THE STRUCTURE OF THE TEETH. 493 







prismatic nucleated cells, the walls of which coalesce, and which form 

 moulds for the deposit of the earthy matter. In the perfectly devel- 

 oped tooth, the thin'parietes of the cells become almost, or entirely, 

 absorbed, and the prismatic earthy casts are blended together as the 

 enamel fibres. On treating a growing tooth with an acid, an exceed- 

 ingly delicate membrane or cuticle is found, covering the entire sur- 

 face, which afterwards becoming calcified and coherent with the ends 

 of the subjacent fibres, forms an impenetrable protective covering to it. 



The crusta petrosa, or cement. Fig. 84, i f , m f , is a thin layer of true 

 bone, which covers the fang, being thinnest next to the enamel, and 

 thickest along the grooves and near the point; it becomes thicker in 

 advanced age, and sometimes fills up the minute opening leading into 

 the pulp cavity. The crusta petrosa contains lacunae and canaliculi; 

 the latter in the deep layers, sometimes anastomose with the termina- 

 tions of the dental tubuli; in its thicker portions, it contains Haver- 

 sian canals, surrounded by concentric lamellae. Its outer surface is 

 firmly attached to a fibro-vascular and sensitive membrane, called the 

 periodontal membrane, which is analogous to a periosteum, and serves 

 to fasten the teeth in the alveoli or sockets of the jaw, being itself 

 united to the periosteal membrane which lines the sockets. 



The dentine gives strength and solidity to the teeth, but being pene- 

 trated by processes of the sensitive pulp, and doubtless subject to 

 nutritive changes, it is liable, when exposed, to suffer pain, and to 

 undergo a process of decay resembling caries, which may even be re- 

 paired by an exudation of dense irregular dentinal substance. The 

 dentine, though very hard, would not bear constant attrition ; hence 

 that singularly hard organized product, the enamel, is provided as a 

 covering to the exposed parts of the teeth. This enamel, however, 

 wears down, as is well seen in the incisor teeth, the primitively sharp, 

 wavy, or notched edge of which soon becomes worn to an even chisel- 

 like border. The enamel often exhibits minute fissures, and, in the 

 depressions between the cusps of the molar teeth, deep cracks, which 

 are the usual seat of commencing caries in the subjacent dentine. As 

 life advances, the crusta petrosa often forms little knobs of bone upon 

 the fangs of the teeth; and after a certain age, a deposit, partly re- 

 sembling dentine and partly bone, named osteo-dentine, or secondary 

 dentine, is sometimes slowly formed in the tooth cavity, whilst the 

 pulp itself necessarily wastes. This deposit is produced, by a conver- 

 sion of the pulp, and serves to strengthen and solidify the tooth, as its 

 crown is being worn away; in time, however, this process ends by cut- 

 ting off the vascular supply of the pulp, and leads to that final stage, 

 in which the remaining parts of the teeth drop out, and leave the eden- 

 tulous jaw of old age. 



The teeth of Man, and of the Mammalia generally, are not parts of 

 the endo-skeleton, but appendages developed in the mucous membrane 

 of the mouth, which, like the armor-plates of the armadillo, the bony 

 scales of the crocodile, and the scales and spines of fishes, all appen- 

 dages of the skin, belong to the exo-skeleton, or dermal skeleton. 



The mode of development of the teeth, and the manner in which the 

 milk teeth are shed, and the permanent teeth are cut, will be described 



