140 DIGESTION. 



shows that the enamel is made up of pentagonal or hexago- 

 nal rods, one end resting npon the subjacent structure, and 

 the other, when there exists but a single layer of enamel, ter- 

 minating just beneath the cuticle of the teeth. The hardness 

 of the enamel varies in different persons. In some it is so soft 

 that in middle 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 further pro- 

 tected by a membrane, from 7Tr J- p- to TJ Vo-o- of an inch in 

 thickness, closely adherent to the enamel, called the cuticle 

 of the enamel. This delicate membrane may be demon- 

 strated 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 resist- 

 ance to reagents, and undoubtedly is very useful in protect- 

 ing 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 re- 

 spects, particularly in its composition, this resembles bone ; 

 but it is much harder, and does not possess the lacunge 

 and canaliculi which are characteristic of the true osseous 

 structure. The dentine bounds and encloses the central cav- 

 ity 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 -zj^-o-o to rsio-o- of an mc h m diameter, with 

 walls of a thickness a little less than their calibre. Their 

 course is slightly wavy or spiral. Commencing at the pulp- 



