MASTICATION. 199 



the neck is the portion, sometimes slightly constricted, situated between the crown and 

 the root, covered by the edge of the gum. Thin sections of the teeth show that they 

 are composed of several distinct structures. 



Enamel of the Teeth. The crown is covered by the enamel, which is by far the 

 hardest structure in the economy. This is white and glistening and is thickest on the 

 lower portion of the tooth, especially over the surfaces which, from being opposed to 

 each other on either jaw, are most exposed to wear. It here exists in several concentric 

 layers. The incrustation of enamel becomes gradually thinner toward the neck, where 

 it ceases. Microscopical examination shows that the enamel is made up of pentagonal or 

 hexagonal rods, one end resting upon the subjacent structure, and the other, when there 

 exists but a single layer of enamel, terminating just beneath the cuticle of the teeth. 

 The hardness of the enamel varies in different persons. In some it is so soft that in mid- 

 dle life it becomes worn away from the opposing surfaces, and occasionally the teeth are 

 worn down almost to the gums ; while in others the enamel remains over the crown of 

 the tooth even in old age. 



The exposed surfaces of the teeth are still farther protected by a membrane, from 

 g^oi7 ^0 f S6oo f an mcn i n thickness, closely adherent to the enamel, called the cuticle 

 of the enamel. This delicate membrane may be demonstrated in thin sections of young 

 teeth by the addition, under the microscope, of weak hydrochloric acid. The acid at- 

 tacks the enamel, producing little bubbles of gas which press out the membrane from 

 the edge of the preparation and thus render it apparent. The cuticle presents a strong 

 resistance to reagents and is undoubtedly very useful in protecting the teeth from the 

 action of acids which may find their way into the mouth. 



Dentine. The largest portion of the teeth is composed of a peculiar structure called 

 dentine, or ivory. In many respects, particularly in its composition, this resembles 

 bone ; but it is much harder, and does not possess the lacunae and canaliculi which are 

 characteristic of the true osseous structure. The dentine bounds and encloses the cen- 

 tral cavity of the tooth, extending in the crown to the enamel and in the root, to the 

 cement. It is formed of a homogeneous fundamental substance, which is penetrated 

 by an immense number of canals radiating from the pulp-cavity toward the exterior. 

 These are called the dentinal tubules or canals. They are from ^-^5-0 to 12 ^ 06 of an 

 inch in diameter, with walls of a thickness a little less than their caliber. Their course 

 is slightly wavy or spiral. Commencing at the pulp-cavity, into which these canals open 

 by innumerable little orifices, they are found to branch and occasionally anastomose, 

 their communications and branches becoming more numerous as they approach the ex- 

 ternal surface of the tooth. The canals of largest diameter are found next the pulp-cav- 

 ity, and they become smaller as they branch. The structure which forms the walls of 

 these tubules is somewhat denser than the intermediate portion, which is sometimes 

 called the inter-tubular substance of the dentine ; but, in some portions of the tooth, the 

 tubules are so numerous that their walls touch each other, and there is, therefore, no 

 inter-tubular substance. Near their origin and near the peripheral terminations of the 

 dentinal tubules, are sometimes found solid globular masses of dentine, called dentine- 

 globules, which irregularly bound triangular or stellate cavities of very variable size. 

 These cavities have been considered as lacunae, like the lacunae of true bone ; but this 

 view is not held by the best and most recent observers. Sometimes these cavities are 

 very numerous and form regular zones near the peripheral termination of the tubules. 

 The dentine is sometimes marked by concentric lines, indicating a lamellated arrange- 

 ment. In the natural condition, the dentinal tubules are filled with a clear liquid, which 

 penetrates from the vascular structures in the pulp-cavity. 



Cement. Covering the dentine of the root, is a thin layer of true bony structure, 

 called the cement, or crusta petrosa. This is thickest at the summit and the deeper por- 

 tions of the root, where it is sometimes lamellated, and it becomes thinner near the neck. 

 It finally becomes continuous with the enamel of the crown, so that the dentine is every- 



