164 



DIGESTION 



Enamel of the Teeth. The crown is covered with 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, espe- 

 cially over the surfaces which, from being opposed to each other on 

 either jaw, are most exposed to wear. It here exists in several con- 

 centric layers. The incrustation of enamel gradually becomes thinner 



toward the neck, where it ceases. The 

 enamel is made up of pentagonal or 

 hexagonal rods, one end resting on the 

 subjacent structure, and the other, when 

 there exists but a single layer of enamel, 

 terminating just beneath the cuticle of 

 the teeth. 



The exposed surfaces of the teeth 

 are still further protected by a mem- 

 brane, pfonF to reriinr of an inch (- 8 to 

 1.7 /*) in thickness, closely adherent to 

 the enamel, called the cuticle of the 

 enamel (Nasmyth's membrane). The 

 cuticle presents a strong resistance to 

 reagents and is useful in protecting the 

 teeth from the action of acids that may 

 find their way into the mouth. 



Dentin. The largest portion of 

 the teeth is composed of dentin. In 

 many respects, particularly in its com- 

 position, this resembles bone ; but it is 

 much harder and does not present the 

 lacunae and canaliculi, which are charac- 

 teristic of true osseous structure. The 

 dentin bounds and encloses the central 

 cavity of the tooth, extending in the 

 crown to the enamel, and in the root, to 

 the cement. It is formed of a homo- 

 geneous fundamental substance, which 

 is penetrated by a large number of canals radiating from the pulp-cavity 

 toward the exterior. These are called the dentinal tubules or canals. 

 They are 2irio^ to 12 Too" of an inch (i to 2 /A) in diameter, with walls 

 of a thickness a little less than their calibre. Their course is slightly 

 wavy or spiral. Beginning at the pulp-cavity, into which these canals 

 open, they are found to branch and occasionally anastomose, their com- 

 munications and branches becoming more frequent as they approach 



Fig. 34. Longitudinal section of a 

 molar tooth of man, X 4 reduced one- 

 half (Sobotta). 



This figure gives a general view of 

 the structure of the tooth. The pulp- 

 cavity is not cut its whole length in the 

 two roots seen in the section. We recog- 

 nize the three main elements of the tooth 

 dentin, enamel and cementum and 

 their division into crown and root. On 

 account of the low magnification, the in- 

 terglobular spaces appear only as a dark 

 zone on the surface of the dentin. 

 C, cementum ; D, dentin ; P, pulp- 

 cavity; S, enamel. 



