176 DIGESTION. [CHAP. xxni. 



in numberless points, and at each point of junction a transparent 

 clear nucleus is visible. 



It is elastic, spongy, loaded with fluid albumen, but destitute of 

 vessels, and it seems perfectly distinct from that columnar struc- 

 ture which appears to be afterwards converted into the enamel. 



In a vertical section of these parts, the enamel-pulp is seen covered 

 with columnar epithelium, the enamel matrix (fig. 153, A, c, B), on 

 the surface towards the dentinal or tooth-pulp ; while, on the op- 

 posite surface, the blood-vessels of the membrane lining the alveolus 

 are seen coming up to, and forming loops immediately under, the 

 enamel-pulp, without penetrating it. It is further remarkable, that 

 short tubes, filled with glandular epithelium, descend among these 

 vessels from the enamel-pulp, and end by blind extremities. How 

 these tubes, which are evidently glandular, can discharge their 

 contents, it is difficult to understand, seeing they appear to open 

 into the substance of the enamel-pulp : but their presence and 

 precise situation we have ascertained to be as we have described 

 them in the molar teeth of the nine months human foetus. 



It is not impossible that the enamel-pulp may perform the me- 

 chanical office of protecting the soft and growing tooth from pressure 

 directed on the gum, and of providing a space in which develop- 

 ment may advance without restraint. 



The next stage is that of ossification, and the earthy matter is 

 first deposited in the homogeneous membrane forming the surface 

 of the dentinal pulp. The most prominent portions of the crown 

 are the first to harden ; and the ossification proceeds inwards by 

 the gradual conversion of the pulp into the dentine, or ivory. The 

 nucleated particles of the pulp nearest the ossifying surface are 

 found arranging themselves in series vertical to that surface ; and 

 it appears, that, in order to form these vertical series, they mul- 

 tiply by transverse division, much as those of bone cartilage are 

 found to do. The earthy matters are then deposited in the in- 

 distinct cells surrounding the nuclei, so as to form the hard and 

 dense walls of the dentinal tubes, as well as in the intercellular sub- 

 stance, so as to form the inter tubular tissue of the perfect tooth. 

 The cells unite endwise, and their nuclei elongate and coalesce 

 in a manner to constitute the cavities of the tubes, and so as often 

 to retain indications of this mode of origin in their permanent form. 

 In all these processes, a striking similarity to those noticed in the 

 ossification of ordinary bone is to be traced. In proportion as 

 the ossification proceeds inwards, so as to occupy the substance 

 of the dentinal pulp, the vessels and nerves which had been deve- 

 loped iu that structure recede, and finally come to occupy the cavity 



