340 



Vertehrata. 



respects^ however, the tooth develops in exactly the same way as in 

 the other case : a papilla grows up to the portion of epithelium that 

 has sunk in, and so on. Dentine has a certain resemblance to 

 bone, consisting of cells and a calcified inter- 

 cellular substance. They differ, however, in 

 that each of the cells of dentine (odontoblasts) 

 possesses only a single long, filamentous 

 process, provided with delicate lateral branches, 

 which run transversely through the whole of 

 the dentine, parallel to neighbouring pro- 

 cesses ; whilst the cell itself, with its nucleus, 

 is never enclosed in the intercellular substance, 

 but lies on the upper surface of the papilla. 

 The dentine is thus provided with numerous 

 delicate canals, each containing a cell process ; 

 for as the dentine increases in thickness 

 the processes gradually elongate. The enamel 

 is a very hard substance, consisting chiefly of calcium phosphate, 

 which in the Mammalia, at least, is composed of thread-like prisms, 

 whilst in many lower forms it appears more homogeneous. It is a 

 cuticular secretion of the lowest layer of epithelial cells.* The 

 fully-formed tooth, which varies considerably in form (though all 

 varieties are modifications of the cone), may have its tip pushed 

 through the mucous membrane by the growth of adjoining parts, 

 and may be attached by its lower end to the subjacent bones 



Fig. 280. Section through 

 a tooth to show the structure 

 of the dentine; diagram- 

 matic, d dentinal tubes, 

 odontoblasts on the inner 

 side of the dentine. — Orig. 



rig. 281. Portion of upper jaw of a, 

 Lizard (Iguana) seen from the inner side ; 

 soft parts removed. Ic jaw bone, to the inner 

 side of which the teeth are firmly ankylosed 

 by a porous mass of bone 6. T' — T^ three 

 teeth which are about to fall out, the lower 

 ends being more or less absorbed (T' least, 

 T^ most) ; i' — l^ the corresponding incom- 

 pletely developed replacement teeth. — Orig. 



(or cartilage) by a fibrous portion of connective tissue, or a small 

 osseous mass, the socket, which develops between the tooth and 

 the bone. In Mammalia and some others, the teeth are placed in 

 alveoli, deep cavities in the bone, into which the lower end of 

 the tooth is sunk. The teeth are, of course, subjected to much, 

 and often rough, usage, and only remain for a limited time in the 

 mouth. They then fall out and new ones are formed ; and thus 



* For the cement, which is only present in Mammalia, see that section, 



