardening salts ; n r , calcified blas- 

 tema; p, p>, stellate nuclei of fully- 

 formed cemental cells. 



