178 DIGESTION. [CHAP. xxm. 



that the pulp is converted into the columns,, as the dentinal pulp is 

 converted into dentine, because the anatomical characters of the 

 pulp are so dissimilar from those of the columns. When first 

 calcified, the enamel rods are loosely aggregated, and easily separate 

 from one another under pressure ; but they gradually become so 

 firmly consolidated by the advance of the calcifying process in their 

 interstices, as to make the finished enamel the most hard and inde- 

 structible of all the products of organization. 



The development of the layer containing the ordinary lacunae of 

 bone, and which, in the human teeth, covers the fang, and is con- 

 tinued a little way within the cavity of the . root, does not seem to 

 have been so accurately studied as that of the dentine and enamel. 

 But this is the less important, as it is in all probability essentially 

 similar to that of bone, which is now pretty well understood. There 

 can be little doubt that a membranous matrix, probably like that of 

 the cranial bones, is laid down as the fang is developed, in which 

 the usual steps of ossification proceed, the lacunae and their cana- 

 liculi being, in our opinion, formed from the corpuscles of the tem- 

 porary matrix. Mr. Nasmyth has described a prolongation of this 

 layer over the entire crown of the tooth, outside the enamel. To 

 understand the formation of such a layer, we must suppose it laid 

 down in a matrix continuous with that which invests the fang, 

 passing over the crown between the enamel-pulp and the wall of 

 the sac inclusive of the lids. The crusta petrosa in the fissures be- 

 tween the enamel of the compound grinders of herbivorous ani- 

 mals must certainly be formed in this way. 



When the ossification of the dentine is so far advanced, that the 

 tooth can sustain with impunity the pressure to which it is destined, 

 and when the enamel is densely calcified, the eruptive stage occurs, 

 in which the tooth makes its way through the gum. This is due to 

 the same laws of development which govern the form and position 

 of other organs. The gum over the sac is absorbed, and the crown 

 of the tooth is forced upwards against it, chiefly by the increasing 

 size of the fang below. 



It may be stated, once for all, that, as the development of the 

 teeth proceeds, so does that of the alveoli, or the bony sockets in 

 which they are lodged; and that, by the time the teeth break through 

 the gums, their walls are sufficiently strong, and embrace the necks 

 of the teeth with firmness enough to furnish a solid basis of support. 

 Their vascular canals are developed, and especially those which con- 

 vey to each tooth its interior supply of vessels and nerves. The 

 gums and alveoli are likewise provided with vessels which play their 



